| @c Copyright (C) 1988,1989,1992,1993,1994,1996,1998,1999,2000,2001,2002 Free Software Foundation, Inc. |
| @c This is part of the GCC manual. |
| @c For copying conditions, see the file gcc.texi. |
| |
| @node C Implementation |
| @chapter C Implementation-defined behavior |
| @cindex implementation-defined behavior, C language |
| |
| A conforming implementation of ISO C is required to document its |
| choice of behavior in each of the areas that are designated |
| ``implementation defined.'' The following lists all such areas, |
| along with the section number from the ISO/IEC 9899:1999 standard. |
| |
| @menu |
| * Translation implementation:: |
| * Environment implementation:: |
| * Identifiers implementation:: |
| * Characters implementation:: |
| * Integers implementation:: |
| * Floating point implementation:: |
| * Arrays and pointers implementation:: |
| * Hints implementation:: |
| * Structures unions enumerations and bit-fields implementation:: |
| * Qualifiers implementation:: |
| * Preprocessing directives implementation:: |
| * Library functions implementation:: |
| * Architecture implementation:: |
| * Locale-specific behavior implementation:: |
| @end menu |
| |
| @node Translation implementation |
| @section Translation |
| |
| @itemize @bullet |
| @item |
| @cite{How a diagnostic is identified (3.10, 5.1.1.3).} |
| |
| @item |
| @cite{Whether each nonempty sequence of white-space characters other than |
| new-line is retained or replaced by one space character in translation |
| phase 3 (5.1.1.2).} |
| @end itemize |
| |
| @node Environment implementation |
| @section Environment |
| |
| The behavior of these points are dependent on the implementation |
| of the C library, and are not defined by GCC itself. |
| |
| @node Identifiers implementation |
| @section Identifiers |
| |
| @itemize @bullet |
| @item |
| @cite{Which additional multibyte characters may appear in identifiers |
| and their correspondence to universal character names (6.4.2).} |
| |
| @item |
| @cite{The number of significant initial characters in an identifier |
| (5.2.4.1, 6.4.2).} |
| @end itemize |
| |
| @node Characters implementation |
| @section Characters |
| |
| @itemize @bullet |
| @item |
| @cite{The number of bits in a byte (3.6).} |
| |
| @item |
| @cite{The values of the members of the execution character set (5.2.1).} |
| |
| @item |
| @cite{The unique value of the member of the execution character set produced |
| for each of the standard alphabetic escape sequences (5.2.2).} |
| |
| @item |
| @cite{The value of a @code{char} object into which has been stored any |
| character other than a member of the basic execution character set (6.2.5).} |
| |
| @item |
| @cite{Which of @code{signed char} or @code{unsigned char} has the same range, |
| representation, and behavior as ``plain'' @code{char} (6.2.5, 6.3.1.1).} |
| |
| @item |
| @cite{The mapping of members of the source character set (in character |
| constants and string literals) to members of the execution character |
| set (6.4.4.4, 5.1.1.2).} |
| |
| @item |
| @cite{The value of an integer character constant containing more than one |
| character or containing a character or escape sequence that does not map |
| to a single-byte execution character (6.4.4.4).} |
| |
| @item |
| @cite{The value of a wide character constant containing more than one |
| multibyte character, or containing a multibyte character or escape |
| sequence not represented in the extended execution character set (6.4.4.4).} |
| |
| @item |
| @cite{The current locale used to convert a wide character constant consisting |
| of a single multibyte character that maps to a member of the extended |
| execution character set into a corresponding wide character code (6.4.4.4).} |
| |
| @item |
| @cite{The current locale used to convert a wide string literal into |
| corresponding wide character codes (6.4.5).} |
| |
| @item |
| @cite{The value of a string literal containing a multibyte character or escape |
| sequence not represented in the execution character set (6.4.5).} |
| @end itemize |
| |
| @node Integers implementation |
| @section Integers |
| |
| @itemize @bullet |
| @item |
| @cite{Any extended integer types that exist in the implementation (6.2.5).} |
| |
| @item |
| @cite{Whether signed integer types are represented using sign and magnitude, |
| two's complement, or one's complement, and whether the extraordinary value |
| is a trap representation or an ordinary value (6.2.6.2).} |
| |
| @item |
| @cite{The rank of any extended integer type relative to another extended |
| integer type with the same precision (6.3.1.1).} |
| |
| @item |
| @cite{The result of, or the signal raised by, converting an integer to a |
| signed integer type when the value cannot be represented in an object of |
| that type (6.3.1.3).} |
| |
| @item |
| @cite{The results of some bitwise operations on signed integers (6.5).} |
| @end itemize |
| |
| @node Floating point implementation |
| @section Floating point |
| |
| @itemize @bullet |
| @item |
| @cite{The accuracy of the floating-point operations and of the library |
| functions in @code{<math.h>} and @code{<complex.h>} that return floating-point |
| results (5.2.4.2.2).} |
| |
| @item |
| @cite{The rounding behaviors characterized by non-standard values |
| of @code{FLT_ROUNDS} @gol |
| (5.2.4.2.2).} |
| |
| @item |
| @cite{The evaluation methods characterized by non-standard negative |
| values of @code{FLT_EVAL_METHOD} (5.2.4.2.2).} |
| |
| @item |
| @cite{The direction of rounding when an integer is converted to a |
| floating-point number that cannot exactly represent the original |
| value (6.3.1.4).} |
| |
| @item |
| @cite{The direction of rounding when a floating-point number is |
| converted to a narrower floating-point number (6.3.1.5).} |
| |
| @item |
| @cite{How the nearest representable value or the larger or smaller |
| representable value immediately adjacent to the nearest representable |
| value is chosen for certain floating constants (6.4.4.2).} |
| |
| @item |
| @cite{Whether and how floating expressions are contracted when not |
| disallowed by the @code{FP_CONTRACT} pragma (6.5).} |
| |
| @item |
| @cite{The default state for the @code{FENV_ACCESS} pragma (7.6.1).} |
| |
| @item |
| @cite{Additional floating-point exceptions, rounding modes, environments, |
| and classifications, and their macro names (7.6, 7.12).} |
| |
| @item |
| @cite{The default state for the @code{FP_CONTRACT} pragma (7.12.2).} |
| |
| @item |
| @cite{Whether the ``inexact'' floating-point exception can be raised |
| when the rounded result actually does equal the mathematical result |
| in an IEC 60559 conformant implementation (F.9).} |
| |
| @item |
| @cite{Whether the ``underflow'' (and ``inexact'') floating-point |
| exception can be raised when a result is tiny but not inexact in an |
| IEC 60559 conformant implementation (F.9).} |
| |
| @end itemize |
| |
| @node Arrays and pointers implementation |
| @section Arrays and pointers |
| |
| @itemize @bullet |
| @item |
| @cite{The result of converting a pointer to an integer or |
| vice versa (6.3.2.3).} |
| |
| A cast from pointer to integer discards most-significant bits if the |
| pointer representation is larger than the integer type, |
| sign-extends@footnote{Future versions of GCC may zero-extend, or use |
| a target-defined @code{ptr_extend} pattern. Do not rely on sign extension.} |
| if the pointer representation is smaller than the integer type, otherwise |
| the bits are unchanged. |
| @c ??? We've always claimed that pointers were unsigned entities. |
| @c Shouldn't we therefore be doing zero-extension? If so, the bug |
| @c is in convert_to_integer, where we call type_for_size and request |
| @c a signed integral type. On the other hand, it might be most useful |
| @c for the target if we extend according to POINTERS_EXTEND_UNSIGNED. |
| |
| A cast from integer to pointer discards most-significant bits if the |
| pointer representation is smaller than the integer type, extends according |
| to the signedness of the integer type if the pointer representation |
| is larger than the integer type, otherwise the bits are unchanged. |
| |
| When casting from pointer to integer and back again, the resulting |
| pointer must reference the same object as the original pointer, otherwise |
| the behavior is undefined. That is, one may not use integer arithmetic to |
| avoid the undefined behavior of pointer arithmetic as proscribed in 6.5.6/8. |
| |
| @item |
| @cite{The size of the result of subtracting two pointers to elements |
| of the same array (6.5.6).} |
| |
| @end itemize |
| |
| @node Hints implementation |
| @section Hints |
| |
| @itemize @bullet |
| @item |
| @cite{The extent to which suggestions made by using the @code{register} |
| storage-class specifier are effective (6.7.1).} |
| |
| @item |
| @cite{The extent to which suggestions made by using the inline function |
| specifier are effective (6.7.4).} |
| |
| @end itemize |
| |
| @node Structures unions enumerations and bit-fields implementation |
| @section Structures, unions, enumerations, and bit-fields |
| |
| @itemize @bullet |
| @item |
| @cite{Whether a ``plain'' int bit-field is treated as a @code{signed int} |
| bit-field or as an @code{unsigned int} bit-field (6.7.2, 6.7.2.1).} |
| |
| @item |
| @cite{Allowable bit-field types other than @code{_Bool}, @code{signed int}, |
| and @code{unsigned int} (6.7.2.1).} |
| |
| @item |
| @cite{Whether a bit-field can straddle a storage-unit boundary (6.7.2.1).} |
| |
| @item |
| @cite{The order of allocation of bit-fields within a unit (6.7.2.1).} |
| |
| @item |
| @cite{The alignment of non-bit-field members of structures (6.7.2.1).} |
| |
| @item |
| @cite{The integer type compatible with each enumerated type (6.7.2.2).} |
| |
| @end itemize |
| |
| @node Qualifiers implementation |
| @section Qualifiers |
| |
| @itemize @bullet |
| @item |
| @cite{What constitutes an access to an object that has volatile-qualified |
| type (6.7.3).} |
| |
| @end itemize |
| |
| @node Preprocessing directives implementation |
| @section Preprocessing directives |
| |
| @itemize @bullet |
| @item |
| @cite{How sequences in both forms of header names are mapped to headers |
| or external source file names (6.4.7).} |
| |
| @item |
| @cite{Whether the value of a character constant in a constant expression |
| that controls conditional inclusion matches the value of the same character |
| constant in the execution character set (6.10.1).} |
| |
| @item |
| @cite{Whether the value of a single-character character constant in a |
| constant expression that controls conditional inclusion may have a |
| negative value (6.10.1).} |
| |
| @item |
| @cite{The places that are searched for an included @samp{<>} delimited |
| header, and how the places are specified or the header is |
| identified (6.10.2).} |
| |
| @item |
| @cite{How the named source file is searched for in an included @samp{""} |
| delimited header (6.10.2).} |
| |
| @item |
| @cite{The method by which preprocessing tokens (possibly resulting from |
| macro expansion) in a @code{#include} directive are combined into a header |
| name (6.10.2).} |
| |
| @item |
| @cite{The nesting limit for @code{#include} processing (6.10.2).} |
| |
| @item |
| @cite{Whether the @samp{#} operator inserts a @samp{\} character before |
| the @samp{\} character that begins a universal character name in a |
| character constant or string literal (6.10.3.2).} |
| |
| @item |
| @cite{The behavior on each recognized non-@code{STDC #pragma} |
| directive (6.10.6).} |
| |
| @item |
| @cite{The definitions for @code{__DATE__} and @code{__TIME__} when |
| respectively, the date and time of translation are not available (6.10.8).} |
| |
| @end itemize |
| |
| @node Library functions implementation |
| @section Library functions |
| |
| The behavior of these points are dependent on the implementation |
| of the C library, and are not defined by GCC itself. |
| |
| @node Architecture implementation |
| @section Architecture |
| |
| @itemize @bullet |
| @item |
| @cite{The values or expressions assigned to the macros specified in the |
| headers @code{<float.h>}, @code{<limits.h>}, and @code{<stdint.h>} |
| (5.2.4.2, 7.18.2, 7.18.3).} |
| |
| @item |
| @cite{The number, order, and encoding of bytes in any object |
| (when not explicitly specified in this International Standard) (6.2.6.1).} |
| |
| @item |
| @cite{The value of the result of the sizeof operator (6.5.3.4).} |
| |
| @end itemize |
| |
| @node Locale-specific behavior implementation |
| @section Locale-specific behavior |
| |
| The behavior of these points are dependent on the implementation |
| of the C library, and are not defined by GCC itself. |
| |
| @node C Extensions |
| @chapter Extensions to the C Language Family |
| @cindex extensions, C language |
| @cindex C language extensions |
| |
| @opindex pedantic |
| GNU C provides several language features not found in ISO standard C@. |
| (The @option{-pedantic} option directs GCC to print a warning message if |
| any of these features is used.) To test for the availability of these |
| features in conditional compilation, check for a predefined macro |
| @code{__GNUC__}, which is always defined under GCC@. |
| |
| These extensions are available in C and Objective-C@. Most of them are |
| also available in C++. @xref{C++ Extensions,,Extensions to the |
| C++ Language}, for extensions that apply @emph{only} to C++. |
| |
| Some features that are in ISO C99 but not C89 or C++ are also, as |
| extensions, accepted by GCC in C89 mode and in C++. |
| |
| @menu |
| * Statement Exprs:: Putting statements and declarations inside expressions. |
| * Local Labels:: Labels local to a statement-expression. |
| * Labels as Values:: Getting pointers to labels, and computed gotos. |
| * Nested Functions:: As in Algol and Pascal, lexical scoping of functions. |
| * Constructing Calls:: Dispatching a call to another function. |
| * Naming Types:: Giving a name to the type of some expression. |
| * Typeof:: @code{typeof}: referring to the type of an expression. |
| * Lvalues:: Using @samp{?:}, @samp{,} and casts in lvalues. |
| * Conditionals:: Omitting the middle operand of a @samp{?:} expression. |
| * Long Long:: Double-word integers---@code{long long int}. |
| * Complex:: Data types for complex numbers. |
| * Hex Floats:: Hexadecimal floating-point constants. |
| * Zero Length:: Zero-length arrays. |
| * Variable Length:: Arrays whose length is computed at run time. |
| * Variadic Macros:: Macros with a variable number of arguments. |
| * Escaped Newlines:: Slightly looser rules for escaped newlines. |
| * Multi-line Strings:: String literals with embedded newlines. |
| * Subscripting:: Any array can be subscripted, even if not an lvalue. |
| * Pointer Arith:: Arithmetic on @code{void}-pointers and function pointers. |
| * Initializers:: Non-constant initializers. |
| * Compound Literals:: Compound literals give structures, unions |
| or arrays as values. |
| * Designated Inits:: Labeling elements of initializers. |
| * Cast to Union:: Casting to union type from any member of the union. |
| * Case Ranges:: `case 1 ... 9' and such. |
| * Mixed Declarations:: Mixing declarations and code. |
| * Function Attributes:: Declaring that functions have no side effects, |
| or that they can never return. |
| * Attribute Syntax:: Formal syntax for attributes. |
| * Function Prototypes:: Prototype declarations and old-style definitions. |
| * C++ Comments:: C++ comments are recognized. |
| * Dollar Signs:: Dollar sign is allowed in identifiers. |
| * Character Escapes:: @samp{\e} stands for the character @key{ESC}. |
| * Variable Attributes:: Specifying attributes of variables. |
| * Type Attributes:: Specifying attributes of types. |
| * Alignment:: Inquiring about the alignment of a type or variable. |
| * Inline:: Defining inline functions (as fast as macros). |
| * Extended Asm:: Assembler instructions with C expressions as operands. |
| (With them you can define ``built-in'' functions.) |
| * Constraints:: Constraints for asm operands |
| * Asm Labels:: Specifying the assembler name to use for a C symbol. |
| * Explicit Reg Vars:: Defining variables residing in specified registers. |
| * Alternate Keywords:: @code{__const__}, @code{__asm__}, etc., for header files. |
| * Incomplete Enums:: @code{enum foo;}, with details to follow. |
| * Function Names:: Printable strings which are the name of the current |
| function. |
| * Return Address:: Getting the return or frame address of a function. |
| * Vector Extensions:: Using vector instructions through built-in functions. |
| * Other Builtins:: Other built-in functions. |
| * Target Builtins:: Built-in functions specific to particular targets. |
| * Pragmas:: Pragmas accepted by GCC. |
| * Unnamed Fields:: Unnamed struct/union fields within structs/unions. |
| @end menu |
| |
| @node Statement Exprs |
| @section Statements and Declarations in Expressions |
| @cindex statements inside expressions |
| @cindex declarations inside expressions |
| @cindex expressions containing statements |
| @cindex macros, statements in expressions |
| |
| @c the above section title wrapped and causes an underfull hbox.. i |
| @c changed it from "within" to "in". --mew 4feb93 |
| |
| A compound statement enclosed in parentheses may appear as an expression |
| in GNU C@. This allows you to use loops, switches, and local variables |
| within an expression. |
| |
| Recall that a compound statement is a sequence of statements surrounded |
| by braces; in this construct, parentheses go around the braces. For |
| example: |
| |
| @example |
| (@{ int y = foo (); int z; |
| if (y > 0) z = y; |
| else z = - y; |
| z; @}) |
| @end example |
| |
| @noindent |
| is a valid (though slightly more complex than necessary) expression |
| for the absolute value of @code{foo ()}. |
| |
| The last thing in the compound statement should be an expression |
| followed by a semicolon; the value of this subexpression serves as the |
| value of the entire construct. (If you use some other kind of statement |
| last within the braces, the construct has type @code{void}, and thus |
| effectively no value.) |
| |
| This feature is especially useful in making macro definitions ``safe'' (so |
| that they evaluate each operand exactly once). For example, the |
| ``maximum'' function is commonly defined as a macro in standard C as |
| follows: |
| |
| @example |
| #define max(a,b) ((a) > (b) ? (a) : (b)) |
| @end example |
| |
| @noindent |
| @cindex side effects, macro argument |
| But this definition computes either @var{a} or @var{b} twice, with bad |
| results if the operand has side effects. In GNU C, if you know the |
| type of the operands (here let's assume @code{int}), you can define |
| the macro safely as follows: |
| |
| @example |
| #define maxint(a,b) \ |
| (@{int _a = (a), _b = (b); _a > _b ? _a : _b; @}) |
| @end example |
| |
| Embedded statements are not allowed in constant expressions, such as |
| the value of an enumeration constant, the width of a bit-field, or |
| the initial value of a static variable. |
| |
| If you don't know the type of the operand, you can still do this, but you |
| must use @code{typeof} (@pxref{Typeof}) or type naming (@pxref{Naming |
| Types}). |
| |
| Statement expressions are not supported fully in G++, and their fate |
| there is unclear. (It is possible that they will become fully supported |
| at some point, or that they will be deprecated, or that the bugs that |
| are present will continue to exist indefinitely.) Presently, statement |
| expressions do not work well as default arguments. |
| |
| In addition, there are semantic issues with statement-expressions in |
| C++. If you try to use statement-expressions instead of inline |
| functions in C++, you may be surprised at the way object destruction is |
| handled. For example: |
| |
| @example |
| #define foo(a) (@{int b = (a); b + 3; @}) |
| @end example |
| |
| @noindent |
| does not work the same way as: |
| |
| @example |
| inline int foo(int a) @{ int b = a; return b + 3; @} |
| @end example |
| |
| @noindent |
| In particular, if the expression passed into @code{foo} involves the |
| creation of temporaries, the destructors for those temporaries will be |
| run earlier in the case of the macro than in the case of the function. |
| |
| These considerations mean that it is probably a bad idea to use |
| statement-expressions of this form in header files that are designed to |
| work with C++. (Note that some versions of the GNU C Library contained |
| header files using statement-expression that lead to precisely this |
| bug.) |
| |
| @node Local Labels |
| @section Locally Declared Labels |
| @cindex local labels |
| @cindex macros, local labels |
| |
| Each statement expression is a scope in which @dfn{local labels} can be |
| declared. A local label is simply an identifier; you can jump to it |
| with an ordinary @code{goto} statement, but only from within the |
| statement expression it belongs to. |
| |
| A local label declaration looks like this: |
| |
| @example |
| __label__ @var{label}; |
| @end example |
| |
| @noindent |
| or |
| |
| @example |
| __label__ @var{label1}, @var{label2}, @dots{}; |
| @end example |
| |
| Local label declarations must come at the beginning of the statement |
| expression, right after the @samp{(@{}, before any ordinary |
| declarations. |
| |
| The label declaration defines the label @emph{name}, but does not define |
| the label itself. You must do this in the usual way, with |
| @code{@var{label}:}, within the statements of the statement expression. |
| |
| The local label feature is useful because statement expressions are |
| often used in macros. If the macro contains nested loops, a @code{goto} |
| can be useful for breaking out of them. However, an ordinary label |
| whose scope is the whole function cannot be used: if the macro can be |
| expanded several times in one function, the label will be multiply |
| defined in that function. A local label avoids this problem. For |
| example: |
| |
| @example |
| #define SEARCH(array, target) \ |
| (@{ \ |
| __label__ found; \ |
| typeof (target) _SEARCH_target = (target); \ |
| typeof (*(array)) *_SEARCH_array = (array); \ |
| int i, j; \ |
| int value; \ |
| for (i = 0; i < max; i++) \ |
| for (j = 0; j < max; j++) \ |
| if (_SEARCH_array[i][j] == _SEARCH_target) \ |
| @{ value = i; goto found; @} \ |
| value = -1; \ |
| found: \ |
| value; \ |
| @}) |
| @end example |
| |
| @node Labels as Values |
| @section Labels as Values |
| @cindex labels as values |
| @cindex computed gotos |
| @cindex goto with computed label |
| @cindex address of a label |
| |
| You can get the address of a label defined in the current function |
| (or a containing function) with the unary operator @samp{&&}. The |
| value has type @code{void *}. This value is a constant and can be used |
| wherever a constant of that type is valid. For example: |
| |
| @example |
| void *ptr; |
| @dots{} |
| ptr = &&foo; |
| @end example |
| |
| To use these values, you need to be able to jump to one. This is done |
| with the computed goto statement@footnote{The analogous feature in |
| Fortran is called an assigned goto, but that name seems inappropriate in |
| C, where one can do more than simply store label addresses in label |
| variables.}, @code{goto *@var{exp};}. For example, |
| |
| @example |
| goto *ptr; |
| @end example |
| |
| @noindent |
| Any expression of type @code{void *} is allowed. |
| |
| One way of using these constants is in initializing a static array that |
| will serve as a jump table: |
| |
| @example |
| static void *array[] = @{ &&foo, &&bar, &&hack @}; |
| @end example |
| |
| Then you can select a label with indexing, like this: |
| |
| @example |
| goto *array[i]; |
| @end example |
| |
| @noindent |
| Note that this does not check whether the subscript is in bounds---array |
| indexing in C never does that. |
| |
| Such an array of label values serves a purpose much like that of the |
| @code{switch} statement. The @code{switch} statement is cleaner, so |
| use that rather than an array unless the problem does not fit a |
| @code{switch} statement very well. |
| |
| Another use of label values is in an interpreter for threaded code. |
| The labels within the interpreter function can be stored in the |
| threaded code for super-fast dispatching. |
| |
| You may not use this mechanism to jump to code in a different function. |
| If you do that, totally unpredictable things will happen. The best way to |
| avoid this is to store the label address only in automatic variables and |
| never pass it as an argument. |
| |
| An alternate way to write the above example is |
| |
| @example |
| static const int array[] = @{ &&foo - &&foo, &&bar - &&foo, |
| &&hack - &&foo @}; |
| goto *(&&foo + array[i]); |
| @end example |
| |
| @noindent |
| This is more friendly to code living in shared libraries, as it reduces |
| the number of dynamic relocations that are needed, and by consequence, |
| allows the data to be read-only. |
| |
| @node Nested Functions |
| @section Nested Functions |
| @cindex nested functions |
| @cindex downward funargs |
| @cindex thunks |
| |
| A @dfn{nested function} is a function defined inside another function. |
| (Nested functions are not supported for GNU C++.) The nested function's |
| name is local to the block where it is defined. For example, here we |
| define a nested function named @code{square}, and call it twice: |
| |
| @example |
| @group |
| foo (double a, double b) |
| @{ |
| double square (double z) @{ return z * z; @} |
| |
| return square (a) + square (b); |
| @} |
| @end group |
| @end example |
| |
| The nested function can access all the variables of the containing |
| function that are visible at the point of its definition. This is |
| called @dfn{lexical scoping}. For example, here we show a nested |
| function which uses an inherited variable named @code{offset}: |
| |
| @example |
| @group |
| bar (int *array, int offset, int size) |
| @{ |
| int access (int *array, int index) |
| @{ return array[index + offset]; @} |
| int i; |
| @dots{} |
| for (i = 0; i < size; i++) |
| @dots{} access (array, i) @dots{} |
| @} |
| @end group |
| @end example |
| |
| Nested function definitions are permitted within functions in the places |
| where variable definitions are allowed; that is, in any block, before |
| the first statement in the block. |
| |
| It is possible to call the nested function from outside the scope of its |
| name by storing its address or passing the address to another function: |
| |
| @example |
| hack (int *array, int size) |
| @{ |
| void store (int index, int value) |
| @{ array[index] = value; @} |
| |
| intermediate (store, size); |
| @} |
| @end example |
| |
| Here, the function @code{intermediate} receives the address of |
| @code{store} as an argument. If @code{intermediate} calls @code{store}, |
| the arguments given to @code{store} are used to store into @code{array}. |
| But this technique works only so long as the containing function |
| (@code{hack}, in this example) does not exit. |
| |
| If you try to call the nested function through its address after the |
| containing function has exited, all hell will break loose. If you try |
| to call it after a containing scope level has exited, and if it refers |
| to some of the variables that are no longer in scope, you may be lucky, |
| but it's not wise to take the risk. If, however, the nested function |
| does not refer to anything that has gone out of scope, you should be |
| safe. |
| |
| GCC implements taking the address of a nested function using a technique |
| called @dfn{trampolines}. A paper describing them is available as |
| |
| @noindent |
| @uref{http://people.debian.org/~karlheg/Usenix88-lexic.pdf}. |
| |
| A nested function can jump to a label inherited from a containing |
| function, provided the label was explicitly declared in the containing |
| function (@pxref{Local Labels}). Such a jump returns instantly to the |
| containing function, exiting the nested function which did the |
| @code{goto} and any intermediate functions as well. Here is an example: |
| |
| @example |
| @group |
| bar (int *array, int offset, int size) |
| @{ |
| __label__ failure; |
| int access (int *array, int index) |
| @{ |
| if (index > size) |
| goto failure; |
| return array[index + offset]; |
| @} |
| int i; |
| @dots{} |
| for (i = 0; i < size; i++) |
| @dots{} access (array, i) @dots{} |
| @dots{} |
| return 0; |
| |
| /* @r{Control comes here from @code{access} |
| if it detects an error.} */ |
| failure: |
| return -1; |
| @} |
| @end group |
| @end example |
| |
| A nested function always has internal linkage. Declaring one with |
| @code{extern} is erroneous. If you need to declare the nested function |
| before its definition, use @code{auto} (which is otherwise meaningless |
| for function declarations). |
| |
| @example |
| bar (int *array, int offset, int size) |
| @{ |
| __label__ failure; |
| auto int access (int *, int); |
| @dots{} |
| int access (int *array, int index) |
| @{ |
| if (index > size) |
| goto failure; |
| return array[index + offset]; |
| @} |
| @dots{} |
| @} |
| @end example |
| |
| @node Constructing Calls |
| @section Constructing Function Calls |
| @cindex constructing calls |
| @cindex forwarding calls |
| |
| Using the built-in functions described below, you can record |
| the arguments a function received, and call another function |
| with the same arguments, without knowing the number or types |
| of the arguments. |
| |
| You can also record the return value of that function call, |
| and later return that value, without knowing what data type |
| the function tried to return (as long as your caller expects |
| that data type). |
| |
| @deftypefn {Built-in Function} {void *} __builtin_apply_args () |
| This built-in function returns a pointer to data |
| describing how to perform a call with the same arguments as were passed |
| to the current function. |
| |
| The function saves the arg pointer register, structure value address, |
| and all registers that might be used to pass arguments to a function |
| into a block of memory allocated on the stack. Then it returns the |
| address of that block. |
| @end deftypefn |
| |
| @deftypefn {Built-in Function} {void *} __builtin_apply (void (*@var{function})(), void *@var{arguments}, size_t @var{size}) |
| This built-in function invokes @var{function} |
| with a copy of the parameters described by @var{arguments} |
| and @var{size}. |
| |
| The value of @var{arguments} should be the value returned by |
| @code{__builtin_apply_args}. The argument @var{size} specifies the size |
| of the stack argument data, in bytes. |
| |
| This function returns a pointer to data describing |
| how to return whatever value was returned by @var{function}. The data |
| is saved in a block of memory allocated on the stack. |
| |
| It is not always simple to compute the proper value for @var{size}. The |
| value is used by @code{__builtin_apply} to compute the amount of data |
| that should be pushed on the stack and copied from the incoming argument |
| area. |
| @end deftypefn |
| |
| @deftypefn {Built-in Function} {void} __builtin_return (void *@var{result}) |
| This built-in function returns the value described by @var{result} from |
| the containing function. You should specify, for @var{result}, a value |
| returned by @code{__builtin_apply}. |
| @end deftypefn |
| |
| @node Naming Types |
| @section Naming an Expression's Type |
| @cindex naming types |
| |
| You can give a name to the type of an expression using a @code{typedef} |
| declaration with an initializer. Here is how to define @var{name} as a |
| type name for the type of @var{exp}: |
| |
| @example |
| typedef @var{name} = @var{exp}; |
| @end example |
| |
| This is useful in conjunction with the statements-within-expressions |
| feature. Here is how the two together can be used to define a safe |
| ``maximum'' macro that operates on any arithmetic type: |
| |
| @example |
| #define max(a,b) \ |
| (@{typedef _ta = (a), _tb = (b); \ |
| _ta _a = (a); _tb _b = (b); \ |
| _a > _b ? _a : _b; @}) |
| @end example |
| |
| @cindex underscores in variables in macros |
| @cindex @samp{_} in variables in macros |
| @cindex local variables in macros |
| @cindex variables, local, in macros |
| @cindex macros, local variables in |
| |
| The reason for using names that start with underscores for the local |
| variables is to avoid conflicts with variable names that occur within the |
| expressions that are substituted for @code{a} and @code{b}. Eventually we |
| hope to design a new form of declaration syntax that allows you to declare |
| variables whose scopes start only after their initializers; this will be a |
| more reliable way to prevent such conflicts. |
| |
| @node Typeof |
| @section Referring to a Type with @code{typeof} |
| @findex typeof |
| @findex sizeof |
| @cindex macros, types of arguments |
| |
| Another way to refer to the type of an expression is with @code{typeof}. |
| The syntax of using of this keyword looks like @code{sizeof}, but the |
| construct acts semantically like a type name defined with @code{typedef}. |
| |
| There are two ways of writing the argument to @code{typeof}: with an |
| expression or with a type. Here is an example with an expression: |
| |
| @example |
| typeof (x[0](1)) |
| @end example |
| |
| @noindent |
| This assumes that @code{x} is an array of pointers to functions; |
| the type described is that of the values of the functions. |
| |
| Here is an example with a typename as the argument: |
| |
| @example |
| typeof (int *) |
| @end example |
| |
| @noindent |
| Here the type described is that of pointers to @code{int}. |
| |
| If you are writing a header file that must work when included in ISO C |
| programs, write @code{__typeof__} instead of @code{typeof}. |
| @xref{Alternate Keywords}. |
| |
| A @code{typeof}-construct can be used anywhere a typedef name could be |
| used. For example, you can use it in a declaration, in a cast, or inside |
| of @code{sizeof} or @code{typeof}. |
| |
| @itemize @bullet |
| @item |
| This declares @code{y} with the type of what @code{x} points to. |
| |
| @example |
| typeof (*x) y; |
| @end example |
| |
| @item |
| This declares @code{y} as an array of such values. |
| |
| @example |
| typeof (*x) y[4]; |
| @end example |
| |
| @item |
| This declares @code{y} as an array of pointers to characters: |
| |
| @example |
| typeof (typeof (char *)[4]) y; |
| @end example |
| |
| @noindent |
| It is equivalent to the following traditional C declaration: |
| |
| @example |
| char *y[4]; |
| @end example |
| |
| To see the meaning of the declaration using @code{typeof}, and why it |
| might be a useful way to write, let's rewrite it with these macros: |
| |
| @example |
| #define pointer(T) typeof(T *) |
| #define array(T, N) typeof(T [N]) |
| @end example |
| |
| @noindent |
| Now the declaration can be rewritten this way: |
| |
| @example |
| array (pointer (char), 4) y; |
| @end example |
| |
| @noindent |
| Thus, @code{array (pointer (char), 4)} is the type of arrays of 4 |
| pointers to @code{char}. |
| @end itemize |
| |
| @node Lvalues |
| @section Generalized Lvalues |
| @cindex compound expressions as lvalues |
| @cindex expressions, compound, as lvalues |
| @cindex conditional expressions as lvalues |
| @cindex expressions, conditional, as lvalues |
| @cindex casts as lvalues |
| @cindex generalized lvalues |
| @cindex lvalues, generalized |
| @cindex extensions, @code{?:} |
| @cindex @code{?:} extensions |
| Compound expressions, conditional expressions and casts are allowed as |
| lvalues provided their operands are lvalues. This means that you can take |
| their addresses or store values into them. |
| |
| Standard C++ allows compound expressions and conditional expressions as |
| lvalues, and permits casts to reference type, so use of this extension |
| is deprecated for C++ code. |
| |
| For example, a compound expression can be assigned, provided the last |
| expression in the sequence is an lvalue. These two expressions are |
| equivalent: |
| |
| @example |
| (a, b) += 5 |
| a, (b += 5) |
| @end example |
| |
| Similarly, the address of the compound expression can be taken. These two |
| expressions are equivalent: |
| |
| @example |
| &(a, b) |
| a, &b |
| @end example |
| |
| A conditional expression is a valid lvalue if its type is not void and the |
| true and false branches are both valid lvalues. For example, these two |
| expressions are equivalent: |
| |
| @example |
| (a ? b : c) = 5 |
| (a ? b = 5 : (c = 5)) |
| @end example |
| |
| A cast is a valid lvalue if its operand is an lvalue. A simple |
| assignment whose left-hand side is a cast works by converting the |
| right-hand side first to the specified type, then to the type of the |
| inner left-hand side expression. After this is stored, the value is |
| converted back to the specified type to become the value of the |
| assignment. Thus, if @code{a} has type @code{char *}, the following two |
| expressions are equivalent: |
| |
| @example |
| (int)a = 5 |
| (int)(a = (char *)(int)5) |
| @end example |
| |
| An assignment-with-arithmetic operation such as @samp{+=} applied to a cast |
| performs the arithmetic using the type resulting from the cast, and then |
| continues as in the previous case. Therefore, these two expressions are |
| equivalent: |
| |
| @example |
| (int)a += 5 |
| (int)(a = (char *)(int) ((int)a + 5)) |
| @end example |
| |
| You cannot take the address of an lvalue cast, because the use of its |
| address would not work out coherently. Suppose that @code{&(int)f} were |
| permitted, where @code{f} has type @code{float}. Then the following |
| statement would try to store an integer bit-pattern where a floating |
| point number belongs: |
| |
| @example |
| *&(int)f = 1; |
| @end example |
| |
| This is quite different from what @code{(int)f = 1} would do---that |
| would convert 1 to floating point and store it. Rather than cause this |
| inconsistency, we think it is better to prohibit use of @samp{&} on a cast. |
| |
| If you really do want an @code{int *} pointer with the address of |
| @code{f}, you can simply write @code{(int *)&f}. |
| |
| @node Conditionals |
| @section Conditionals with Omitted Operands |
| @cindex conditional expressions, extensions |
| @cindex omitted middle-operands |
| @cindex middle-operands, omitted |
| @cindex extensions, @code{?:} |
| @cindex @code{?:} extensions |
| |
| The middle operand in a conditional expression may be omitted. Then |
| if the first operand is nonzero, its value is the value of the conditional |
| expression. |
| |
| Therefore, the expression |
| |
| @example |
| x ? : y |
| @end example |
| |
| @noindent |
| has the value of @code{x} if that is nonzero; otherwise, the value of |
| @code{y}. |
| |
| This example is perfectly equivalent to |
| |
| @example |
| x ? x : y |
| @end example |
| |
| @cindex side effect in ?: |
| @cindex ?: side effect |
| @noindent |
| In this simple case, the ability to omit the middle operand is not |
| especially useful. When it becomes useful is when the first operand does, |
| or may (if it is a macro argument), contain a side effect. Then repeating |
| the operand in the middle would perform the side effect twice. Omitting |
| the middle operand uses the value already computed without the undesirable |
| effects of recomputing it. |
| |
| @node Long Long |
| @section Double-Word Integers |
| @cindex @code{long long} data types |
| @cindex double-word arithmetic |
| @cindex multiprecision arithmetic |
| @cindex @code{LL} integer suffix |
| @cindex @code{ULL} integer suffix |
| |
| ISO C99 supports data types for integers that are at least 64 bits wide, |
| and as an extension GCC supports them in C89 mode and in C++. |
| Simply write @code{long long int} for a signed integer, or |
| @code{unsigned long long int} for an unsigned integer. To make an |
| integer constant of type @code{long long int}, add the suffix @samp{LL} |
| to the integer. To make an integer constant of type @code{unsigned long |
| long int}, add the suffix @samp{ULL} to the integer. |
| |
| You can use these types in arithmetic like any other integer types. |
| Addition, subtraction, and bitwise boolean operations on these types |
| are open-coded on all types of machines. Multiplication is open-coded |
| if the machine supports fullword-to-doubleword a widening multiply |
| instruction. Division and shifts are open-coded only on machines that |
| provide special support. The operations that are not open-coded use |
| special library routines that come with GCC@. |
| |
| There may be pitfalls when you use @code{long long} types for function |
| arguments, unless you declare function prototypes. If a function |
| expects type @code{int} for its argument, and you pass a value of type |
| @code{long long int}, confusion will result because the caller and the |
| subroutine will disagree about the number of bytes for the argument. |
| Likewise, if the function expects @code{long long int} and you pass |
| @code{int}. The best way to avoid such problems is to use prototypes. |
| |
| @node Complex |
| @section Complex Numbers |
| @cindex complex numbers |
| @cindex @code{_Complex} keyword |
| @cindex @code{__complex__} keyword |
| |
| ISO C99 supports complex floating data types, and as an extension GCC |
| supports them in C89 mode and in C++, and supports complex integer data |
| types which are not part of ISO C99. You can declare complex types |
| using the keyword @code{_Complex}. As an extension, the older GNU |
| keyword @code{__complex__} is also supported. |
| |
| For example, @samp{_Complex double x;} declares @code{x} as a |
| variable whose real part and imaginary part are both of type |
| @code{double}. @samp{_Complex short int y;} declares @code{y} to |
| have real and imaginary parts of type @code{short int}; this is not |
| likely to be useful, but it shows that the set of complex types is |
| complete. |
| |
| To write a constant with a complex data type, use the suffix @samp{i} or |
| @samp{j} (either one; they are equivalent). For example, @code{2.5fi} |
| has type @code{_Complex float} and @code{3i} has type |
| @code{_Complex int}. Such a constant always has a pure imaginary |
| value, but you can form any complex value you like by adding one to a |
| real constant. This is a GNU extension; if you have an ISO C99 |
| conforming C library (such as GNU libc), and want to construct complex |
| constants of floating type, you should include @code{<complex.h>} and |
| use the macros @code{I} or @code{_Complex_I} instead. |
| |
| @cindex @code{__real__} keyword |
| @cindex @code{__imag__} keyword |
| To extract the real part of a complex-valued expression @var{exp}, write |
| @code{__real__ @var{exp}}. Likewise, use @code{__imag__} to |
| extract the imaginary part. This is a GNU extension; for values of |
| floating type, you should use the ISO C99 functions @code{crealf}, |
| @code{creal}, @code{creall}, @code{cimagf}, @code{cimag} and |
| @code{cimagl}, declared in @code{<complex.h>} and also provided as |
| built-in functions by GCC@. |
| |
| @cindex complex conjugation |
| The operator @samp{~} performs complex conjugation when used on a value |
| with a complex type. This is a GNU extension; for values of |
| floating type, you should use the ISO C99 functions @code{conjf}, |
| @code{conj} and @code{conjl}, declared in @code{<complex.h>} and also |
| provided as built-in functions by GCC@. |
| |
| GCC can allocate complex automatic variables in a noncontiguous |
| fashion; it's even possible for the real part to be in a register while |
| the imaginary part is on the stack (or vice-versa). None of the |
| supported debugging info formats has a way to represent noncontiguous |
| allocation like this, so GCC describes a noncontiguous complex |
| variable as if it were two separate variables of noncomplex type. |
| If the variable's actual name is @code{foo}, the two fictitious |
| variables are named @code{foo$real} and @code{foo$imag}. You can |
| examine and set these two fictitious variables with your debugger. |
| |
| A future version of GDB will know how to recognize such pairs and treat |
| them as a single variable with a complex type. |
| |
| @node Hex Floats |
| @section Hex Floats |
| @cindex hex floats |
| |
| ISO C99 supports floating-point numbers written not only in the usual |
| decimal notation, such as @code{1.55e1}, but also numbers such as |
| @code{0x1.fp3} written in hexadecimal format. As a GNU extension, GCC |
| supports this in C89 mode (except in some cases when strictly |
| conforming) and in C++. In that format the |
| @samp{0x} hex introducer and the @samp{p} or @samp{P} exponent field are |
| mandatory. The exponent is a decimal number that indicates the power of |
| 2 by which the significant part will be multiplied. Thus @samp{0x1.f} is |
| @tex |
| $1 {15\over16}$, |
| @end tex |
| @ifnottex |
| 1 15/16, |
| @end ifnottex |
| @samp{p3} multiplies it by 8, and the value of @code{0x1.fp3} |
| is the same as @code{1.55e1}. |
| |
| Unlike for floating-point numbers in the decimal notation the exponent |
| is always required in the hexadecimal notation. Otherwise the compiler |
| would not be able to resolve the ambiguity of, e.g., @code{0x1.f}. This |
| could mean @code{1.0f} or @code{1.9375} since @samp{f} is also the |
| extension for floating-point constants of type @code{float}. |
| |
| @node Zero Length |
| @section Arrays of Length Zero |
| @cindex arrays of length zero |
| @cindex zero-length arrays |
| @cindex length-zero arrays |
| @cindex flexible array members |
| |
| Zero-length arrays are allowed in GNU C@. They are very useful as the |
| last element of a structure which is really a header for a variable-length |
| object: |
| |
| @example |
| struct line @{ |
| int length; |
| char contents[0]; |
| @}; |
| |
| struct line *thisline = (struct line *) |
| malloc (sizeof (struct line) + this_length); |
| thisline->length = this_length; |
| @end example |
| |
| In ISO C89, you would have to give @code{contents} a length of 1, which |
| means either you waste space or complicate the argument to @code{malloc}. |
| |
| In ISO C99, you would use a @dfn{flexible array member}, which is |
| slightly different in syntax and semantics: |
| |
| @itemize @bullet |
| @item |
| Flexible array members are written as @code{contents[]} without |
| the @code{0}. |
| |
| @item |
| Flexible array members have incomplete type, and so the @code{sizeof} |
| operator may not be applied. As a quirk of the original implementation |
| of zero-length arrays, @code{sizeof} evaluates to zero. |
| |
| @item |
| Flexible array members may only appear as the last member of a |
| @code{struct} that is otherwise non-empty. |
| @end itemize |
| |
| GCC versions before 3.0 allowed zero-length arrays to be statically |
| initialized, as if they were flexible arrays. In addition to those |
| cases that were useful, it also allowed initializations in situations |
| that would corrupt later data. Non-empty initialization of zero-length |
| arrays is now treated like any case where there are more initializer |
| elements than the array holds, in that a suitable warning about "excess |
| elements in array" is given, and the excess elements (all of them, in |
| this case) are ignored. |
| |
| Instead GCC allows static initialization of flexible array members. |
| This is equivalent to defining a new structure containing the original |
| structure followed by an array of sufficient size to contain the data. |
| I.e.@: in the following, @code{f1} is constructed as if it were declared |
| like @code{f2}. |
| |
| @example |
| struct f1 @{ |
| int x; int y[]; |
| @} f1 = @{ 1, @{ 2, 3, 4 @} @}; |
| |
| struct f2 @{ |
| struct f1 f1; int data[3]; |
| @} f2 = @{ @{ 1 @}, @{ 2, 3, 4 @} @}; |
| @end example |
| |
| @noindent |
| The convenience of this extension is that @code{f1} has the desired |
| type, eliminating the need to consistently refer to @code{f2.f1}. |
| |
| This has symmetry with normal static arrays, in that an array of |
| unknown size is also written with @code{[]}. |
| |
| Of course, this extension only makes sense if the extra data comes at |
| the end of a top-level object, as otherwise we would be overwriting |
| data at subsequent offsets. To avoid undue complication and confusion |
| with initialization of deeply nested arrays, we simply disallow any |
| non-empty initialization except when the structure is the top-level |
| object. For example: |
| |
| @example |
| struct foo @{ int x; int y[]; @}; |
| struct bar @{ struct foo z; @}; |
| |
| struct foo a = @{ 1, @{ 2, 3, 4 @} @}; // @r{Valid.} |
| struct bar b = @{ @{ 1, @{ 2, 3, 4 @} @} @}; // @r{Invalid.} |
| struct bar c = @{ @{ 1, @{ @} @} @}; // @r{Valid.} |
| struct foo d[1] = @{ @{ 1 @{ 2, 3, 4 @} @} @}; // @r{Invalid.} |
| @end example |
| |
| @node Variable Length |
| @section Arrays of Variable Length |
| @cindex variable-length arrays |
| @cindex arrays of variable length |
| @cindex VLAs |
| |
| Variable-length automatic arrays are allowed in ISO C99, and as an |
| extension GCC accepts them in C89 mode and in C++. (However, GCC's |
| implementation of variable-length arrays does not yet conform in detail |
| to the ISO C99 standard.) These arrays are |
| declared like any other automatic arrays, but with a length that is not |
| a constant expression. The storage is allocated at the point of |
| declaration and deallocated when the brace-level is exited. For |
| example: |
| |
| @example |
| FILE * |
| concat_fopen (char *s1, char *s2, char *mode) |
| @{ |
| char str[strlen (s1) + strlen (s2) + 1]; |
| strcpy (str, s1); |
| strcat (str, s2); |
| return fopen (str, mode); |
| @} |
| @end example |
| |
| @cindex scope of a variable length array |
| @cindex variable-length array scope |
| @cindex deallocating variable length arrays |
| Jumping or breaking out of the scope of the array name deallocates the |
| storage. Jumping into the scope is not allowed; you get an error |
| message for it. |
| |
| @cindex @code{alloca} vs variable-length arrays |
| You can use the function @code{alloca} to get an effect much like |
| variable-length arrays. The function @code{alloca} is available in |
| many other C implementations (but not in all). On the other hand, |
| variable-length arrays are more elegant. |
| |
| There are other differences between these two methods. Space allocated |
| with @code{alloca} exists until the containing @emph{function} returns. |
| The space for a variable-length array is deallocated as soon as the array |
| name's scope ends. (If you use both variable-length arrays and |
| @code{alloca} in the same function, deallocation of a variable-length array |
| will also deallocate anything more recently allocated with @code{alloca}.) |
| |
| You can also use variable-length arrays as arguments to functions: |
| |
| @example |
| struct entry |
| tester (int len, char data[len][len]) |
| @{ |
| @dots{} |
| @} |
| @end example |
| |
| The length of an array is computed once when the storage is allocated |
| and is remembered for the scope of the array in case you access it with |
| @code{sizeof}. |
| |
| If you want to pass the array first and the length afterward, you can |
| use a forward declaration in the parameter list---another GNU extension. |
| |
| @example |
| struct entry |
| tester (int len; char data[len][len], int len) |
| @{ |
| @dots{} |
| @} |
| @end example |
| |
| @cindex parameter forward declaration |
| The @samp{int len} before the semicolon is a @dfn{parameter forward |
| declaration}, and it serves the purpose of making the name @code{len} |
| known when the declaration of @code{data} is parsed. |
| |
| You can write any number of such parameter forward declarations in the |
| parameter list. They can be separated by commas or semicolons, but the |
| last one must end with a semicolon, which is followed by the ``real'' |
| parameter declarations. Each forward declaration must match a ``real'' |
| declaration in parameter name and data type. ISO C99 does not support |
| parameter forward declarations. |
| |
| @node Variadic Macros |
| @section Macros with a Variable Number of Arguments. |
| @cindex variable number of arguments |
| @cindex macro with variable arguments |
| @cindex rest argument (in macro) |
| @cindex variadic macros |
| |
| In the ISO C standard of 1999, a macro can be declared to accept a |
| variable number of arguments much as a function can. The syntax for |
| defining the macro is similar to that of a function. Here is an |
| example: |
| |
| @example |
| #define debug(format, ...) fprintf (stderr, format, __VA_ARGS__) |
| @end example |
| |
| Here @samp{@dots{}} is a @dfn{variable argument}. In the invocation of |
| such a macro, it represents the zero or more tokens until the closing |
| parenthesis that ends the invocation, including any commas. This set of |
| tokens replaces the identifier @code{__VA_ARGS__} in the macro body |
| wherever it appears. See the CPP manual for more information. |
| |
| GCC has long supported variadic macros, and used a different syntax that |
| allowed you to give a name to the variable arguments just like any other |
| argument. Here is an example: |
| |
| @example |
| #define debug(format, args...) fprintf (stderr, format, args) |
| @end example |
| |
| This is in all ways equivalent to the ISO C example above, but arguably |
| more readable and descriptive. |
| |
| GNU CPP has two further variadic macro extensions, and permits them to |
| be used with either of the above forms of macro definition. |
| |
| In standard C, you are not allowed to leave the variable argument out |
| entirely; but you are allowed to pass an empty argument. For example, |
| this invocation is invalid in ISO C, because there is no comma after |
| the string: |
| |
| @example |
| debug ("A message") |
| @end example |
| |
| GNU CPP permits you to completely omit the variable arguments in this |
| way. In the above examples, the compiler would complain, though since |
| the expansion of the macro still has the extra comma after the format |
| string. |
| |
| To help solve this problem, CPP behaves specially for variable arguments |
| used with the token paste operator, @samp{##}. If instead you write |
| |
| @example |
| #define debug(format, ...) fprintf (stderr, format, ## __VA_ARGS__) |
| @end example |
| |
| and if the variable arguments are omitted or empty, the @samp{##} |
| operator causes the preprocessor to remove the comma before it. If you |
| do provide some variable arguments in your macro invocation, GNU CPP |
| does not complain about the paste operation and instead places the |
| variable arguments after the comma. Just like any other pasted macro |
| argument, these arguments are not macro expanded. |
| |
| @node Escaped Newlines |
| @section Slightly Looser Rules for Escaped Newlines |
| @cindex escaped newlines |
| @cindex newlines (escaped) |
| |
| Recently, the preprocessor has relaxed its treatment of escaped |
| newlines. Previously, the newline had to immediately follow a |
| backslash. The current implementation allows whitespace in the form of |
| spaces, horizontal and vertical tabs, and form feeds between the |
| backslash and the subsequent newline. The preprocessor issues a |
| warning, but treats it as a valid escaped newline and combines the two |
| lines to form a single logical line. This works within comments and |
| tokens, including multi-line strings, as well as between tokens. |
| Comments are @emph{not} treated as whitespace for the purposes of this |
| relaxation, since they have not yet been replaced with spaces. |
| |
| @node Multi-line Strings |
| @section String Literals with Embedded Newlines |
| @cindex multi-line string literals |
| |
| As an extension, GNU CPP permits string literals to cross multiple lines |
| without escaping the embedded newlines. Each embedded newline is |
| replaced with a single @samp{\n} character in the resulting string |
| literal, regardless of what form the newline took originally. |
| |
| CPP currently allows such strings in directives as well (other than the |
| @samp{#include} family). This is deprecated and will eventually be |
| removed. |
| |
| @node Subscripting |
| @section Non-Lvalue Arrays May Have Subscripts |
| @cindex subscripting |
| @cindex arrays, non-lvalue |
| |
| @cindex subscripting and function values |
| In ISO C99, arrays that are not lvalues still decay to pointers, and |
| may be subscripted, although they may not be modified or used after |
| the next sequence point and the unary @samp{&} operator may not be |
| applied to them. As an extension, GCC allows such arrays to be |
| subscripted in C89 mode, though otherwise they do not decay to |
| pointers outside C99 mode. For example, |
| this is valid in GNU C though not valid in C89: |
| |
| @example |
| @group |
| struct foo @{int a[4];@}; |
| |
| struct foo f(); |
| |
| bar (int index) |
| @{ |
| return f().a[index]; |
| @} |
| @end group |
| @end example |
| |
| @node Pointer Arith |
| @section Arithmetic on @code{void}- and Function-Pointers |
| @cindex void pointers, arithmetic |
| @cindex void, size of pointer to |
| @cindex function pointers, arithmetic |
| @cindex function, size of pointer to |
| |
| In GNU C, addition and subtraction operations are supported on pointers to |
| @code{void} and on pointers to functions. This is done by treating the |
| size of a @code{void} or of a function as 1. |
| |
| A consequence of this is that @code{sizeof} is also allowed on @code{void} |
| and on function types, and returns 1. |
| |
| @opindex Wpointer-arith |
| The option @option{-Wpointer-arith} requests a warning if these extensions |
| are used. |
| |
| @node Initializers |
| @section Non-Constant Initializers |
| @cindex initializers, non-constant |
| @cindex non-constant initializers |
| |
| As in standard C++ and ISO C99, the elements of an aggregate initializer for an |
| automatic variable are not required to be constant expressions in GNU C@. |
| Here is an example of an initializer with run-time varying elements: |
| |
| @example |
| foo (float f, float g) |
| @{ |
| float beat_freqs[2] = @{ f-g, f+g @}; |
| @dots{} |
| @} |
| @end example |
| |
| @node Compound Literals |
| @section Compound Literals |
| @cindex constructor expressions |
| @cindex initializations in expressions |
| @cindex structures, constructor expression |
| @cindex expressions, constructor |
| @cindex compound literals |
| @c The GNU C name for what C99 calls compound literals was "constructor expressions". |
| |
| ISO C99 supports compound literals. A compound literal looks like |
| a cast containing an initializer. Its value is an object of the |
| type specified in the cast, containing the elements specified in |
| the initializer; it is an lvalue. As an extension, GCC supports |
| compound literals in C89 mode and in C++. |
| |
| Usually, the specified type is a structure. Assume that |
| @code{struct foo} and @code{structure} are declared as shown: |
| |
| @example |
| struct foo @{int a; char b[2];@} structure; |
| @end example |
| |
| @noindent |
| Here is an example of constructing a @code{struct foo} with a compound literal: |
| |
| @example |
| structure = ((struct foo) @{x + y, 'a', 0@}); |
| @end example |
| |
| @noindent |
| This is equivalent to writing the following: |
| |
| @example |
| @{ |
| struct foo temp = @{x + y, 'a', 0@}; |
| structure = temp; |
| @} |
| @end example |
| |
| You can also construct an array. If all the elements of the compound literal |
| are (made up of) simple constant expressions, suitable for use in |
| initializers of objects of static storage duration, then the compound |
| literal can be coerced to a pointer to its first element and used in |
| such an initializer, as shown here: |
| |
| @example |
| char **foo = (char *[]) @{ "x", "y", "z" @}; |
| @end example |
| |
| Compound literals for scalar types and union types are is |
| also allowed, but then the compound literal is equivalent |
| to a cast. |
| |
| As a GNU extension, GCC allows initialization of objects with static storage |
| duration by compound literals (which is not possible in ISO C99, because |
| the initializer is not a constant). |
| It is handled as if the object was initialized only with the bracket |
| enclosed list if compound literal's and object types match. |
| The initializer list of the compound literal must be constant. |
| If the object being initialized has array type of unknown size, the size is |
| determined by compound literal size. |
| |
| @example |
| static struct foo x = (struct foo) @{1, 'a', 'b'@}; |
| static int y[] = (int []) @{1, 2, 3@}; |
| static int z[] = (int [3]) @{1@}; |
| @end example |
| |
| @noindent |
| The above lines are equivalent to the following: |
| @example |
| static struct foo x = @{1, 'a', 'b'@}; |
| static int y[] = @{1, 2, 3@}; |
| static int z[] = @{1, 0, 0@}; |
| @end example |
| |
| @node Designated Inits |
| @section Designated Initializers |
| @cindex initializers with labeled elements |
| @cindex labeled elements in initializers |
| @cindex case labels in initializers |
| @cindex designated initializers |
| |
| Standard C89 requires the elements of an initializer to appear in a fixed |
| order, the same as the order of the elements in the array or structure |
| being initialized. |
| |
| In ISO C99 you can give the elements in any order, specifying the array |
| indices or structure field names they apply to, and GNU C allows this as |
| an extension in C89 mode as well. This extension is not |
| implemented in GNU C++. |
| |
| To specify an array index, write |
| @samp{[@var{index}] =} before the element value. For example, |
| |
| @example |
| int a[6] = @{ [4] = 29, [2] = 15 @}; |
| @end example |
| |
| @noindent |
| is equivalent to |
| |
| @example |
| int a[6] = @{ 0, 0, 15, 0, 29, 0 @}; |
| @end example |
| |
| @noindent |
| The index values must be constant expressions, even if the array being |
| initialized is automatic. |
| |
| An alternative syntax for this which has been obsolete since GCC 2.5 but |
| GCC still accepts is to write @samp{[@var{index}]} before the element |
| value, with no @samp{=}. |
| |
| To initialize a range of elements to the same value, write |
| @samp{[@var{first} ... @var{last}] = @var{value}}. This is a GNU |
| extension. For example, |
| |
| @example |
| int widths[] = @{ [0 ... 9] = 1, [10 ... 99] = 2, [100] = 3 @}; |
| @end example |
| |
| @noindent |
| If the value in it has side-effects, the side-effects will happen only once, |
| not for each initialized field by the range initializer. |
| |
| @noindent |
| Note that the length of the array is the highest value specified |
| plus one. |
| |
| In a structure initializer, specify the name of a field to initialize |
| with @samp{.@var{fieldname} =} before the element value. For example, |
| given the following structure, |
| |
| @example |
| struct point @{ int x, y; @}; |
| @end example |
| |
| @noindent |
| the following initialization |
| |
| @example |
| struct point p = @{ .y = yvalue, .x = xvalue @}; |
| @end example |
| |
| @noindent |
| is equivalent to |
| |
| @example |
| struct point p = @{ xvalue, yvalue @}; |
| @end example |
| |
| Another syntax which has the same meaning, obsolete since GCC 2.5, is |
| @samp{@var{fieldname}:}, as shown here: |
| |
| @example |
| struct point p = @{ y: yvalue, x: xvalue @}; |
| @end example |
| |
| @cindex designators |
| The @samp{[@var{index}]} or @samp{.@var{fieldname}} is known as a |
| @dfn{designator}. You can also use a designator (or the obsolete colon |
| syntax) when initializing a union, to specify which element of the union |
| should be used. For example, |
| |
| @example |
| union foo @{ int i; double d; @}; |
| |
| union foo f = @{ .d = 4 @}; |
| @end example |
| |
| @noindent |
| will convert 4 to a @code{double} to store it in the union using |
| the second element. By contrast, casting 4 to type @code{union foo} |
| would store it into the union as the integer @code{i}, since it is |
| an integer. (@xref{Cast to Union}.) |
| |
| You can combine this technique of naming elements with ordinary C |
| initialization of successive elements. Each initializer element that |
| does not have a designator applies to the next consecutive element of the |
| array or structure. For example, |
| |
| @example |
| int a[6] = @{ [1] = v1, v2, [4] = v4 @}; |
| @end example |
| |
| @noindent |
| is equivalent to |
| |
| @example |
| int a[6] = @{ 0, v1, v2, 0, v4, 0 @}; |
| @end example |
| |
| Labeling the elements of an array initializer is especially useful |
| when the indices are characters or belong to an @code{enum} type. |
| For example: |
| |
| @example |
| int whitespace[256] |
| = @{ [' '] = 1, ['\t'] = 1, ['\h'] = 1, |
| ['\f'] = 1, ['\n'] = 1, ['\r'] = 1 @}; |
| @end example |
| |
| @cindex designator lists |
| You can also write a series of @samp{.@var{fieldname}} and |
| @samp{[@var{index}]} designators before an @samp{=} to specify a |
| nested subobject to initialize; the list is taken relative to the |
| subobject corresponding to the closest surrounding brace pair. For |
| example, with the @samp{struct point} declaration above: |
| |
| @example |
| struct point ptarray[10] = @{ [2].y = yv2, [2].x = xv2, [0].x = xv0 @}; |
| @end example |
| |
| @noindent |
| If the same field is initialized multiple times, it will have value from |
| the last initialization. If any such overridden initialization has |
| side-effect, it is unspecified whether the side-effect happens or not. |
| Currently, gcc will discard them and issue a warning. |
| |
| @node Case Ranges |
| @section Case Ranges |
| @cindex case ranges |
| @cindex ranges in case statements |
| |
| You can specify a range of consecutive values in a single @code{case} label, |
| like this: |
| |
| @example |
| case @var{low} ... @var{high}: |
| @end example |
| |
| @noindent |
| This has the same effect as the proper number of individual @code{case} |
| labels, one for each integer value from @var{low} to @var{high}, inclusive. |
| |
| This feature is especially useful for ranges of ASCII character codes: |
| |
| @example |
| case 'A' ... 'Z': |
| @end example |
| |
| @strong{Be careful:} Write spaces around the @code{...}, for otherwise |
| it may be parsed wrong when you use it with integer values. For example, |
| write this: |
| |
| @example |
| case 1 ... 5: |
| @end example |
| |
| @noindent |
| rather than this: |
| |
| @example |
| case 1...5: |
| @end example |
| |
| @node Cast to Union |
| @section Cast to a Union Type |
| @cindex cast to a union |
| @cindex union, casting to a |
| |
| A cast to union type is similar to other casts, except that the type |
| specified is a union type. You can specify the type either with |
| @code{union @var{tag}} or with a typedef name. A cast to union is actually |
| a constructor though, not a cast, and hence does not yield an lvalue like |
| normal casts. (@xref{Compound Literals}.) |
| |
| The types that may be cast to the union type are those of the members |
| of the union. Thus, given the following union and variables: |
| |
| @example |
| union foo @{ int i; double d; @}; |
| int x; |
| double y; |
| @end example |
| |
| @noindent |
| both @code{x} and @code{y} can be cast to type @code{union foo}. |
| |
| Using the cast as the right-hand side of an assignment to a variable of |
| union type is equivalent to storing in a member of the union: |
| |
| @example |
| union foo u; |
| @dots{} |
| u = (union foo) x @equiv{} u.i = x |
| u = (union foo) y @equiv{} u.d = y |
| @end example |
| |
| You can also use the union cast as a function argument: |
| |
| @example |
| void hack (union foo); |
| @dots{} |
| hack ((union foo) x); |
| @end example |
| |
| @node Mixed Declarations |
| @section Mixed Declarations and Code |
| @cindex mixed declarations and code |
| @cindex declarations, mixed with code |
| @cindex code, mixed with declarations |
| |
| ISO C99 and ISO C++ allow declarations and code to be freely mixed |
| within compound statements. As an extension, GCC also allows this in |
| C89 mode. For example, you could do: |
| |
| @example |
| int i; |
| @dots{} |
| i++; |
| int j = i + 2; |
| @end example |
| |
| Each identifier is visible from where it is declared until the end of |
| the enclosing block. |
| |
| @node Function Attributes |
| @section Declaring Attributes of Functions |
| @cindex function attributes |
| @cindex declaring attributes of functions |
| @cindex functions that never return |
| @cindex functions that have no side effects |
| @cindex functions in arbitrary sections |
| @cindex functions that behave like malloc |
| @cindex @code{volatile} applied to function |
| @cindex @code{const} applied to function |
| @cindex functions with @code{printf}, @code{scanf}, @code{strftime} or @code{strfmon} style arguments |
| @cindex functions that are passed arguments in registers on the 386 |
| @cindex functions that pop the argument stack on the 386 |
| @cindex functions that do not pop the argument stack on the 386 |
| |
| In GNU C, you declare certain things about functions called in your program |
| which help the compiler optimize function calls and check your code more |
| carefully. |
| |
| The keyword @code{__attribute__} allows you to specify special |
| attributes when making a declaration. This keyword is followed by an |
| attribute specification inside double parentheses. The following |
| attributes are currently defined for functions on all targets: |
| @code{noreturn}, @code{noinline}, @code{always_inline}, |
| @code{pure}, @code{const}, |
| @code{format}, @code{format_arg}, @code{no_instrument_function}, |
| @code{section}, @code{constructor}, @code{destructor}, @code{used}, |
| @code{unused}, @code{deprecated}, @code{weak}, @code{malloc}, and |
| @code{alias}. Several other attributes are defined for functions on |
| particular target systems. Other attributes, including @code{section} |
| are supported for variables declarations (@pxref{Variable Attributes}) |
| and for types (@pxref{Type Attributes}). |
| |
| You may also specify attributes with @samp{__} preceding and following |
| each keyword. This allows you to use them in header files without |
| being concerned about a possible macro of the same name. For example, |
| you may use @code{__noreturn__} instead of @code{noreturn}. |
| |
| @xref{Attribute Syntax}, for details of the exact syntax for using |
| attributes. |
| |
| @table @code |
| @cindex @code{noreturn} function attribute |
| @item noreturn |
| A few standard library functions, such as @code{abort} and @code{exit}, |
| cannot return. GCC knows this automatically. Some programs define |
| their own functions that never return. You can declare them |
| @code{noreturn} to tell the compiler this fact. For example, |
| |
| @smallexample |
| @group |
| void fatal () __attribute__ ((noreturn)); |
| |
| void |
| fatal (@dots{}) |
| @{ |
| @dots{} /* @r{Print error message.} */ @dots{} |
| exit (1); |
| @} |
| @end group |
| @end smallexample |
| |
| The @code{noreturn} keyword tells the compiler to assume that |
| @code{fatal} cannot return. It can then optimize without regard to what |
| would happen if @code{fatal} ever did return. This makes slightly |
| better code. More importantly, it helps avoid spurious warnings of |
| uninitialized variables. |
| |
| Do not assume that registers saved by the calling function are |
| restored before calling the @code{noreturn} function. |
| |
| It does not make sense for a @code{noreturn} function to have a return |
| type other than @code{void}. |
| |
| The attribute @code{noreturn} is not implemented in GCC versions |
| earlier than 2.5. An alternative way to declare that a function does |
| not return, which works in the current version and in some older |
| versions, is as follows: |
| |
| @smallexample |
| typedef void voidfn (); |
| |
| volatile voidfn fatal; |
| @end smallexample |
| |
| @cindex @code{noinline} function attribute |
| @item noinline |
| This function attribute prevents a function from being considered for |
| inlining. |
| |
| @cindex @code{always_inline} function attribute |
| @item always_inline |
| Generally, functions are not inlined unless optimization is specified. |
| For functions declared inline, this attribute inlines the function even |
| if no optimization level was specified. |
| |
| @cindex @code{pure} function attribute |
| @item pure |
| Many functions have no effects except the return value and their |
| return value depends only on the parameters and/or global variables. |
| Such a function can be subject |
| to common subexpression elimination and loop optimization just as an |
| arithmetic operator would be. These functions should be declared |
| with the attribute @code{pure}. For example, |
| |
| @smallexample |
| int square (int) __attribute__ ((pure)); |
| @end smallexample |
| |
| @noindent |
| says that the hypothetical function @code{square} is safe to call |
| fewer times than the program says. |
| |
| Some of common examples of pure functions are @code{strlen} or @code{memcmp}. |
| Interesting non-pure functions are functions with infinite loops or those |
| depending on volatile memory or other system resource, that may change between |
| two consecutive calls (such as @code{feof} in a multithreading environment). |
| |
| The attribute @code{pure} is not implemented in GCC versions earlier |
| than 2.96. |
| @cindex @code{const} function attribute |
| @item const |
| Many functions do not examine any values except their arguments, and |
| have no effects except the return value. Basically this is just slightly |
| more strict class than the @code{pure} attribute above, since function is not |
| allowed to read global memory. |
| |
| @cindex pointer arguments |
| Note that a function that has pointer arguments and examines the data |
| pointed to must @emph{not} be declared @code{const}. Likewise, a |
| function that calls a non-@code{const} function usually must not be |
| @code{const}. It does not make sense for a @code{const} function to |
| return @code{void}. |
| |
| The attribute @code{const} is not implemented in GCC versions earlier |
| than 2.5. An alternative way to declare that a function has no side |
| effects, which works in the current version and in some older versions, |
| is as follows: |
| |
| @smallexample |
| typedef int intfn (); |
| |
| extern const intfn square; |
| @end smallexample |
| |
| This approach does not work in GNU C++ from 2.6.0 on, since the language |
| specifies that the @samp{const} must be attached to the return value. |
| |
| |
| @item format (@var{archetype}, @var{string-index}, @var{first-to-check}) |
| @cindex @code{format} function attribute |
| @opindex Wformat |
| The @code{format} attribute specifies that a function takes @code{printf}, |
| @code{scanf}, @code{strftime} or @code{strfmon} style arguments which |
| should be type-checked against a format string. For example, the |
| declaration: |
| |
| @smallexample |
| extern int |
| my_printf (void *my_object, const char *my_format, ...) |
| __attribute__ ((format (printf, 2, 3))); |
| @end smallexample |
| |
| @noindent |
| causes the compiler to check the arguments in calls to @code{my_printf} |
| for consistency with the @code{printf} style format string argument |
| @code{my_format}. |
| |
| The parameter @var{archetype} determines how the format string is |
| interpreted, and should be @code{printf}, @code{scanf}, @code{strftime} |
| or @code{strfmon}. (You can also use @code{__printf__}, |
| @code{__scanf__}, @code{__strftime__} or @code{__strfmon__}.) The |
| parameter @var{string-index} specifies which argument is the format |
| string argument (starting from 1), while @var{first-to-check} is the |
| number of the first argument to check against the format string. For |
| functions where the arguments are not available to be checked (such as |
| @code{vprintf}), specify the third parameter as zero. In this case the |
| compiler only checks the format string for consistency. For |
| @code{strftime} formats, the third parameter is required to be zero. |
| |
| In the example above, the format string (@code{my_format}) is the second |
| argument of the function @code{my_print}, and the arguments to check |
| start with the third argument, so the correct parameters for the format |
| attribute are 2 and 3. |
| |
| @opindex ffreestanding |
| The @code{format} attribute allows you to identify your own functions |
| which take format strings as arguments, so that GCC can check the |
| calls to these functions for errors. The compiler always (unless |
| @option{-ffreestanding} is used) checks formats |
| for the standard library functions @code{printf}, @code{fprintf}, |
| @code{sprintf}, @code{scanf}, @code{fscanf}, @code{sscanf}, @code{strftime}, |
| @code{vprintf}, @code{vfprintf} and @code{vsprintf} whenever such |
| warnings are requested (using @option{-Wformat}), so there is no need to |
| modify the header file @file{stdio.h}. In C99 mode, the functions |
| @code{snprintf}, @code{vsnprintf}, @code{vscanf}, @code{vfscanf} and |
| @code{vsscanf} are also checked. Except in strictly conforming C |
| standard modes, the X/Open function @code{strfmon} is also checked as |
| are @code{printf_unlocked} and @code{fprintf_unlocked}. |
| @xref{C Dialect Options,,Options Controlling C Dialect}. |
| |
| @item format_arg (@var{string-index}) |
| @cindex @code{format_arg} function attribute |
| @opindex Wformat-nonliteral |
| The @code{format_arg} attribute specifies that a function takes a format |
| string for a @code{printf}, @code{scanf}, @code{strftime} or |
| @code{strfmon} style function and modifies it (for example, to translate |
| it into another language), so the result can be passed to a |
| @code{printf}, @code{scanf}, @code{strftime} or @code{strfmon} style |
| function (with the remaining arguments to the format function the same |
| as they would have been for the unmodified string). For example, the |
| declaration: |
| |
| @smallexample |
| extern char * |
| my_dgettext (char *my_domain, const char *my_format) |
| __attribute__ ((format_arg (2))); |
| @end smallexample |
| |
| @noindent |
| causes the compiler to check the arguments in calls to a @code{printf}, |
| @code{scanf}, @code{strftime} or @code{strfmon} type function, whose |
| format string argument is a call to the @code{my_dgettext} function, for |
| consistency with the format string argument @code{my_format}. If the |
| @code{format_arg} attribute had not been specified, all the compiler |
| could tell in such calls to format functions would be that the format |
| string argument is not constant; this would generate a warning when |
| @option{-Wformat-nonliteral} is used, but the calls could not be checked |
| without the attribute. |
| |
| The parameter @var{string-index} specifies which argument is the format |
| string argument (starting from 1). |
| |
| The @code{format-arg} attribute allows you to identify your own |
| functions which modify format strings, so that GCC can check the |
| calls to @code{printf}, @code{scanf}, @code{strftime} or @code{strfmon} |
| type function whose operands are a call to one of your own function. |
| The compiler always treats @code{gettext}, @code{dgettext}, and |
| @code{dcgettext} in this manner except when strict ISO C support is |
| requested by @option{-ansi} or an appropriate @option{-std} option, or |
| @option{-ffreestanding} is used. @xref{C Dialect Options,,Options |
| Controlling C Dialect}. |
| |
| @item no_instrument_function |
| @cindex @code{no_instrument_function} function attribute |
| @opindex finstrument-functions |
| If @option{-finstrument-functions} is given, profiling function calls will |
| be generated at entry and exit of most user-compiled functions. |
| Functions with this attribute will not be so instrumented. |
| |
| @item section ("@var{section-name}") |
| @cindex @code{section} function attribute |
| Normally, the compiler places the code it generates in the @code{text} section. |
| Sometimes, however, you need additional sections, or you need certain |
| particular functions to appear in special sections. The @code{section} |
| attribute specifies that a function lives in a particular section. |
| For example, the declaration: |
| |
| @smallexample |
| extern void foobar (void) __attribute__ ((section ("bar"))); |
| @end smallexample |
| |
| @noindent |
| puts the function @code{foobar} in the @code{bar} section. |
| |
| Some file formats do not support arbitrary sections so the @code{section} |
| attribute is not available on all platforms. |
| If you need to map the entire contents of a module to a particular |
| section, consider using the facilities of the linker instead. |
| |
| @item constructor |
| @itemx destructor |
| @cindex @code{constructor} function attribute |
| @cindex @code{destructor} function attribute |
| The @code{constructor} attribute causes the function to be called |
| automatically before execution enters @code{main ()}. Similarly, the |
| @code{destructor} attribute causes the function to be called |
| automatically after @code{main ()} has completed or @code{exit ()} has |
| been called. Functions with these attributes are useful for |
| initializing data that will be used implicitly during the execution of |
| the program. |
| |
| These attributes are not currently implemented for Objective-C@. |
| |
| @cindex @code{unused} attribute. |
| @item unused |
| This attribute, attached to a function, means that the function is meant |
| to be possibly unused. GCC will not produce a warning for this |
| function. GNU C++ does not currently support this attribute as |
| definitions without parameters are valid in C++. |
| |
| @cindex @code{used} attribute. |
| @item used |
| This attribute, attached to a function, means that code must be emitted |
| for the function even if it appears that the function is not referenced. |
| This is useful, for example, when the function is referenced only in |
| inline assembly. |
| |
| @cindex @code{deprecated} attribute. |
| @item deprecated |
| The @code{deprecated} attribute results in a warning if the function |
| is used anywhere in the source file. This is useful when identifying |
| functions that are expected to be removed in a future version of a |
| program. The warning also includes the location of the declaration |
| of the deprecated function, to enable users to easily find further |
| information about why the function is deprecated, or what they should |
| do instead. Note that the warnings only occurs for uses: |
| |
| @smallexample |
| int old_fn () __attribute__ ((deprecated)); |
| int old_fn (); |
| int (*fn_ptr)() = old_fn; |
| @end smallexample |
| |
| results in a warning on line 3 but not line 2. |
| |
| The @code{deprecated} attribute can also be used for variables and |
| types (@pxref{Variable Attributes}, @pxref{Type Attributes}.) |
| |
| @item weak |
| @cindex @code{weak} attribute |
| The @code{weak} attribute causes the declaration to be emitted as a weak |
| symbol rather than a global. This is primarily useful in defining |
| library functions which can be overridden in user code, though it can |
| also be used with non-function declarations. Weak symbols are supported |
| for ELF targets, and also for a.out targets when using the GNU assembler |
| and linker. |
| |
| @item malloc |
| @cindex @code{malloc} attribute |
| The @code{malloc} attribute is used to tell the compiler that a function |
| may be treated as if it were the malloc function. The compiler assumes |
| that calls to malloc result in a pointers that cannot alias anything. |
| This will often improve optimization. |
| |
| @item alias ("@var{target}") |
| @cindex @code{alias} attribute |
| The @code{alias} attribute causes the declaration to be emitted as an |
| alias for another symbol, which must be specified. For instance, |
| |
| @smallexample |
| void __f () @{ /* @r{Do something.} */; @} |
| void f () __attribute__ ((weak, alias ("__f"))); |
| @end smallexample |
| |
| declares @samp{f} to be a weak alias for @samp{__f}. In C++, the |
| mangled name for the target must be used. |
| |
| Not all target machines support this attribute. |
| |
| @item visibility ("@var{visibility_type}") |
| @cindex @code{visibility} attribute |
| The @code{visibility} attribute on ELF targets causes the declaration |
| to be emitted with hidden, protected or internal visibility. |
| |
| @smallexample |
| void __attribute__ ((visibility ("protected"))) |
| f () @{ /* @r{Do something.} */; @} |
| int i __attribute__ ((visibility ("hidden"))); |
| @end smallexample |
| |
| See the ELF gABI for complete details, but the short story is |
| |
| @table @dfn |
| @item hidden |
| Hidden visibility indicates that the symbol will not be placed into |
| the dynamic symbol table, so no other @dfn{module} (executable or |
| shared library) can reference it directly. |
| |
| @item protected |
| Protected visibility indicates that the symbol will be placed in the |
| dynamic symbol table, but that references within the defining module |
| will bind to the local symbol. That is, the symbol cannot be overridden |
| by another module. |
| |
| @item internal |
| Internal visibility is like hidden visibility, but with additional |
| processor specific semantics. Unless otherwise specified by the psABI, |
| gcc defines internal visibility to mean that the function is @emph{never} |
| called from another module. Note that hidden symbols, while then cannot |
| be referenced directly by other modules, can be referenced indirectly via |
| function pointers. By indicating that a symbol cannot be called from |
| outside the module, gcc may for instance omit the load of a PIC register |
| since it is known that the calling function loaded the correct value. |
| @end table |
| |
| Not all ELF targets support this attribute. |
| |
| @item regparm (@var{number}) |
| @cindex functions that are passed arguments in registers on the 386 |
| On the Intel 386, the @code{regparm} attribute causes the compiler to |
| pass up to @var{number} integer arguments in registers EAX, |
| EDX, and ECX instead of on the stack. Functions that take a |
| variable number of arguments will continue to be passed all of their |
| arguments on the stack. |
| |
| @item stdcall |
| @cindex functions that pop the argument stack on the 386 |
| On the Intel 386, the @code{stdcall} attribute causes the compiler to |
| assume that the called function will pop off the stack space used to |
| pass arguments, unless it takes a variable number of arguments. |
| |
| The PowerPC compiler for Windows NT currently ignores the @code{stdcall} |
| attribute. |
| |
| @item cdecl |
| @cindex functions that do pop the argument stack on the 386 |
| @opindex mrtd |
| On the Intel 386, the @code{cdecl} attribute causes the compiler to |
| assume that the calling function will pop off the stack space used to |
| pass arguments. This is |
| useful to override the effects of the @option{-mrtd} switch. |
| |
| The PowerPC compiler for Windows NT currently ignores the @code{cdecl} |
| attribute. |
| |
| @item longcall |
| @cindex functions called via pointer on the RS/6000 and PowerPC |
| On the RS/6000 and PowerPC, the @code{longcall} attribute causes the |
| compiler to always call the function via a pointer, so that functions |
| which reside further than 64 megabytes (67,108,864 bytes) from the |
| current location can be called. |
| |
| @item long_call/short_call |
| @cindex indirect calls on ARM |
| This attribute allows to specify how to call a particular function on |
| ARM@. Both attributes override the @option{-mlong-calls} (@pxref{ARM Options}) |
| command line switch and @code{#pragma long_calls} settings. The |
| @code{long_call} attribute causes the compiler to always call the |
| function by first loading its address into a register and then using the |
| contents of that register. The @code{short_call} attribute always places |
| the offset to the function from the call site into the @samp{BL} |
| instruction directly. |
| |
| @item dllimport |
| @cindex functions which are imported from a dll on PowerPC Windows NT |
| On the PowerPC running Windows NT, the @code{dllimport} attribute causes |
| the compiler to call the function via a global pointer to the function |
| pointer that is set up by the Windows NT dll library. The pointer name |
| is formed by combining @code{__imp_} and the function name. |
| |
| @item dllexport |
| @cindex functions which are exported from a dll on PowerPC Windows NT |
| On the PowerPC running Windows NT, the @code{dllexport} attribute causes |
| the compiler to provide a global pointer to the function pointer, so |
| that it can be called with the @code{dllimport} attribute. The pointer |
| name is formed by combining @code{__imp_} and the function name. |
| |
| @item exception (@var{except-func} [, @var{except-arg}]) |
| @cindex functions which specify exception handling on PowerPC Windows NT |
| On the PowerPC running Windows NT, the @code{exception} attribute causes |
| the compiler to modify the structured exception table entry it emits for |
| the declared function. The string or identifier @var{except-func} is |
| placed in the third entry of the structured exception table. It |
| represents a function, which is called by the exception handling |
| mechanism if an exception occurs. If it was specified, the string or |
| identifier @var{except-arg} is placed in the fourth entry of the |
| structured exception table. |
| |
| @item function_vector |
| @cindex calling functions through the function vector on the H8/300 processors |
| Use this attribute on the H8/300 and H8/300H to indicate that the specified |
| function should be called through the function vector. Calling a |
| function through the function vector will reduce code size, however; |
| the function vector has a limited size (maximum 128 entries on the H8/300 |
| and 64 entries on the H8/300H) and shares space with the interrupt vector. |
| |
| You must use GAS and GLD from GNU binutils version 2.7 or later for |
| this attribute to work correctly. |
| |
| @item interrupt |
| @cindex interrupt handler functions |
| Use this attribute on the ARM, AVR, M32R/D and Xstormy16 ports to indicate |
| that the specified function is an interrupt handler. The compiler will |
| generate function entry and exit sequences suitable for use in an |
| interrupt handler when this attribute is present. |
| |
| Note, interrupt handlers for the H8/300, H8/300H and SH processors can |
| be specified via the @code{interrupt_handler} attribute. |
| |
| Note, on the AVR interrupts will be enabled inside the function. |
| |
| Note, for the ARM you can specify the kind of interrupt to be handled by |
| adding an optional parameter to the interrupt attribute like this: |
| |
| @smallexample |
| void f () __attribute__ ((interrupt ("IRQ"))); |
| @end smallexample |
| |
| Permissible values for this parameter are: IRQ, FIQ, SWI, ABORT and UNDEF@. |
| |
| @item interrupt_handler |
| @cindex interrupt handler functions on the H8/300 and SH processors |
| Use this attribute on the H8/300, H8/300H and SH to indicate that the |
| specified function is an interrupt handler. The compiler will generate |
| function entry and exit sequences suitable for use in an interrupt |
| handler when this attribute is present. |
| |
| @item sp_switch |
| Use this attribute on the SH to indicate an @code{interrupt_handler} |
| function should switch to an alternate stack. It expects a string |
| argument that names a global variable holding the address of the |
| alternate stack. |
| |
| @smallexample |
| void *alt_stack; |
| void f () __attribute__ ((interrupt_handler, |
| sp_switch ("alt_stack"))); |
| @end smallexample |
| |
| @item trap_exit |
| Use this attribute on the SH for an @code{interrupt_handle} to return using |
| @code{trapa} instead of @code{rte}. This attribute expects an integer |
| argument specifying the trap number to be used. |
| |
| @item eightbit_data |
| @cindex eight bit data on the H8/300 and H8/300H |
| Use this attribute on the H8/300 and H8/300H to indicate that the specified |
| variable should be placed into the eight bit data section. |
| The compiler will generate more efficient code for certain operations |
| on data in the eight bit data area. Note the eight bit data area is limited to |
| 256 bytes of data. |
| |
| You must use GAS and GLD from GNU binutils version 2.7 or later for |
| this attribute to work correctly. |
| |
| @item tiny_data |
| @cindex tiny data section on the H8/300H |
| Use this attribute on the H8/300H to indicate that the specified |
| variable should be placed into the tiny data section. |
| The compiler will generate more efficient code for loads and stores |
| on data in the tiny data section. Note the tiny data area is limited to |
| slightly under 32kbytes of data. |
| |
| @item signal |
| @cindex signal handler functions on the AVR processors |
| Use this attribute on the AVR to indicate that the specified |
| function is an signal handler. The compiler will generate function |
| entry and exit sequences suitable for use in an signal handler when this |
| attribute is present. Interrupts will be disabled inside function. |
| |
| @item naked |
| @cindex function without a prologue/epilogue code |
| Use this attribute on the ARM or AVR ports to indicate that the specified |
| function do not need prologue/epilogue sequences generated by the |
| compiler. It is up to the programmer to provide these sequences. |
| |
| @item model (@var{model-name}) |
| @cindex function addressability on the M32R/D |
| Use this attribute on the M32R/D to set the addressability of an object, |
| and the code generated for a function. |
| The identifier @var{model-name} is one of @code{small}, @code{medium}, |
| or @code{large}, representing each of the code models. |
| |
| Small model objects live in the lower 16MB of memory (so that their |
| addresses can be loaded with the @code{ld24} instruction), and are |
| callable with the @code{bl} instruction. |
| |
| Medium model objects may live anywhere in the 32-bit address space (the |
| compiler will generate @code{seth/add3} instructions to load their addresses), |
| and are callable with the @code{bl} instruction. |
| |
| Large model objects may live anywhere in the 32-bit address space (the |
| compiler will generate @code{seth/add3} instructions to load their addresses), |
| and may not be reachable with the @code{bl} instruction (the compiler will |
| generate the much slower @code{seth/add3/jl} instruction sequence). |
| |
| @end table |
| |
| You can specify multiple attributes in a declaration by separating them |
| by commas within the double parentheses or by immediately following an |
| attribute declaration with another attribute declaration. |
| |
| @cindex @code{#pragma}, reason for not using |
| @cindex pragma, reason for not using |
| Some people object to the @code{__attribute__} feature, suggesting that |
| ISO C's @code{#pragma} should be used instead. At the time |
| @code{__attribute__} was designed, there were two reasons for not doing |
| this. |
| |
| @enumerate |
| @item |
| It is impossible to generate @code{#pragma} commands from a macro. |
| |
| @item |
| There is no telling what the same @code{#pragma} might mean in another |
| compiler. |
| @end enumerate |
| |
| These two reasons applied to almost any application that might have been |
| proposed for @code{#pragma}. It was basically a mistake to use |
| @code{#pragma} for @emph{anything}. |
| |
| The ISO C99 standard includes @code{_Pragma}, which now allows pragmas |
| to be generated from macros. In addition, a @code{#pragma GCC} |
| namespace is now in use for GCC-specific pragmas. However, it has been |
| found convenient to use @code{__attribute__} to achieve a natural |
| attachment of attributes to their corresponding declarations, whereas |
| @code{#pragma GCC} is of use for constructs that do not naturally form |
| part of the grammar. @xref{Other Directives,,Miscellaneous |
| Preprocessing Directives, cpp, The C Preprocessor}. |
| |
| @node Attribute Syntax |
| @section Attribute Syntax |
| @cindex attribute syntax |
| |
| This section describes the syntax with which @code{__attribute__} may be |
| used, and the constructs to which attribute specifiers bind, for the C |
| language. Some details may vary for C++ and Objective-C@. Because of |
| infelicities in the grammar for attributes, some forms described here |
| may not be successfully parsed in all cases. |
| |
| There are some problems with the semantics of attributes in C++. For |
| example, there are no manglings for attributes, although they may affect |
| code generation, so problems may arise when attributed types are used in |
| conjunction with templates or overloading. Similarly, @code{typeid} |
| does not distinguish between types with different attributes. Support |
| for attributes in C++ may be restricted in future to attributes on |
| declarations only, but not on nested declarators. |
| |
| @xref{Function Attributes}, for details of the semantics of attributes |
| applying to functions. @xref{Variable Attributes}, for details of the |
| semantics of attributes applying to variables. @xref{Type Attributes}, |
| for details of the semantics of attributes applying to structure, union |
| and enumerated types. |
| |
| An @dfn{attribute specifier} is of the form |
| @code{__attribute__ ((@var{attribute-list}))}. An @dfn{attribute list} |
| is a possibly empty comma-separated sequence of @dfn{attributes}, where |
| each attribute is one of the following: |
| |
| @itemize @bullet |
| @item |
| Empty. Empty attributes are ignored. |
| |
| @item |
| A word (which may be an identifier such as @code{unused}, or a reserved |
| word such as @code{const}). |
| |
| @item |
| A word, followed by, in parentheses, parameters for the attribute. |
| These parameters take one of the following forms: |
| |
| @itemize @bullet |
| @item |
| An identifier. For example, @code{mode} attributes use this form. |
| |
| @item |
| An identifier followed by a comma and a non-empty comma-separated list |
| of expressions. For example, @code{format} attributes use this form. |
| |
| @item |
| A possibly empty comma-separated list of expressions. For example, |
| @code{format_arg} attributes use this form with the list being a single |
| integer constant expression, and @code{alias} attributes use this form |
| with the list being a single string constant. |
| @end itemize |
| @end itemize |
| |
| An @dfn{attribute specifier list} is a sequence of one or more attribute |
| specifiers, not separated by any other tokens. |
| |
| An attribute specifier list may appear after the colon following a |
| label, other than a @code{case} or @code{default} label. The only |
| attribute it makes sense to use after a label is @code{unused}. This |
| feature is intended for code generated by programs which contains labels |
| that may be unused but which is compiled with @option{-Wall}. It would |
| not normally be appropriate to use in it human-written code, though it |
| could be useful in cases where the code that jumps to the label is |
| contained within an @code{#ifdef} conditional. |
| |
| An attribute specifier list may appear as part of a @code{struct}, |
| @code{union} or @code{enum} specifier. It may go either immediately |
| after the @code{struct}, @code{union} or @code{enum} keyword, or after |
| the closing brace. It is ignored if the content of the structure, union |
| or enumerated type is not defined in the specifier in which the |
| attribute specifier list is used---that is, in usages such as |
| @code{struct __attribute__((foo)) bar} with no following opening brace. |
| Where attribute specifiers follow the closing brace, they are considered |
| to relate to the structure, union or enumerated type defined, not to any |
| enclosing declaration the type specifier appears in, and the type |
| defined is not complete until after the attribute specifiers. |
| @c Otherwise, there would be the following problems: a shift/reduce |
| @c conflict between attributes binding the struct/union/enum and |
| @c binding to the list of specifiers/qualifiers; and "aligned" |
| @c attributes could use sizeof for the structure, but the size could be |
| @c changed later by "packed" attributes. |
| |
| Otherwise, an attribute specifier appears as part of a declaration, |
| counting declarations of unnamed parameters and type names, and relates |
| to that declaration (which may be nested in another declaration, for |
| example in the case of a parameter declaration), or to a particular declarator |
| within a declaration. Where an |
| attribute specifier is applied to a parameter declared as a function or |
| an array, it should apply to the function or array rather than the |
| pointer to which the parameter is implicitly converted, but this is not |
| yet correctly implemented. |
| |
| Any list of specifiers and qualifiers at the start of a declaration may |
| contain attribute specifiers, whether or not such a list may in that |
| context contain storage class specifiers. (Some attributes, however, |
| are essentially in the nature of storage class specifiers, and only make |
| sense where storage class specifiers may be used; for example, |
| @code{section}.) There is one necessary limitation to this syntax: the |
| first old-style parameter declaration in a function definition cannot |
| begin with an attribute specifier, because such an attribute applies to |
| the function instead by syntax described below (which, however, is not |
| yet implemented in this case). In some other cases, attribute |
| specifiers are permitted by this grammar but not yet supported by the |
| compiler. All attribute specifiers in this place relate to the |
| declaration as a whole. In the obsolescent usage where a type of |
| @code{int} is implied by the absence of type specifiers, such a list of |
| specifiers and qualifiers may be an attribute specifier list with no |
| other specifiers or qualifiers. |
| |
| An attribute specifier list may appear immediately before a declarator |
| (other than the first) in a comma-separated list of declarators in a |
| declaration of more than one identifier using a single list of |
| specifiers and qualifiers. Such attribute specifiers apply |
| only to the identifier before whose declarator they appear. For |
| example, in |
| |
| @smallexample |
| __attribute__((noreturn)) void d0 (void), |
| __attribute__((format(printf, 1, 2))) d1 (const char *, ...), |
| d2 (void) |
| @end smallexample |
| |
| @noindent |
| the @code{noreturn} attribute applies to all the functions |
| declared; the @code{format} attribute only applies to @code{d1}. |
| |
| An attribute specifier list may appear immediately before the comma, |
| @code{=} or semicolon terminating the declaration of an identifier other |
| than a function definition. At present, such attribute specifiers apply |
| to the declared object or function, but in future they may attach to the |
| outermost adjacent declarator. In simple cases there is no difference, |
| but, for example, in |
| |
| @smallexample |
| void (****f)(void) __attribute__((noreturn)); |
| @end smallexample |
| |
| @noindent |
| at present the @code{noreturn} attribute applies to @code{f}, which |
| causes a warning since @code{f} is not a function, but in future it may |
| apply to the function @code{****f}. The precise semantics of what |
| attributes in such cases will apply to are not yet specified. Where an |
| assembler name for an object or function is specified (@pxref{Asm |
| Labels}), at present the attribute must follow the @code{asm} |
| specification; in future, attributes before the @code{asm} specification |
| may apply to the adjacent declarator, and those after it to the declared |
| object or function. |
| |
| An attribute specifier list may, in future, be permitted to appear after |
| the declarator in a function definition (before any old-style parameter |
| declarations or the function body). |
| |
| Attribute specifiers may be mixed with type qualifiers appearing inside |
| the @code{[]} of a parameter array declarator, in the C99 construct by |
| which such qualifiers are applied to the pointer to which the array is |
| implicitly converted. Such attribute specifiers apply to the pointer, |
| not to the array, but at present this is not implemented and they are |
| ignored. |
| |
| An attribute specifier list may appear at the start of a nested |
| declarator. At present, there are some limitations in this usage: the |
| attributes correctly apply to the declarator, but for most individual |
| attributes the semantics this implies are not implemented. |
| When attribute specifiers follow the @code{*} of a pointer |
| declarator, they may be mixed with any type qualifiers present. |
| The following describes the formal semantics of this syntax. It will make the |
| most sense if you are familiar with the formal specification of |
| declarators in the ISO C standard. |
| |
| Consider (as in C99 subclause 6.7.5 paragraph 4) a declaration @code{T |
| D1}, where @code{T} contains declaration specifiers that specify a type |
| @var{Type} (such as @code{int}) and @code{D1} is a declarator that |
| contains an identifier @var{ident}. The type specified for @var{ident} |
| for derived declarators whose type does not include an attribute |
| specifier is as in the ISO C standard. |
| |
| If @code{D1} has the form @code{( @var{attribute-specifier-list} D )}, |
| and the declaration @code{T D} specifies the type |
| ``@var{derived-declarator-type-list} @var{Type}'' for @var{ident}, then |
| @code{T D1} specifies the type ``@var{derived-declarator-type-list} |
| @var{attribute-specifier-list} @var{Type}'' for @var{ident}. |
| |
| If @code{D1} has the form @code{* |
| @var{type-qualifier-and-attribute-specifier-list} D}, and the |
| declaration @code{T D} specifies the type |
| ``@var{derived-declarator-type-list} @var{Type}'' for @var{ident}, then |
| @code{T D1} specifies the type ``@var{derived-declarator-type-list} |
| @var{type-qualifier-and-attribute-specifier-list} @var{Type}'' for |
| @var{ident}. |
| |
| For example, |
| |
| @smallexample |
| void (__attribute__((noreturn)) ****f) (void); |
| @end smallexample |
| |
| @noindent |
| specifies the type ``pointer to pointer to pointer to pointer to |
| non-returning function returning @code{void}''. As another example, |
| |
| @smallexample |
| char *__attribute__((aligned(8))) *f; |
| @end smallexample |
| |
| @noindent |
| specifies the type ``pointer to 8-byte-aligned pointer to @code{char}''. |
| Note again that this does not work with most attributes; for example, |
| the usage of @samp{aligned} and @samp{noreturn} attributes given above |
| is not yet supported. |
| |
| For compatibility with existing code written for compiler versions that |
| did not implement attributes on nested declarators, some laxity is |
| allowed in the placing of attributes. If an attribute that only applies |
| to types is applied to a declaration, it will be treated as applying to |
| the type of that declaration. If an attribute that only applies to |
| declarations is applied to the type of a declaration, it will be treated |
| as applying to that declaration; and, for compatibility with code |
| placing the attributes immediately before the identifier declared, such |
| an attribute applied to a function return type will be treated as |
| applying to the function type, and such an attribute applied to an array |
| element type will be treated as applying to the array type. If an |
| attribute that only applies to function types is applied to a |
| pointer-to-function type, it will be treated as applying to the pointer |
| target type; if such an attribute is applied to a function return type |
| that is not a pointer-to-function type, it will be treated as applying |
| to the function type. |
| |
| @node Function Prototypes |
| @section Prototypes and Old-Style Function Definitions |
| @cindex function prototype declarations |
| @cindex old-style function definitions |
| @cindex promotion of formal parameters |
| |
| GNU C extends ISO C to allow a function prototype to override a later |
| old-style non-prototype definition. Consider the following example: |
| |
| @example |
| /* @r{Use prototypes unless the compiler is old-fashioned.} */ |
| #ifdef __STDC__ |
| #define P(x) x |
| #else |
| #define P(x) () |
| #endif |
| |
| /* @r{Prototype function declaration.} */ |
| int isroot P((uid_t)); |
| |
| /* @r{Old-style function definition.} */ |
| int |
| isroot (x) /* ??? lossage here ??? */ |
| uid_t x; |
| @{ |
| return x == 0; |
| @} |
| @end example |
| |
| Suppose the type @code{uid_t} happens to be @code{short}. ISO C does |
| not allow this example, because subword arguments in old-style |
| non-prototype definitions are promoted. Therefore in this example the |
| function definition's argument is really an @code{int}, which does not |
| match the prototype argument type of @code{short}. |
| |
| This restriction of ISO C makes it hard to write code that is portable |
| to traditional C compilers, because the programmer does not know |
| whether the @code{uid_t} type is @code{short}, @code{int}, or |
| @code{long}. Therefore, in cases like these GNU C allows a prototype |
| to override a later old-style definition. More precisely, in GNU C, a |
| function prototype argument type overrides the argument type specified |
| by a later old-style definition if the former type is the same as the |
| latter type before promotion. Thus in GNU C the above example is |
| equivalent to the following: |
| |
| @example |
| int isroot (uid_t); |
| |
| int |
| isroot (uid_t x) |
| @{ |
| return x == 0; |
| @} |
| @end example |
| |
| @noindent |
| GNU C++ does not support old-style function definitions, so this |
| extension is irrelevant. |
| |
| @node C++ Comments |
| @section C++ Style Comments |
| @cindex // |
| @cindex C++ comments |
| @cindex comments, C++ style |
| |
| In GNU C, you may use C++ style comments, which start with @samp{//} and |
| continue until the end of the line. Many other C implementations allow |
| such comments, and they are included in the 1999 C standard. However, |
| C++ style comments are not recognized if you specify an @option{-std} |
| option specifying a version of ISO C before C99, or @option{-ansi} |
| (equivalent to @option{-std=c89}). |
| |
| @node Dollar Signs |
| @section Dollar Signs in Identifier Names |
| @cindex $ |
| @cindex dollar signs in identifier names |
| @cindex identifier names, dollar signs in |
| |
| In GNU C, you may normally use dollar signs in identifier names. |
| This is because many traditional C implementations allow such identifiers. |
| However, dollar signs in identifiers are not supported on a few target |
| machines, typically because the target assembler does not allow them. |
| |
| @node Character Escapes |
| @section The Character @key{ESC} in Constants |
| |
| You can use the sequence @samp{\e} in a string or character constant to |
| stand for the ASCII character @key{ESC}. |
| |
| @node Alignment |
| @section Inquiring on Alignment of Types or Variables |
| @cindex alignment |
| @cindex type alignment |
| @cindex variable alignment |
| |
| The keyword @code{__alignof__} allows you to inquire about how an object |
| is aligned, or the minimum alignment usually required by a type. Its |
| syntax is just like @code{sizeof}. |
| |
| For example, if the target machine requires a @code{double} value to be |
| aligned on an 8-byte boundary, then @code{__alignof__ (double)} is 8. |
| This is true on many RISC machines. On more traditional machine |
| designs, @code{__alignof__ (double)} is 4 or even 2. |
| |
| Some machines never actually require alignment; they allow reference to any |
| data type even at an odd addresses. For these machines, @code{__alignof__} |
| reports the @emph{recommended} alignment of a type. |
| |
| If the operand of @code{__alignof__} is an lvalue rather than a type, |
| its value is the required alignment for its type, taking into account |
| any minimum alignment specified with GCC's @code{__attribute__} |
| extension (@pxref{Variable Attributes}). For example, after this |
| declaration: |
| |
| @example |
| struct foo @{ int x; char y; @} foo1; |
| @end example |
| |
| @noindent |
| the value of @code{__alignof__ (foo1.y)} is 1, even though its actual |
| alignment is probably 2 or 4, the same as @code{__alignof__ (int)}. |
| |
| It is an error to ask for the alignment of an incomplete type. |
| |
| @node Variable Attributes |
| @section Specifying Attributes of Variables |
| @cindex attribute of variables |
| @cindex variable attributes |
| |
| The keyword @code{__attribute__} allows you to specify special |
| attributes of variables or structure fields. This keyword is followed |
| by an attribute specification inside double parentheses. Ten |
| attributes are currently defined for variables: @code{aligned}, |
| @code{mode}, @code{nocommon}, @code{packed}, @code{section}, |
| @code{transparent_union}, @code{unused}, @code{deprecated}, |
| @code{vector_size}, and @code{weak}. Some other attributes are defined |
| for variables on particular target systems. Other attributes are |
| available for functions (@pxref{Function Attributes}) and for types |
| (@pxref{Type Attributes}). Other front ends might define more |
| attributes (@pxref{C++ Extensions,,Extensions to the C++ Language}). |
| |
| You may also specify attributes with @samp{__} preceding and following |
| each keyword. This allows you to use them in header files without |
| being concerned about a possible macro of the same name. For example, |
| you may use @code{__aligned__} instead of @code{aligned}. |
| |
| @xref{Attribute Syntax}, for details of the exact syntax for using |
| attributes. |
| |
| @table @code |
| @cindex @code{aligned} attribute |
| @item aligned (@var{alignment}) |
| This attribute specifies a minimum alignment for the variable or |
| structure field, measured in bytes. For example, the declaration: |
| |
| @smallexample |
| int x __attribute__ ((aligned (16))) = 0; |
| @end smallexample |
| |
| @noindent |
| causes the compiler to allocate the global variable @code{x} on a |
| 16-byte boundary. On a 68040, this could be used in conjunction with |
| an @code{asm} expression to access the @code{move16} instruction which |
| requires 16-byte aligned operands. |
| |
| You can also specify the alignment of structure fields. For example, to |
| create a double-word aligned @code{int} pair, you could write: |
| |
| @smallexample |
| struct foo @{ int x[2] __attribute__ ((aligned (8))); @}; |
| @end smallexample |
| |
| @noindent |
| This is an alternative to creating a union with a @code{double} member |
| that forces the union to be double-word aligned. |
| |
| It is not possible to specify the alignment of functions; the alignment |
| of functions is determined by the machine's requirements and cannot be |
| changed. You cannot specify alignment for a typedef name because such a |
| name is just an alias, not a distinct type. |
| |
| As in the preceding examples, you can explicitly specify the alignment |
| (in bytes) that you wish the compiler to use for a given variable or |
| structure field. Alternatively, you can leave out the alignment factor |
| and just ask the compiler to align a variable or field to the maximum |
| useful alignment for the target machine you are compiling for. For |
| example, you could write: |
| |
| @smallexample |
| short array[3] __attribute__ ((aligned)); |
| @end smallexample |
| |
| Whenever you leave out the alignment factor in an @code{aligned} attribute |
| specification, the compiler automatically sets the alignment for the declared |
| variable or field to the largest alignment which is ever used for any data |
| type on the target machine you are compiling for. Doing this can often make |
| copy operations more efficient, because the compiler can use whatever |
| instructions copy the biggest chunks of memory when performing copies to |
| or from the variables or fields that you have aligned this way. |
| |
| The @code{aligned} attribute can only increase the alignment; but you |
| can decrease it by specifying @code{packed} as well. See below. |
| |
| Note that the effectiveness of @code{aligned} attributes may be limited |
| by inherent limitations in your linker. On many systems, the linker is |
| only able to arrange for variables to be aligned up to a certain maximum |
| alignment. (For some linkers, the maximum supported alignment may |
| be very very small.) If your linker is only able to align variables |
| up to a maximum of 8 byte alignment, then specifying @code{aligned(16)} |
| in an @code{__attribute__} will still only provide you with 8 byte |
| alignment. See your linker documentation for further information. |
| |
| @item mode (@var{mode}) |
| @cindex @code{mode} attribute |
| This attribute specifies the data type for the declaration---whichever |
| type corresponds to the mode @var{mode}. This in effect lets you |
| request an integer or floating point type according to its width. |
| |
| You may also specify a mode of @samp{byte} or @samp{__byte__} to |
| indicate the mode corresponding to a one-byte integer, @samp{word} or |
| @samp{__word__} for the mode of a one-word integer, and @samp{pointer} |
| or @samp{__pointer__} for the mode used to represent pointers. |
| |
| @item nocommon |
| @cindex @code{nocommon} attribute |
| @opindex fno-common |
| This attribute specifies requests GCC not to place a variable |
| ``common'' but instead to allocate space for it directly. If you |
| specify the @option{-fno-common} flag, GCC will do this for all |
| variables. |
| |
| Specifying the @code{nocommon} attribute for a variable provides an |
| initialization of zeros. A variable may only be initialized in one |
| source file. |
| |
| @item packed |
| @cindex @code{packed} attribute |
| The @code{packed} attribute specifies that a variable or structure field |
| should have the smallest possible alignment---one byte for a variable, |
| and one bit for a field, unless you specify a larger value with the |
| @code{aligned} attribute. |
| |
| Here is a structure in which the field @code{x} is packed, so that it |
| immediately follows @code{a}: |
| |
| @example |
| struct foo |
| @{ |
| char a; |
| int x[2] __attribute__ ((packed)); |
| @}; |
| @end example |
| |
| @item section ("@var{section-name}") |
| @cindex @code{section} variable attribute |
| Normally, the compiler places the objects it generates in sections like |
| @code{data} and @code{bss}. Sometimes, however, you need additional sections, |
| or you need certain particular variables to appear in special sections, |
| for example to map to special hardware. The @code{section} |
| attribute specifies that a variable (or function) lives in a particular |
| section. For example, this small program uses several specific section names: |
| |
| @smallexample |
| struct duart a __attribute__ ((section ("DUART_A"))) = @{ 0 @}; |
| struct duart b __attribute__ ((section ("DUART_B"))) = @{ 0 @}; |
| char stack[10000] __attribute__ ((section ("STACK"))) = @{ 0 @}; |
| int init_data __attribute__ ((section ("INITDATA"))) = 0; |
| |
| main() |
| @{ |
| /* Initialize stack pointer */ |
| init_sp (stack + sizeof (stack)); |
| |
| /* Initialize initialized data */ |
| memcpy (&init_data, &data, &edata - &data); |
| |
| /* Turn on the serial ports */ |
| init_duart (&a); |
| init_duart (&b); |
| @} |
| @end smallexample |
| |
| @noindent |
| Use the @code{section} attribute with an @emph{initialized} definition |
| of a @emph{global} variable, as shown in the example. GCC issues |
| a warning and otherwise ignores the @code{section} attribute in |
| uninitialized variable declarations. |
| |
| You may only use the @code{section} attribute with a fully initialized |
| global definition because of the way linkers work. The linker requires |
| each object be defined once, with the exception that uninitialized |
| variables tentatively go in the @code{common} (or @code{bss}) section |
| and can be multiply ``defined''. You can force a variable to be |
| initialized with the @option{-fno-common} flag or the @code{nocommon} |
| attribute. |
| |
| Some file formats do not support arbitrary sections so the @code{section} |
| attribute is not available on all platforms. |
| If you need to map the entire contents of a module to a particular |
| section, consider using the facilities of the linker instead. |
| |
| @item shared |
| @cindex @code{shared} variable attribute |
| On Windows NT, in addition to putting variable definitions in a named |
| section, the section can also be shared among all running copies of an |
| executable or DLL@. For example, this small program defines shared data |
| by putting it in a named section @code{shared} and marking the section |
| shareable: |
| |
| @smallexample |
| int foo __attribute__((section ("shared"), shared)) = 0; |
| |
| int |
| main() |
| @{ |
| /* Read and write foo. All running |
| copies see the same value. */ |
| return 0; |
| @} |
| @end smallexample |
| |
| @noindent |
| You may only use the @code{shared} attribute along with @code{section} |
| attribute with a fully initialized global definition because of the way |
| linkers work. See @code{section} attribute for more information. |
| |
| The @code{shared} attribute is only available on Windows NT@. |
| |
| @item transparent_union |
| This attribute, attached to a function parameter which is a union, means |
| that the corresponding argument may have the type of any union member, |
| but the argument is passed as if its type were that of the first union |
| member. For more details see @xref{Type Attributes}. You can also use |
| this attribute on a @code{typedef} for a union data type; then it |
| applies to all function parameters with that type. |
| |
| @item unused |
| This attribute, attached to a variable, means that the variable is meant |
| to be possibly unused. GCC will not produce a warning for this |
| variable. |
| |
| @item deprecated |
| The @code{deprecated} attribute results in a warning if the variable |
| is used anywhere in the source file. This is useful when identifying |
| variables that are expected to be removed in a future version of a |
| program. The warning also includes the location of the declaration |
| of the deprecated variable, to enable users to easily find further |
| information about why the variable is deprecated, or what they should |
| do instead. Note that the warnings only occurs for uses: |
| |
| @smallexample |
| extern int old_var __attribute__ ((deprecated)); |
| extern int old_var; |
| int new_fn () @{ return old_var; @} |
| @end smallexample |
| |
| results in a warning on line 3 but not line 2. |
| |
| The @code{deprecated} attribute can also be used for functions and |
| types (@pxref{Function Attributes}, @pxref{Type Attributes}.) |
| |
| @item vector_size (@var{bytes}) |
| This attribute specifies the vector size for the variable, measured in |
| bytes. For example, the declaration: |
| |
| @smallexample |
| int foo __attribute__ ((vector_size (16))); |
| @end smallexample |
| |
| @noindent |
| causes the compiler to set the mode for @code{foo}, to be 16 bytes, |
| divided into @code{int} sized units. Assuming a 32-bit int (a vector of |
| 4 units of 4 bytes), the corresponding mode of @code{foo} will be V4SI@. |
| |
| This attribute is only applicable to integral and float scalars, |
| although arrays, pointers, and function return values are allowed in |
| conjunction with this construct. |
| |
| Aggregates with this attribute are invalid, even if they are of the same |
| size as a corresponding scalar. For example, the declaration: |
| |
| @smallexample |
| struct S @{ int a; @}; |
| struct S __attribute__ ((vector_size (16))) foo; |
| @end smallexample |
| |
| @noindent |
| is invalid even if the size of the structure is the same as the size of |
| the @code{int}. |
| |
| @item weak |
| The @code{weak} attribute is described in @xref{Function Attributes}. |
| |
| @item model (@var{model-name}) |
| @cindex variable addressability on the M32R/D |
| Use this attribute on the M32R/D to set the addressability of an object. |
| The identifier @var{model-name} is one of @code{small}, @code{medium}, |
| or @code{large}, representing each of the code models. |
| |
| Small model objects live in the lower 16MB of memory (so that their |
| addresses can be loaded with the @code{ld24} instruction). |
| |
| Medium and large model objects may live anywhere in the 32-bit address space |
| (the compiler will generate @code{seth/add3} instructions to load their |
| addresses). |
| |
| @end table |
| |
| To specify multiple attributes, separate them by commas within the |
| double parentheses: for example, @samp{__attribute__ ((aligned (16), |
| packed))}. |
| |
| @node Type Attributes |
| @section Specifying Attributes of Types |
| @cindex attribute of types |
| @cindex type attributes |
| |
| The keyword @code{__attribute__} allows you to specify special |
| attributes of @code{struct} and @code{union} types when you define such |
| types. This keyword is followed by an attribute specification inside |
| double parentheses. Five attributes are currently defined for types: |
| @code{aligned}, @code{packed}, @code{transparent_union}, @code{unused}, |
| and @code{deprecated}. Other attributes are defined for functions |
| (@pxref{Function Attributes}) and for variables (@pxref{Variable Attributes}). |
| |
| You may also specify any one of these attributes with @samp{__} |
| preceding and following its keyword. This allows you to use these |
| attributes in header files without being concerned about a possible |
| macro of the same name. For example, you may use @code{__aligned__} |
| instead of @code{aligned}. |
| |
| You may specify the @code{aligned} and @code{transparent_union} |
| attributes either in a @code{typedef} declaration or just past the |
| closing curly brace of a complete enum, struct or union type |
| @emph{definition} and the @code{packed} attribute only past the closing |
| brace of a definition. |
| |
| You may also specify attributes between the enum, struct or union |
| tag and the name of the type rather than after the closing brace. |
| |
| @xref{Attribute Syntax}, for details of the exact syntax for using |
| attributes. |
| |
| @table @code |
| @cindex @code{aligned} attribute |
| @item aligned (@var{alignment}) |
| This attribute specifies a minimum alignment (in bytes) for variables |
| of the specified type. For example, the declarations: |
| |
| @smallexample |
| struct S @{ short f[3]; @} __attribute__ ((aligned (8))); |
| typedef int more_aligned_int __attribute__ ((aligned (8))); |
| @end smallexample |
| |
| @noindent |
| force the compiler to insure (as far as it can) that each variable whose |
| type is @code{struct S} or @code{more_aligned_int} will be allocated and |
| aligned @emph{at least} on a 8-byte boundary. On a Sparc, having all |
| variables of type @code{struct S} aligned to 8-byte boundaries allows |
| the compiler to use the @code{ldd} and @code{std} (doubleword load and |
| store) instructions when copying one variable of type @code{struct S} to |
| another, thus improving run-time efficiency. |
| |
| Note that the alignment of any given @code{struct} or @code{union} type |
| is required by the ISO C standard to be at least a perfect multiple of |
| the lowest common multiple of the alignments of all of the members of |
| the @code{struct} or @code{union} in question. This means that you @emph{can} |
| effectively adjust the alignment of a @code{struct} or @code{union} |
| type by attaching an @code{aligned} attribute to any one of the members |
| of such a type, but the notation illustrated in the example above is a |
| more obvious, intuitive, and readable way to request the compiler to |
| adjust the alignment of an entire @code{struct} or @code{union} type. |
| |
| As in the preceding example, you can explicitly specify the alignment |
| (in bytes) that you wish the compiler to use for a given @code{struct} |
| or @code{union} type. Alternatively, you can leave out the alignment factor |
| and just ask the compiler to align a type to the maximum |
| useful alignment for the target machine you are compiling for. For |
| example, you could write: |
| |
| @smallexample |
| struct S @{ short f[3]; @} __attribute__ ((aligned)); |
| @end smallexample |
| |
| Whenever you leave out the alignment factor in an @code{aligned} |
| attribute specification, the compiler automatically sets the alignment |
| for the type to the largest alignment which is ever used for any data |
| type on the target machine you are compiling for. Doing this can often |
| make copy operations more efficient, because the compiler can use |
| whatever instructions copy the biggest chunks of memory when performing |
| copies to or from the variables which have types that you have aligned |
| this way. |
| |
| In the example above, if the size of each @code{short} is 2 bytes, then |
| the size of the entire @code{struct S} type is 6 bytes. The smallest |
| power of two which is greater than or equal to that is 8, so the |
| compiler sets the alignment for the entire @code{struct S} type to 8 |
| bytes. |
| |
| Note that although you can ask the compiler to select a time-efficient |
| alignment for a given type and then declare only individual stand-alone |
| objects of that type, the compiler's ability to select a time-efficient |
| alignment is primarily useful only when you plan to create arrays of |
| variables having the relevant (efficiently aligned) type. If you |
| declare or use arrays of variables of an efficiently-aligned type, then |
| it is likely that your program will also be doing pointer arithmetic (or |
| subscripting, which amounts to the same thing) on pointers to the |
| relevant type, and the code that the compiler generates for these |
| pointer arithmetic operations will often be more efficient for |
| efficiently-aligned types than for other types. |
| |
| The @code{aligned} attribute can only increase the alignment; but you |
| can decrease it by specifying @code{packed} as well. See below. |
| |
| Note that the effectiveness of @code{aligned} attributes may be limited |
| by inherent limitations in your linker. On many systems, the linker is |
| only able to arrange for variables to be aligned up to a certain maximum |
| alignment. (For some linkers, the maximum supported alignment may |
| be very very small.) If your linker is only able to align variables |
| up to a maximum of 8 byte alignment, then specifying @code{aligned(16)} |
| in an @code{__attribute__} will still only provide you with 8 byte |
| alignment. See your linker documentation for further information. |
| |
| @item packed |
| This attribute, attached to an @code{enum}, @code{struct}, or |
| @code{union} type definition, specified that the minimum required memory |
| be used to represent the type. |
| |
| @opindex fshort-enums |
| Specifying this attribute for @code{struct} and @code{union} types is |
| equivalent to specifying the @code{packed} attribute on each of the |
| structure or union members. Specifying the @option{-fshort-enums} |
| flag on the line is equivalent to specifying the @code{packed} |
| attribute on all @code{enum} definitions. |
| |
| You may only specify this attribute after a closing curly brace on an |
| @code{enum} definition, not in a @code{typedef} declaration, unless that |
| declaration also contains the definition of the @code{enum}. |
| |
| @item transparent_union |
| This attribute, attached to a @code{union} type definition, indicates |
| that any function parameter having that union type causes calls to that |
| function to be treated in a special way. |
| |
| First, the argument corresponding to a transparent union type can be of |
| any type in the union; no cast is required. Also, if the union contains |
| a pointer type, the corresponding argument can be a null pointer |
| constant or a void pointer expression; and if the union contains a void |
| pointer type, the corresponding argument can be any pointer expression. |
| If the union member type is a pointer, qualifiers like @code{const} on |
| the referenced type must be respected, just as with normal pointer |
| conversions. |
| |
| Second, the argument is passed to the function using the calling |
| conventions of first member of the transparent union, not the calling |
| conventions of the union itself. All members of the union must have the |
| same machine representation; this is necessary for this argument passing |
| to work properly. |
| |
| Transparent unions are designed for library functions that have multiple |
| interfaces for compatibility reasons. For example, suppose the |
| @code{wait} function must accept either a value of type @code{int *} to |
| comply with Posix, or a value of type @code{union wait *} to comply with |
| the 4.1BSD interface. If @code{wait}'s parameter were @code{void *}, |
| @code{wait} would accept both kinds of arguments, but it would also |
| accept any other pointer type and this would make argument type checking |
| less useful. Instead, @code{<sys/wait.h>} might define the interface |
| as follows: |
| |
| @smallexample |
| typedef union |
| @{ |
| int *__ip; |
| union wait *__up; |
| @} wait_status_ptr_t __attribute__ ((__transparent_union__)); |
| |
| pid_t wait (wait_status_ptr_t); |
| @end smallexample |
| |
| This interface allows either @code{int *} or @code{union wait *} |
| arguments to be passed, using the @code{int *} calling convention. |
| The program can call @code{wait} with arguments of either type: |
| |
| @example |
| int w1 () @{ int w; return wait (&w); @} |
| int w2 () @{ union wait w; return wait (&w); @} |
| @end example |
| |
| With this interface, @code{wait}'s implementation might look like this: |
| |
| @example |
| pid_t wait (wait_status_ptr_t p) |
| @{ |
| return waitpid (-1, p.__ip, 0); |
| @} |
| @end example |
| |
| @item unused |
| When attached to a type (including a @code{union} or a @code{struct}), |
| this attribute means that variables of that type are meant to appear |
| possibly unused. GCC will not produce a warning for any variables of |
| that type, even if the variable appears to do nothing. This is often |
| the case with lock or thread classes, which are usually defined and then |
| not referenced, but contain constructors and destructors that have |
| nontrivial bookkeeping functions. |
| |
| @item deprecated |
| The @code{deprecated} attribute results in a warning if the type |
| is used anywhere in the source file. This is useful when identifying |
| types that are expected to be removed in a future version of a program. |
| If possible, the warning also includes the location of the declaration |
| of the deprecated type, to enable users to easily find further |
| information about why the type is deprecated, or what they should do |
| instead. Note that the warnings only occur for uses and then only |
| if the type is being applied to an identifier that itself is not being |
| declared as deprecated. |
| |
| @smallexample |
| typedef int T1 __attribute__ ((deprecated)); |
| T1 x; |
| typedef T1 T2; |
| T2 y; |
| typedef T1 T3 __attribute__ ((deprecated)); |
| T3 z __attribute__ ((deprecated)); |
| @end smallexample |
| |
| results in a warning on line 2 and 3 but not lines 4, 5, or 6. No |
| warning is issued for line 4 because T2 is not explicitly |
| deprecated. Line 5 has no warning because T3 is explicitly |
| deprecated. Similarly for line 6. |
| |
| The @code{deprecated} attribute can also be used for functions and |
| variables (@pxref{Function Attributes}, @pxref{Variable Attributes}.) |
| |
| @end table |
| |
| To specify multiple attributes, separate them by commas within the |
| double parentheses: for example, @samp{__attribute__ ((aligned (16), |
| packed))}. |
| |
| @node Inline |
| @section An Inline Function is As Fast As a Macro |
| @cindex inline functions |
| @cindex integrating function code |
| @cindex open coding |
| @cindex macros, inline alternative |
| |
| By declaring a function @code{inline}, you can direct GCC to |
| integrate that function's code into the code for its callers. This |
| makes execution faster by eliminating the function-call overhead; in |
| addition, if any of the actual argument values are constant, their known |
| values may permit simplifications at compile time so that not all of the |
| inline function's code needs to be included. The effect on code size is |
| less predictable; object code may be larger or smaller with function |
| inlining, depending on the particular case. Inlining of functions is an |
| optimization and it really ``works'' only in optimizing compilation. If |
| you don't use @option{-O}, no function is really inline. |
| |
| Inline functions are included in the ISO C99 standard, but there are |
| currently substantial differences between what GCC implements and what |
| the ISO C99 standard requires. |
| |
| To declare a function inline, use the @code{inline} keyword in its |
| declaration, like this: |
| |
| @example |
| inline int |
| inc (int *a) |
| @{ |
| (*a)++; |
| @} |
| @end example |
| |
| (If you are writing a header file to be included in ISO C programs, write |
| @code{__inline__} instead of @code{inline}. @xref{Alternate Keywords}.) |
| You can also make all ``simple enough'' functions inline with the option |
| @option{-finline-functions}. |
| |
| @opindex Winline |
| Note that certain usages in a function definition can make it unsuitable |
| for inline substitution. Among these usages are: use of varargs, use of |
| alloca, use of variable sized data types (@pxref{Variable Length}), |
| use of computed goto (@pxref{Labels as Values}), use of nonlocal goto, |
| and nested functions (@pxref{Nested Functions}). Using @option{-Winline} |
| will warn when a function marked @code{inline} could not be substituted, |
| and will give the reason for the failure. |
| |
| Note that in C and Objective-C, unlike C++, the @code{inline} keyword |
| does not affect the linkage of the function. |
| |
| @cindex automatic @code{inline} for C++ member fns |
| @cindex @code{inline} automatic for C++ member fns |
| @cindex member fns, automatically @code{inline} |
| @cindex C++ member fns, automatically @code{inline} |
| @opindex fno-default-inline |
| GCC automatically inlines member functions defined within the class |
| body of C++ programs even if they are not explicitly declared |
| @code{inline}. (You can override this with @option{-fno-default-inline}; |
| @pxref{C++ Dialect Options,,Options Controlling C++ Dialect}.) |
| |
| @cindex inline functions, omission of |
| @opindex fkeep-inline-functions |
| When a function is both inline and @code{static}, if all calls to the |
| function are integrated into the caller, and the function's address is |
| never used, then the function's own assembler code is never referenced. |
| In this case, GCC does not actually output assembler code for the |
| function, unless you specify the option @option{-fkeep-inline-functions}. |
| Some calls cannot be integrated for various reasons (in particular, |
| calls that precede the function's definition cannot be integrated, and |
| neither can recursive calls within the definition). If there is a |
| nonintegrated call, then the function is compiled to assembler code as |
| usual. The function must also be compiled as usual if the program |
| refers to its address, because that can't be inlined. |
| |
| @cindex non-static inline function |
| When an inline function is not @code{static}, then the compiler must assume |
| that there may be calls from other source files; since a global symbol can |
| be defined only once in any program, the function must not be defined in |
| the other source files, so the calls therein cannot be integrated. |
| Therefore, a non-@code{static} inline function is always compiled on its |
| own in the usual fashion. |
| |
| If you specify both @code{inline} and @code{extern} in the function |
| definition, then the definition is used only for inlining. In no case |
| is the function compiled on its own, not even if you refer to its |
| address explicitly. Such an address becomes an external reference, as |
| if you had only declared the function, and had not defined it. |
| |
| This combination of @code{inline} and @code{extern} has almost the |
| effect of a macro. The way to use it is to put a function definition in |
| a header file with these keywords, and put another copy of the |
| definition (lacking @code{inline} and @code{extern}) in a library file. |
| The definition in the header file will cause most calls to the function |
| to be inlined. If any uses of the function remain, they will refer to |
| the single copy in the library. |
| |
| For future compatibility with when GCC implements ISO C99 semantics for |
| inline functions, it is best to use @code{static inline} only. (The |
| existing semantics will remain available when @option{-std=gnu89} is |
| specified, but eventually the default will be @option{-std=gnu99} and |
| that will implement the C99 semantics, though it does not do so yet.) |
| |
| GCC does not inline any functions when not optimizing unless you specify |
| the @samp{always_inline} attribute for the function, like this: |
| |
| @example |
| /* Prototype. */ |
| inline void foo (const char) __attribute__((always_inline)); |
| @end example |
| |
| @node Extended Asm |
| @section Assembler Instructions with C Expression Operands |
| @cindex extended @code{asm} |
| @cindex @code{asm} expressions |
| @cindex assembler instructions |
| @cindex registers |
| |
| In an assembler instruction using @code{asm}, you can specify the |
| operands of the instruction using C expressions. This means you need not |
| guess which registers or memory locations will contain the data you want |
| to use. |
| |
| You must specify an assembler instruction template much like what |
| appears in a machine description, plus an operand constraint string for |
| each operand. |
| |
| For example, here is how to use the 68881's @code{fsinx} instruction: |
| |
| @example |
| asm ("fsinx %1,%0" : "=f" (result) : "f" (angle)); |
| @end example |
| |
| @noindent |
| Here @code{angle} is the C expression for the input operand while |
| @code{result} is that of the output operand. Each has @samp{"f"} as its |
| operand constraint, saying that a floating point register is required. |
| The @samp{=} in @samp{=f} indicates that the operand is an output; all |
| output operands' constraints must use @samp{=}. The constraints use the |
| same language used in the machine description (@pxref{Constraints}). |
| |
| Each operand is described by an operand-constraint string followed by |
| the C expression in parentheses. A colon separates the assembler |
| template from the first output operand and another separates the last |
| output operand from the first input, if any. Commas separate the |
| operands within each group. The total number of operands is currently |
| limited to 30; this limitation may be lifted in some future version of |
| GCC. |
| |
| If there are no output operands but there are input operands, you must |
| place two consecutive colons surrounding the place where the output |
| operands would go. |
| |
| As of GCC version 3.1, it is also possible to specify input and output |
| operands using symbolic names which can be referenced within the |
| assembler code. These names are specified inside square brackets |
| preceding the constraint string, and can be referenced inside the |
| assembler code using @code{%[@var{name}]} instead of a percentage sign |
| followed by the operand number. Using named operands the above example |
| could look like: |
| |
| @example |
| asm ("fsinx %[angle],%[output]" |
| : [output] "=f" (result) |
| : [angle] "f" (angle)); |
| @end example |
| |
| @noindent |
| Note that the symbolic operand names have no relation whatsoever to |
| other C identifiers. You may use any name you like, even those of |
| existing C symbols, but must ensure that no two operands within the same |
| assembler construct use the same symbolic name. |
| |
| Output operand expressions must be lvalues; the compiler can check this. |
| The input operands need not be lvalues. The compiler cannot check |
| whether the operands have data types that are reasonable for the |
| instruction being executed. It does not parse the assembler instruction |
| template and does not know what it means or even whether it is valid |
| assembler input. The extended @code{asm} feature is most often used for |
| machine instructions the compiler itself does not know exist. If |
| the output expression cannot be directly addressed (for example, it is a |
| bit-field), your constraint must allow a register. In that case, GCC |
| will use the register as the output of the @code{asm}, and then store |
| that register into the output. |
| |
| The ordinary output operands must be write-only; GCC will assume that |
| the values in these operands before the instruction are dead and need |
| not be generated. Extended asm supports input-output or read-write |
| operands. Use the constraint character @samp{+} to indicate such an |
| operand and list it with the output operands. |
| |
| When the constraints for the read-write operand (or the operand in which |
| only some of the bits are to be changed) allows a register, you may, as |
| an alternative, logically split its function into two separate operands, |
| one input operand and one write-only output operand. The connection |
| between them is expressed by constraints which say they need to be in |
| the same location when the instruction executes. You can use the same C |
| expression for both operands, or different expressions. For example, |
| here we write the (fictitious) @samp{combine} instruction with |
| @code{bar} as its read-only source operand and @code{foo} as its |
| read-write destination: |
| |
| @example |
| asm ("combine %2,%0" : "=r" (foo) : "0" (foo), "g" (bar)); |
| @end example |
| |
| @noindent |
| The constraint @samp{"0"} for operand 1 says that it must occupy the |
| same location as operand 0. A number in constraint is allowed only in |
| an input operand and it must refer to an output operand. |
| |
| Only a number in the constraint can guarantee that one operand will be in |
| the same place as another. The mere fact that @code{foo} is the value |
| of both operands is not enough to guarantee that they will be in the |
| same place in the generated assembler code. The following would not |
| work reliably: |
| |
| @example |
| asm ("combine %2,%0" : "=r" (foo) : "r" (foo), "g" (bar)); |
| @end example |
| |
| Various optimizations or reloading could cause operands 0 and 1 to be in |
| different registers; GCC knows no reason not to do so. For example, the |
| compiler might find a copy of the value of @code{foo} in one register and |
| use it for operand 1, but generate the output operand 0 in a different |
| register (copying it afterward to @code{foo}'s own address). Of course, |
| since the register for operand 1 is not even mentioned in the assembler |
| code, the result will not work, but GCC can't tell that. |
| |
| As of GCC version 3.1, one may write @code{[@var{name}]} instead of |
| the operand number for a matching constraint. For example: |
| |
| @example |
| asm ("cmoveq %1,%2,%[result]" |
| : [result] "=r"(result) |
| : "r" (test), "r"(new), "[result]"(old)); |
| @end example |
| |
| Some instructions clobber specific hard registers. To describe this, |
| write a third colon after the input operands, followed by the names of |
| the clobbered hard registers (given as strings). Here is a realistic |
| example for the VAX: |
| |
| @example |
| asm volatile ("movc3 %0,%1,%2" |
| : /* no outputs */ |
| : "g" (from), "g" (to), "g" (count) |
| : "r0", "r1", "r2", "r3", "r4", "r5"); |
| @end example |
| |
| You may not write a clobber description in a way that overlaps with an |
| input or output operand. For example, you may not have an operand |
| describing a register class with one member if you mention that register |
| in the clobber list. There is no way for you to specify that an input |
| operand is modified without also specifying it as an output |
| operand. Note that if all the output operands you specify are for this |
| purpose (and hence unused), you will then also need to specify |
| @code{volatile} for the @code{asm} construct, as described below, to |
| prevent GCC from deleting the @code{asm} statement as unused. |
| |
| If you refer to a particular hardware register from the assembler code, |
| you will probably have to list the register after the third colon to |
| tell the compiler the register's value is modified. In some assemblers, |
| the register names begin with @samp{%}; to produce one @samp{%} in the |
| assembler code, you must write @samp{%%} in the input. |
| |
| If your assembler instruction can alter the condition code register, add |
| @samp{cc} to the list of clobbered registers. GCC on some machines |
| represents the condition codes as a specific hardware register; |
| @samp{cc} serves to name this register. On other machines, the |
| condition code is handled differently, and specifying @samp{cc} has no |
| effect. But it is valid no matter what the machine. |
| |
| If your assembler instruction modifies memory in an unpredictable |
| fashion, add @samp{memory} to the list of clobbered registers. This |
| will cause GCC to not keep memory values cached in registers across |
| the assembler instruction. You will also want to add the |
| @code{volatile} keyword if the memory affected is not listed in the |
| inputs or outputs of the @code{asm}, as the @samp{memory} clobber does |
| not count as a side-effect of the @code{asm}. |
| |
| You can put multiple assembler instructions together in a single |
| @code{asm} template, separated by the characters normally used in assembly |
| code for the system. A combination that works in most places is a newline |
| to break the line, plus a tab character to move to the instruction field |
| (written as @samp{\n\t}). Sometimes semicolons can be used, if the |
| assembler allows semicolons as a line-breaking character. Note that some |
| assembler dialects use semicolons to start a comment. |
| The input operands are guaranteed not to use any of the clobbered |
| registers, and neither will the output operands' addresses, so you can |
| read and write the clobbered registers as many times as you like. Here |
| is an example of multiple instructions in a template; it assumes the |
| subroutine @code{_foo} accepts arguments in registers 9 and 10: |
| |
| @example |
| asm ("movl %0,r9\n\tmovl %1,r10\n\tcall _foo" |
| : /* no outputs */ |
| : "g" (from), "g" (to) |
| : "r9", "r10"); |
| @end example |
| |
| Unless an output operand has the @samp{&} constraint modifier, GCC |
| may allocate it in the same register as an unrelated input operand, on |
| the assumption the inputs are consumed before the outputs are produced. |
| This assumption may be false if the assembler code actually consists of |
| more than one instruction. In such a case, use @samp{&} for each output |
| operand that may not overlap an input. @xref{Modifiers}. |
| |
| If you want to test the condition code produced by an assembler |
| instruction, you must include a branch and a label in the @code{asm} |
| construct, as follows: |
| |
| @example |
| asm ("clr %0\n\tfrob %1\n\tbeq 0f\n\tmov #1,%0\n0:" |
| : "g" (result) |
| : "g" (input)); |
| @end example |
| |
| @noindent |
| This assumes your assembler supports local labels, as the GNU assembler |
| and most Unix assemblers do. |
| |
| Speaking of labels, jumps from one @code{asm} to another are not |
| supported. The compiler's optimizers do not know about these jumps, and |
| therefore they cannot take account of them when deciding how to |
| optimize. |
| |
| @cindex macros containing @code{asm} |
| Usually the most convenient way to use these @code{asm} instructions is to |
| encapsulate them in macros that look like functions. For example, |
| |
| @example |
| #define sin(x) \ |
| (@{ double __value, __arg = (x); \ |
| asm ("fsinx %1,%0": "=f" (__value): "f" (__arg)); \ |
| __value; @}) |
| @end example |
| |
| @noindent |
| Here the variable @code{__arg} is used to make sure that the instruction |
| operates on a proper @code{double} value, and to accept only those |
| arguments @code{x} which can convert automatically to a @code{double}. |
| |
| Another way to make sure the instruction operates on the correct data |
| type is to use a cast in the @code{asm}. This is different from using a |
| variable @code{__arg} in that it converts more different types. For |
| example, if the desired type were @code{int}, casting the argument to |
| @code{int} would accept a pointer with no complaint, while assigning the |
| argument to an @code{int} variable named @code{__arg} would warn about |
| using a pointer unless the caller explicitly casts it. |
| |
| If an @code{asm} has output operands, GCC assumes for optimization |
| purposes the instruction has no side effects except to change the output |
| operands. This does not mean instructions with a side effect cannot be |
| used, but you must be careful, because the compiler may eliminate them |
| if the output operands aren't used, or move them out of loops, or |
| replace two with one if they constitute a common subexpression. Also, |
| if your instruction does have a side effect on a variable that otherwise |
| appears not to change, the old value of the variable may be reused later |
| if it happens to be found in a register. |
| |
| You can prevent an @code{asm} instruction from being deleted, moved |
| significantly, or combined, by writing the keyword @code{volatile} after |
| the @code{asm}. For example: |
| |
| @example |
| #define get_and_set_priority(new) \ |
| (@{ int __old; \ |
| asm volatile ("get_and_set_priority %0, %1" \ |
| : "=g" (__old) : "g" (new)); \ |
| __old; @}) |
| @end example |
| |
| @noindent |
| If you write an @code{asm} instruction with no outputs, GCC will know |
| the instruction has side-effects and will not delete the instruction or |
| move it outside of loops. |
| |
| The @code{volatile} keyword indicates that the instruction has |
| important side-effects. GCC will not delete a volatile @code{asm} if |
| it is reachable. (The instruction can still be deleted if GCC can |
| prove that control-flow will never reach the location of the |
| instruction.) In addition, GCC will not reschedule instructions |
| across a volatile @code{asm} instruction. For example: |
| |
| @example |
| *(volatile int *)addr = foo; |
| asm volatile ("eieio" : : ); |
| @end example |
| |
| @noindent |
| Assume @code{addr} contains the address of a memory mapped device |
| register. The PowerPC @code{eieio} instruction (Enforce In-order |
| Execution of I/O) tells the CPU to make sure that the store to that |
| device register happens before it issues any other I/O@. |
| |
| Note that even a volatile @code{asm} instruction can be moved in ways |
| that appear insignificant to the compiler, such as across jump |
| instructions. You can't expect a sequence of volatile @code{asm} |
| instructions to remain perfectly consecutive. If you want consecutive |
| output, use a single @code{asm}. Also, GCC will perform some |
| optimizations across a volatile @code{asm} instruction; GCC does not |
| ``forget everything'' when it encounters a volatile @code{asm} |
| instruction the way some other compilers do. |
| |
| An @code{asm} instruction without any operands or clobbers (an ``old |
| style'' @code{asm}) will be treated identically to a volatile |
| @code{asm} instruction. |
| |
| It is a natural idea to look for a way to give access to the condition |
| code left by the assembler instruction. However, when we attempted to |
| implement this, we found no way to make it work reliably. The problem |
| is that output operands might need reloading, which would result in |
| additional following ``store'' instructions. On most machines, these |
| instructions would alter the condition code before there was time to |
| test it. This problem doesn't arise for ordinary ``test'' and |
| ``compare'' instructions because they don't have any output operands. |
| |
| For reasons similar to those described above, it is not possible to give |
| an assembler instruction access to the condition code left by previous |
| instructions. |
| |
| If you are writing a header file that should be includable in ISO C |
| programs, write @code{__asm__} instead of @code{asm}. @xref{Alternate |
| Keywords}. |
| |
| @subsection i386 floating point asm operands |
| |
| There are several rules on the usage of stack-like regs in |
| asm_operands insns. These rules apply only to the operands that are |
| stack-like regs: |
| |
| @enumerate |
| @item |
| Given a set of input regs that die in an asm_operands, it is |
| necessary to know which are implicitly popped by the asm, and |
| which must be explicitly popped by gcc. |
| |
| An input reg that is implicitly popped by the asm must be |
| explicitly clobbered, unless it is constrained to match an |
| output operand. |
| |
| @item |
| For any input reg that is implicitly popped by an asm, it is |
| necessary to know how to adjust the stack to compensate for the pop. |
| If any non-popped input is closer to the top of the reg-stack than |
| the implicitly popped reg, it would not be possible to know what the |
| stack looked like---it's not clear how the rest of the stack ``slides |
| up''. |
| |
| All implicitly popped input regs must be closer to the top of |
| the reg-stack than any input that is not implicitly popped. |
| |
| It is possible that if an input dies in an insn, reload might |
| use the input reg for an output reload. Consider this example: |
| |
| @example |
| asm ("foo" : "=t" (a) : "f" (b)); |
| @end example |
| |
| This asm says that input B is not popped by the asm, and that |
| the asm pushes a result onto the reg-stack, i.e., the stack is one |
| deeper after the asm than it was before. But, it is possible that |
| reload will think that it can use the same reg for both the input and |
| the output, if input B dies in this insn. |
| |
| If any input operand uses the @code{f} constraint, all output reg |
| constraints must use the @code{&} earlyclobber. |
| |
| The asm above would be written as |
| |
| @example |
| asm ("foo" : "=&t" (a) : "f" (b)); |
| @end example |
| |
| @item |
| Some operands need to be in particular places on the stack. All |
| output operands fall in this category---there is no other way to |
| know which regs the outputs appear in unless the user indicates |
| this in the constraints. |
| |
| Output operands must specifically indicate which reg an output |
| appears in after an asm. @code{=f} is not allowed: the operand |
| constraints must select a class with a single reg. |
| |
| @item |
| Output operands may not be ``inserted'' between existing stack regs. |
| Since no 387 opcode uses a read/write operand, all output operands |
| are dead before the asm_operands, and are pushed by the asm_operands. |
| It makes no sense to push anywhere but the top of the reg-stack. |
| |
| Output operands must start at the top of the reg-stack: output |
| operands may not ``skip'' a reg. |
| |
| @item |
| Some asm statements may need extra stack space for internal |
| calculations. This can be guaranteed by clobbering stack registers |
| unrelated to the inputs and outputs. |
| |
| @end enumerate |
| |
| Here are a couple of reasonable asms to want to write. This asm |
| takes one input, which is internally popped, and produces two outputs. |
| |
| @example |
| asm ("fsincos" : "=t" (cos), "=u" (sin) : "0" (inp)); |
| @end example |
| |
| This asm takes two inputs, which are popped by the @code{fyl2xp1} opcode, |
| and replaces them with one output. The user must code the @code{st(1)} |
| clobber for reg-stack.c to know that @code{fyl2xp1} pops both inputs. |
| |
| @example |
| asm ("fyl2xp1" : "=t" (result) : "0" (x), "u" (y) : "st(1)"); |
| @end example |
| |
| @include md.texi |
| |
| @node Asm Labels |
| @section Controlling Names Used in Assembler Code |
| @cindex assembler names for identifiers |
| @cindex names used in assembler code |
| @cindex identifiers, names in assembler code |
| |
| You can specify the name to be used in the assembler code for a C |
| function or variable by writing the @code{asm} (or @code{__asm__}) |
| keyword after the declarator as follows: |
| |
| @example |
| int foo asm ("myfoo") = 2; |
| @end example |
| |
| @noindent |
| This specifies that the name to be used for the variable @code{foo} in |
| the assembler code should be @samp{myfoo} rather than the usual |
| @samp{_foo}. |
| |
| On systems where an underscore is normally prepended to the name of a C |
| function or variable, this feature allows you to define names for the |
| linker that do not start with an underscore. |
| |
| It does not make sense to use this feature with a non-static local |
| variable since such variables do not have assembler names. If you are |
| trying to put the variable in a particular register, see @ref{Explicit |
| Reg Vars}. GCC presently accepts such code with a warning, but will |
| probably be changed to issue an error, rather than a warning, in the |
| future. |
| |
| You cannot use @code{asm} in this way in a function @emph{definition}; but |
| you can get the same effect by writing a declaration for the function |
| before its definition and putting @code{asm} there, like this: |
| |
| @example |
| extern func () asm ("FUNC"); |
| |
| func (x, y) |
| int x, y; |
| @dots{} |
| @end example |
| |
| It is up to you to make sure that the assembler names you choose do not |
| conflict with any other assembler symbols. Also, you must not use a |
| register name; that would produce completely invalid assembler code. GCC |
| does not as yet have the ability to store static variables in registers. |
| Perhaps that will be added. |
| |
| @node Explicit Reg Vars |
| @section Variables in Specified Registers |
| @cindex explicit register variables |
| @cindex variables in specified registers |
| @cindex specified registers |
| @cindex registers, global allocation |
| |
| GNU C allows you to put a few global variables into specified hardware |
| registers. You can also specify the register in which an ordinary |
| register variable should be allocated. |
| |
| @itemize @bullet |
| @item |
| Global register variables reserve registers throughout the program. |
| This may be useful in programs such as programming language |
| interpreters which have a couple of global variables that are accessed |
| very often. |
| |
| @item |
| Local register variables in specific registers do not reserve the |
| registers. The compiler's data flow analysis is capable of determining |
| where the specified registers contain live values, and where they are |
| available for other uses. Stores into local register variables may be deleted |
| when they appear to be dead according to dataflow analysis. References |
| to local register variables may be deleted or moved or simplified. |
| |
| These local variables are sometimes convenient for use with the extended |
| @code{asm} feature (@pxref{Extended Asm}), if you want to write one |
| output of the assembler instruction directly into a particular register. |
| (This will work provided the register you specify fits the constraints |
| specified for that operand in the @code{asm}.) |
| @end itemize |
| |
| @menu |
| * Global Reg Vars:: |
| * Local Reg Vars:: |
| @end menu |
| |
| @node Global Reg Vars |
| @subsection Defining Global Register Variables |
| @cindex global register variables |
| @cindex registers, global variables in |
| |
| You can define a global register variable in GNU C like this: |
| |
| @example |
| register int *foo asm ("a5"); |
| @end example |
| |
| @noindent |
| Here @code{a5} is the name of the register which should be used. Choose a |
| register which is normally saved and restored by function calls on your |
| machine, so that library routines will not clobber it. |
| |
| Naturally the register name is cpu-dependent, so you would need to |
| conditionalize your program according to cpu type. The register |
| @code{a5} would be a good choice on a 68000 for a variable of pointer |
| type. On machines with register windows, be sure to choose a ``global'' |
| register that is not affected magically by the function call mechanism. |
| |
| In addition, operating systems on one type of cpu may differ in how they |
| name the registers; then you would need additional conditionals. For |
| example, some 68000 operating systems call this register @code{%a5}. |
| |
| Eventually there may be a way of asking the compiler to choose a register |
| automatically, but first we need to figure out how it should choose and |
| how to enable you to guide the choice. No solution is evident. |
| |
| Defining a global register variable in a certain register reserves that |
| register entirely for this use, at least within the current compilation. |
| The register will not be allocated for any other purpose in the functions |
| in the current compilation. The register will not be saved and restored by |
| these functions. Stores into this register are never deleted even if they |
| would appear to be dead, but references may be deleted or moved or |
| simplified. |
| |
| It is not safe to access the global register variables from signal |
| handlers, or from more than one thread of control, because the system |
| library routines may temporarily use the register for other things (unless |
| you recompile them specially for the task at hand). |
| |
| @cindex @code{qsort}, and global register variables |
| It is not safe for one function that uses a global register variable to |
| call another such function @code{foo} by way of a third function |
| @code{lose} that was compiled without knowledge of this variable (i.e.@: in a |
| different source file in which the variable wasn't declared). This is |
| because @code{lose} might save the register and put some other value there. |
| For example, you can't expect a global register variable to be available in |
| the comparison-function that you pass to @code{qsort}, since @code{qsort} |
| might have put something else in that register. (If you are prepared to |
| recompile @code{qsort} with the same global register variable, you can |
| solve this problem.) |
| |
| If you want to recompile @code{qsort} or other source files which do not |
| actually use your global register variable, so that they will not use that |
| register for any other purpose, then it suffices to specify the compiler |
| option @option{-ffixed-@var{reg}}. You need not actually add a global |
| register declaration to their source code. |
| |
| A function which can alter the value of a global register variable cannot |
| safely be called from a function compiled without this variable, because it |
| could clobber the value the caller expects to find there on return. |
| Therefore, the function which is the entry point into the part of the |
| program that uses the global register variable must explicitly save and |
| restore the value which belongs to its caller. |
| |
| @cindex register variable after @code{longjmp} |
| @cindex global register after @code{longjmp} |
| @cindex value after @code{longjmp} |
| @findex longjmp |
| @findex setjmp |
| On most machines, @code{longjmp} will restore to each global register |
| variable the value it had at the time of the @code{setjmp}. On some |
| machines, however, @code{longjmp} will not change the value of global |
| register variables. To be portable, the function that called @code{setjmp} |
| should make other arrangements to save the values of the global register |
| variables, and to restore them in a @code{longjmp}. This way, the same |
| thing will happen regardless of what @code{longjmp} does. |
| |
| All global register variable declarations must precede all function |
| definitions. If such a declaration could appear after function |
| definitions, the declaration would be too late to prevent the register from |
| being used for other purposes in the preceding functions. |
| |
| Global register variables may not have initial values, because an |
| executable file has no means to supply initial contents for a register. |
| |
| On the Sparc, there are reports that g3 @dots{} g7 are suitable |
| registers, but certain library functions, such as @code{getwd}, as well |
| as the subroutines for division and remainder, modify g3 and g4. g1 and |
| g2 are local temporaries. |
| |
| On the 68000, a2 @dots{} a5 should be suitable, as should d2 @dots{} d7. |
| Of course, it will not do to use more than a few of those. |
| |
| @node Local Reg Vars |
| @subsection Specifying Registers for Local Variables |
| @cindex local variables, specifying registers |
| @cindex specifying registers for local variables |
| @cindex registers for local variables |
| |
| You can define a local register variable with a specified register |
| like this: |
| |
| @example |
| register int *foo asm ("a5"); |
| @end example |
| |
| @noindent |
| Here @code{a5} is the name of the register which should be used. Note |
| that this is the same syntax used for defining global register |
| variables, but for a local variable it would appear within a function. |
| |
| Naturally the register name is cpu-dependent, but this is not a |
| problem, since specific registers are most often useful with explicit |
| assembler instructions (@pxref{Extended Asm}). Both of these things |
| generally require that you conditionalize your program according to |
| cpu type. |
| |
| In addition, operating systems on one type of cpu may differ in how they |
| name the registers; then you would need additional conditionals. For |
| example, some 68000 operating systems call this register @code{%a5}. |
| |
| Defining such a register variable does not reserve the register; it |
| remains available for other uses in places where flow control determines |
| the variable's value is not live. However, these registers are made |
| unavailable for use in the reload pass; excessive use of this feature |
| leaves the compiler too few available registers to compile certain |
| functions. |
| |
| This option does not guarantee that GCC will generate code that has |
| this variable in the register you specify at all times. You may not |
| code an explicit reference to this register in an @code{asm} statement |
| and assume it will always refer to this variable. |
| |
| Stores into local register variables may be deleted when they appear to be dead |
| according to dataflow analysis. References to local register variables may |
| be deleted or moved or simplified. |
| |
| @node Alternate Keywords |
| @section Alternate Keywords |
| @cindex alternate keywords |
| @cindex keywords, alternate |
| |
| @option{-ansi} and the various @option{-std} options disable certain |
| keywords. This causes trouble when you want to use GNU C extensions, or |
| a general-purpose header file that should be usable by all programs, |
| including ISO C programs. The keywords @code{asm}, @code{typeof} and |
| @code{inline} are not available in programs compiled with |
| @option{-ansi} or @option{-std} (although @code{inline} can be used in a |
| program compiled with @option{-std=c99}). The ISO C99 keyword |
| @code{restrict} is only available when @option{-std=gnu99} (which will |
| eventually be the default) or @option{-std=c99} (or the equivalent |
| @option{-std=iso9899:1999}) is used. |
| |
| The way to solve these problems is to put @samp{__} at the beginning and |
| end of each problematical keyword. For example, use @code{__asm__} |
| instead of @code{asm}, and @code{__inline__} instead of @code{inline}. |
| |
| Other C compilers won't accept these alternative keywords; if you want to |
| compile with another compiler, you can define the alternate keywords as |
| macros to replace them with the customary keywords. It looks like this: |
| |
| @example |
| #ifndef __GNUC__ |
| #define __asm__ asm |
| #endif |
| @end example |
| |
| @findex __extension__ |
| @opindex pedantic |
| @option{-pedantic} and other options cause warnings for many GNU C extensions. |
| You can |
| prevent such warnings within one expression by writing |
| @code{__extension__} before the expression. @code{__extension__} has no |
| effect aside from this. |
| |
| @node Incomplete Enums |
| @section Incomplete @code{enum} Types |
| |
| You can define an @code{enum} tag without specifying its possible values. |
| This results in an incomplete type, much like what you get if you write |
| @code{struct foo} without describing the elements. A later declaration |
| which does specify the possible values completes the type. |
| |
| You can't allocate variables or storage using the type while it is |
| incomplete. However, you can work with pointers to that type. |
| |
| This extension may not be very useful, but it makes the handling of |
| @code{enum} more consistent with the way @code{struct} and @code{union} |
| are handled. |
| |
| This extension is not supported by GNU C++. |
| |
| @node Function Names |
| @section Function Names as Strings |
| @cindex @code{__FUNCTION__} identifier |
| @cindex @code{__PRETTY_FUNCTION__} identifier |
| @cindex @code{__func__} identifier |
| |
| GCC predefines two magic identifiers to hold the name of the current |
| function. The identifier @code{__FUNCTION__} holds the name of the function |
| as it appears in the source. The identifier @code{__PRETTY_FUNCTION__} |
| holds the name of the function pretty printed in a language specific |
| fashion. |
| |
| These names are always the same in a C function, but in a C++ function |
| they may be different. For example, this program: |
| |
| @smallexample |
| extern "C" @{ |
| extern int printf (char *, ...); |
| @} |
| |
| class a @{ |
| public: |
| sub (int i) |
| @{ |
| printf ("__FUNCTION__ = %s\n", __FUNCTION__); |
| printf ("__PRETTY_FUNCTION__ = %s\n", __PRETTY_FUNCTION__); |
| @} |
| @}; |
| |
| int |
| main (void) |
| @{ |
| a ax; |
| ax.sub (0); |
| return 0; |
| @} |
| @end smallexample |
| |
| @noindent |
| gives this output: |
| |
| @smallexample |
| __FUNCTION__ = sub |
| __PRETTY_FUNCTION__ = int a::sub (int) |
| @end smallexample |
| |
| The compiler automagically replaces the identifiers with a string |
| literal containing the appropriate name. Thus, they are neither |
| preprocessor macros, like @code{__FILE__} and @code{__LINE__}, nor |
| variables. This means that they catenate with other string literals, and |
| that they can be used to initialize char arrays. For example |
| |
| @smallexample |
| char here[] = "Function " __FUNCTION__ " in " __FILE__; |
| @end smallexample |
| |
| On the other hand, @samp{#ifdef __FUNCTION__} does not have any special |
| meaning inside a function, since the preprocessor does not do anything |
| special with the identifier @code{__FUNCTION__}. |
| |
| Note that these semantics are deprecated, and that GCC 3.2 will handle |
| @code{__FUNCTION__} and @code{__PRETTY_FUNCTION__} the same way as |
| @code{__func__}. @code{__func__} is defined by the ISO standard C99: |
| |
| @display |
| The identifier @code{__func__} is implicitly declared by the translator |
| as if, immediately following the opening brace of each function |
| definition, the declaration |
| |
| @smallexample |
| static const char __func__[] = "function-name"; |
| @end smallexample |
| |
| appeared, where function-name is the name of the lexically-enclosing |
| function. This name is the unadorned name of the function. |
| @end display |
| |
| By this definition, @code{__func__} is a variable, not a string literal. |
| In particular, @code{__func__} does not catenate with other string |
| literals. |
| |
| In @code{C++}, @code{__FUNCTION__} and @code{__PRETTY_FUNCTION__} are |
| variables, declared in the same way as @code{__func__}. |
| |
| @node Return Address |
| @section Getting the Return or Frame Address of a Function |
| |
| These functions may be used to get information about the callers of a |
| function. |
| |
| @deftypefn {Built-in Function} {void *} __builtin_return_address (unsigned int @var{level}) |
| This function returns the return address of the current function, or of |
| one of its callers. The @var{level} argument is number of frames to |
| scan up the call stack. A value of @code{0} yields the return address |
| of the current function, a value of @code{1} yields the return address |
| of the caller of the current function, and so forth. |
| |
| The @var{level} argument must be a constant integer. |
| |
| On some machines it may be impossible to determine the return address of |
| any function other than the current one; in such cases, or when the top |
| of the stack has been reached, this function will return @code{0} or a |
| random value. In addition, @code{__builtin_frame_address} may be used |
| to determine if the top of the stack has been reached. |
| |
| This function should only be used with a nonzero argument for debugging |
| purposes. |
| @end deftypefn |
| |
| @deftypefn {Built-in Function} {void *} __builtin_frame_address (unsigned int @var{level}) |
| This function is similar to @code{__builtin_return_address}, but it |
| returns the address of the function frame rather than the return address |
| of the function. Calling @code{__builtin_frame_address} with a value of |
| @code{0} yields the frame address of the current function, a value of |
| @code{1} yields the frame address of the caller of the current function, |
| and so forth. |
| |
| The frame is the area on the stack which holds local variables and saved |
| registers. The frame address is normally the address of the first word |
| pushed on to the stack by the function. However, the exact definition |
| depends upon the processor and the calling convention. If the processor |
| has a dedicated frame pointer register, and the function has a frame, |
| then @code{__builtin_frame_address} will return the value of the frame |
| pointer register. |
| |
| On some machines it may be impossible to determine the frame address of |
| any function other than the current one; in such cases, or when the top |
| of the stack has been reached, this function will return @code{0} if |
| the first frame pointer is properly initialized by the startup code. |
| |
| This function should only be used with a nonzero argument for debugging |
| purposes. |
| @end deftypefn |
| |
| @node Vector Extensions |
| @section Using vector instructions through built-in functions |
| |
| On some targets, the instruction set contains SIMD vector instructions that |
| operate on multiple values contained in one large register at the same time. |
| For example, on the i386 the MMX, 3Dnow! and SSE extensions can be used |
| this way. |
| |
| The first step in using these extensions is to provide the necessary data |
| types. This should be done using an appropriate @code{typedef}: |
| |
| @example |
| typedef int v4si __attribute__ ((mode(V4SI))); |
| @end example |
| |
| The base type @code{int} is effectively ignored by the compiler, the |
| actual properties of the new type @code{v4si} are defined by the |
| @code{__attribute__}. It defines the machine mode to be used; for vector |
| types these have the form @code{V@var{n}@var{B}}; @var{n} should be the |
| number of elements in the vector, and @var{B} should be the base mode of the |
| individual elements. The following can be used as base modes: |
| |
| @table @code |
| @item QI |
| An integer that is as wide as the smallest addressable unit, usually 8 bits. |
| @item HI |
| An integer, twice as wide as a QI mode integer, usually 16 bits. |
| @item SI |
| An integer, four times as wide as a QI mode integer, usually 32 bits. |
| @item DI |
| An integer, eight times as wide as a QI mode integer, usually 64 bits. |
| @item SF |
| A floating point value, as wide as a SI mode integer, usually 32 bits. |
| @item DF |
| A floating point value, as wide as a DI mode integer, usually 64 bits. |
| @end table |
| |
| Not all base types or combinations are always valid; which modes can be used |
| is determined by the target machine. For example, if targetting the i386 MMX |
| extensions, only @code{V8QI}, @code{V4HI} and @code{V2SI} are allowed modes. |
| |
| There are no @code{V1xx} vector modes - they would be identical to the |
| corresponding base mode. |
| |
| There is no distinction between signed and unsigned vector modes. This |
| distinction is made by the operations that perform on the vectors, not |
| by the data type. |
| |
| The types defined in this manner are somewhat special, they cannot be |
| used with most normal C operations (i.e., a vector addition can @emph{not} |
| be represented by a normal addition of two vector type variables). You |
| can declare only variables and use them in function calls and returns, as |
| well as in assignments and some casts. It is possible to cast from one |
| vector type to another, provided they are of the same size (in fact, you |
| can also cast vectors to and from other datatypes of the same size). |
| |
| A port that supports vector operations provides a set of built-in functions |
| that can be used to operate on vectors. For example, a function to add two |
| vectors and multiply the result by a third could look like this: |
| |
| @example |
| v4si f (v4si a, v4si b, v4si c) |
| @{ |
| v4si tmp = __builtin_addv4si (a, b); |
| return __builtin_mulv4si (tmp, c); |
| @} |
| |
| @end example |
| |
| @node Other Builtins |
| @section Other built-in functions provided by GCC |
| @cindex built-in functions |
| @findex __builtin_isgreater |
| @findex __builtin_isgreaterequal |
| @findex __builtin_isless |
| @findex __builtin_islessequal |
| @findex __builtin_islessgreater |
| @findex __builtin_isunordered |
| @findex abort |
| @findex abs |
| @findex alloca |
| @findex bcmp |
| @findex bzero |
| @findex cimag |
| @findex cimagf |
| @findex cimagl |
| @findex conj |
| @findex conjf |
| @findex conjl |
| @findex cos |
| @findex cosf |
| @findex cosl |
| @findex creal |
| @findex crealf |
| @findex creall |
| @findex exit |
| @findex _exit |
| @findex _Exit |
| @findex fabs |
| @findex fabsf |
| @findex fabsl |
| @findex ffs |
| @findex fprintf |
| @findex fprintf_unlocked |
| @findex fputs |
| @findex fputs_unlocked |
| @findex imaxabs |
| @findex index |
| @findex labs |
| @findex llabs |
| @findex memcmp |
| @findex memcpy |
| @findex memset |
| @findex printf |
| @findex printf_unlocked |
| @findex rindex |
| @findex sin |
| @findex sinf |
| @findex sinl |
| @findex sqrt |
| @findex sqrtf |
| @findex sqrtl |
| @findex strcat |
| @findex strchr |
| @findex strcmp |
| @findex strcpy |
| @findex strcspn |
| @findex strlen |
| @findex strncat |
| @findex strncmp |
| @findex strncpy |
| @findex strpbrk |
| @findex strrchr |
| @findex strspn |
| @findex strstr |
| |
| GCC provides a large number of built-in functions other than the ones |
| mentioned above. Some of these are for internal use in the processing |
| of exceptions or variable-length argument lists and will not be |
| documented here because they may change from time to time; we do not |
| recommend general use of these functions. |
| |
| The remaining functions are provided for optimization purposes. |
| |
| @opindex fno-builtin |
| GCC includes built-in versions of many of the functions in the standard |
| C library. The versions prefixed with @code{__builtin_} will always be |
| treated as having the same meaning as the C library function even if you |
| specify the @option{-fno-builtin} option. (@pxref{C Dialect Options}) |
| Many of these functions are only optimized in certain cases; if they are |
| not optimized in a particular case, a call to the library function will |
| be emitted. |
| |
| @opindex ansi |
| @opindex std |
| The functions @code{abort}, @code{exit}, @code{_Exit} and @code{_exit} |
| are recognized and presumed not to return, but otherwise are not built |
| in. @code{_exit} is not recognized in strict ISO C mode (@option{-ansi}, |
| @option{-std=c89} or @option{-std=c99}). @code{_Exit} is not recognized in |
| strict C89 mode (@option{-ansi} or @option{-std=c89}). |
| |
| Outside strict ISO C mode, the functions @code{alloca}, @code{bcmp}, |
| @code{bzero}, @code{index}, @code{rindex}, @code{ffs}, @code{fputs_unlocked}, |
| @code{printf_unlocked} and @code{fprintf_unlocked} may be handled as |
| built-in functions. All these functions have corresponding versions |
| prefixed with @code{__builtin_}, which may be used even in strict C89 |
| mode. |
| |
| The ISO C99 functions @code{conj}, @code{conjf}, @code{conjl}, |
| @code{creal}, @code{crealf}, @code{creall}, @code{cimag}, @code{cimagf}, |
| @code{cimagl}, @code{llabs} and @code{imaxabs} are handled as built-in |
| functions except in strict ISO C89 mode. There are also built-in |
| versions of the ISO C99 functions @code{cosf}, @code{cosl}, |
| @code{fabsf}, @code{fabsl}, @code{sinf}, @code{sinl}, @code{sqrtf}, and |
| @code{sqrtl}, that are recognized in any mode since ISO C89 reserves |
| these names for the purpose to which ISO C99 puts them. All these |
| functions have corresponding versions prefixed with @code{__builtin_}. |
| |
| The ISO C89 functions @code{abs}, @code{cos}, @code{fabs}, |
| @code{fprintf}, @code{fputs}, @code{labs}, @code{memcmp}, @code{memcpy}, |
| @code{memset}, @code{printf}, @code{sin}, @code{sqrt}, @code{strcat}, |
| @code{strchr}, @code{strcmp}, @code{strcpy}, @code{strcspn}, |
| @code{strlen}, @code{strncat}, @code{strncmp}, @code{strncpy}, |
| @code{strpbrk}, @code{strrchr}, @code{strspn}, and @code{strstr} are all |
| recognized as built-in functions unless @option{-fno-builtin} is |
| specified (or @option{-fno-builtin-@var{function}} is specified for an |
| individual function). All of these functions have corresponding |
| versions prefixed with @code{__builtin_}. |
| |
| GCC provides built-in versions of the ISO C99 floating point comparison |
| macros that avoid raising exceptions for unordered operands. They have |
| the same names as the standard macros ( @code{isgreater}, |
| @code{isgreaterequal}, @code{isless}, @code{islessequal}, |
| @code{islessgreater}, and @code{isunordered}) , with @code{__builtin_} |
| prefixed. We intend for a library implementor to be able to simply |
| @code{#define} each standard macro to its built-in equivalent. |
| |
| @deftypefn {Built-in Function} int __builtin_types_compatible_p (@var{type1}, @var{type2}) |
| |
| You can use the built-in function @code{__builtin_types_compatible_p} to |
| determine whether two types are the same. |
| |
| This built-in function returns 1 if the unqualified versions of the |
| types @var{type1} and @var{type2} (which are types, not expressions) are |
| compatible, 0 otherwise. The result of this built-in function can be |
| used in integer constant expressions. |
| |
| This built-in function ignores top level qualifiers (e.g., @code{const}, |
| @code{volatile}). For example, @code{int} is equivalent to @code{const |
| int}. |
| |
| The type @code{int[]} and @code{int[5]} are compatible. On the other |
| hand, @code{int} and @code{char *} are not compatible, even if the size |
| of their types, on the particular architecture are the same. Also, the |
| amount of pointer indirection is taken into account when determining |
| similarity. Consequently, @code{short *} is not similar to |
| @code{short **}. Furthermore, two types that are typedefed are |
| considered compatible if their underlying types are compatible. |
| |
| An @code{enum} type is considered to be compatible with another |
| @code{enum} type. For example, @code{enum @{foo, bar@}} is similar to |
| @code{enum @{hot, dog@}}. |
| |
| You would typically use this function in code whose execution varies |
| depending on the arguments' types. For example: |
| |
| @smallexample |
| #define foo(x) \ |
| (@{ \ |
| typeof (x) tmp; \ |
| if (__builtin_types_compatible_p (typeof (x), long double)) \ |
| tmp = foo_long_double (tmp); \ |
| else if (__builtin_types_compatible_p (typeof (x), double)) \ |
| tmp = foo_double (tmp); \ |
| else if (__builtin_types_compatible_p (typeof (x), float)) \ |
| tmp = foo_float (tmp); \ |
| else \ |
| abort (); \ |
| tmp; \ |
| @}) |
| @end smallexample |
| |
| @emph{Note:} This construct is only available for C. |
| |
| @end deftypefn |
| |
| @deftypefn {Built-in Function} @var{type} __builtin_choose_expr (@var{const_exp}, @var{exp1}, @var{exp2}) |
| |
| You can use the built-in function @code{__builtin_choose_expr} to |
| evaluate code depending on the value of a constant expression. This |
| built-in function returns @var{exp1} if @var{const_exp}, which is a |
| constant expression that must be able to be determined at compile time, |
| is nonzero. Otherwise it returns 0. |
| |
| This built-in function is analogous to the @samp{? :} operator in C, |
| except that the expression returned has its type unaltered by promotion |
| rules. Also, the built-in function does not evaluate the expression |
| that was not chosen. For example, if @var{const_exp} evaluates to true, |
| @var{exp2} is not evaluated even if it has side-effects. |
| |
| This built-in function can return an lvalue if the chosen argument is an |
| lvalue. |
| |
| If @var{exp1} is returned, the return type is the same as @var{exp1}'s |
| type. Similarly, if @var{exp2} is returned, its return type is the same |
| as @var{exp2}. |
| |
| Example: |
| |
| @smallexample |
| #define foo(x) \ |
| __builtin_choose_expr (__builtin_types_compatible_p (typeof (x), double), \ |
| foo_double (x), \ |
| __builtin_choose_expr (__builtin_types_compatible_p (typeof (x), float), \ |
| foo_float (x), \ |
| /* @r{The void expression results in a compile-time error} \ |
| @r{when assigning the result to something.} */ \ |
| (void)0)) |
| @end smallexample |
| |
| @emph{Note:} This construct is only available for C. Furthermore, the |
| unused expression (@var{exp1} or @var{exp2} depending on the value of |
| @var{const_exp}) may still generate syntax errors. This may change in |
| future revisions. |
| |
| @end deftypefn |
| |
| @deftypefn {Built-in Function} int __builtin_constant_p (@var{exp}) |
| You can use the built-in function @code{__builtin_constant_p} to |
| determine if a value is known to be constant at compile-time and hence |
| that GCC can perform constant-folding on expressions involving that |
| value. The argument of the function is the value to test. The function |
| returns the integer 1 if the argument is known to be a compile-time |
| constant and 0 if it is not known to be a compile-time constant. A |
| return of 0 does not indicate that the value is @emph{not} a constant, |
| but merely that GCC cannot prove it is a constant with the specified |
| value of the @option{-O} option. |
| |
| You would typically use this function in an embedded application where |
| memory was a critical resource. If you have some complex calculation, |
| you may want it to be folded if it involves constants, but need to call |
| a function if it does not. For example: |
| |
| @smallexample |
| #define Scale_Value(X) \ |
| (__builtin_constant_p (X) \ |
| ? ((X) * SCALE + OFFSET) : Scale (X)) |
| @end smallexample |
| |
| You may use this built-in function in either a macro or an inline |
| function. However, if you use it in an inlined function and pass an |
| argument of the function as the argument to the built-in, GCC will |
| never return 1 when you call the inline function with a string constant |
| or compound literal (@pxref{Compound Literals}) and will not return 1 |
| when you pass a constant numeric value to the inline function unless you |
| specify the @option{-O} option. |
| |
| You may also use @code{__builtin_constant_p} in initializers for static |
| data. For instance, you can write |
| |
| @smallexample |
| static const int table[] = @{ |
| __builtin_constant_p (EXPRESSION) ? (EXPRESSION) : -1, |
| /* ... */ |
| @}; |
| @end smallexample |
| |
| @noindent |
| This is an acceptable initializer even if @var{EXPRESSION} is not a |
| constant expression. GCC must be more conservative about evaluating the |
| built-in in this case, because it has no opportunity to perform |
| optimization. |
| |
| Previous versions of GCC did not accept this built-in in data |
| initializers. The earliest version where it is completely safe is |
| 3.0.1. |
| @end deftypefn |
| |
| @deftypefn {Built-in Function} long __builtin_expect (long @var{exp}, long @var{c}) |
| @opindex fprofile-arcs |
| You may use @code{__builtin_expect} to provide the compiler with |
| branch prediction information. In general, you should prefer to |
| use actual profile feedback for this (@option{-fprofile-arcs}), as |
| programmers are notoriously bad at predicting how their programs |
| actually perform. However, there are applications in which this |
| data is hard to collect. |
| |
| The return value is the value of @var{exp}, which should be an |
| integral expression. The value of @var{c} must be a compile-time |
| constant. The semantics of the built-in are that it is expected |
| that @var{exp} == @var{c}. For example: |
| |
| @smallexample |
| if (__builtin_expect (x, 0)) |
| foo (); |
| @end smallexample |
| |
| @noindent |
| would indicate that we do not expect to call @code{foo}, since |
| we expect @code{x} to be zero. Since you are limited to integral |
| expressions for @var{exp}, you should use constructions such as |
| |
| @smallexample |
| if (__builtin_expect (ptr != NULL, 1)) |
| error (); |
| @end smallexample |
| |
| @noindent |
| when testing pointer or floating-point values. |
| @end deftypefn |
| |
| @deftypefn {Built-in Function} void __builtin_prefetch (const void *@var{addr}, ...) |
| This function is used to minimize cache-miss latency by moving data into |
| a cache before it is accessed. |
| You can insert calls to @code{__builtin_prefetch} into code for which |
| you know addresses of data in memory that is likely to be accessed soon. |
| If the target supports them, data prefetch instructions will be generated. |
| If the prefetch is done early enough before the access then the data will |
| be in the cache by the time it is accessed. |
| |
| The value of @var{addr} is the address of the memory to prefetch. |
| There are two optional arguments, @var{rw} and @var{locality}. |
| The value of @var{rw} is a compile-time constant one or zero; one |
| means that the prefetch is preparing for a write to the memory address |
| and zero, the default, means that the prefetch is preparing for a read. |
| The value @var{locality} must be a compile-time constant integer between |
| zero and three. A value of zero means that the data has no temporal |
| locality, so it need not be left in the cache after the access. A value |
| of three means that the data has a high degree of temporal locality and |
| should be left in all levels of cache possible. Values of one and two |
| mean, respectively, a low or moderate degree of temporal locality. The |
| default is three. |
| |
| @smallexample |
| for (i = 0; i < n; i++) |
| @{ |
| a[i] = a[i] + b[i]; |
| __builtin_prefetch (&a[i+j], 1, 1); |
| __builtin_prefetch (&b[i+j], 0, 1); |
| /* ... */ |
| @} |
| @end smallexample |
| |
| Data prefetch does not generate faults if @var{addr} is invalid, but |
| the address expression itself must be valid. For example, a prefetch |
| of @code{p->next} will not fault if @code{p->next} is not a valid |
| address, but evaluation will fault if @code{p} is not a valid address. |
| |
| If the target does not support data prefetch, the address expression |
| is evaluated if it includes side effects but no other code is generated |
| and GCC does not issue a warning. |
| @end deftypefn |
| |
| @node Target Builtins |
| @section Built-in Functions Specific to Particular Target Machines |
| |
| On some target machines, GCC supports many built-in functions specific |
| to those machines. Generally these generate calls to specific machine |
| instructions, but allow the compiler to schedule those calls. |
| |
| @menu |
| * X86 Built-in Functions:: |
| * PowerPC AltiVec Built-in Functions:: |
| @end menu |
| |
| @node X86 Built-in Functions |
| @subsection X86 Built-in Functions |
| |
| These built-in functions are available for the i386 and x86-64 family |
| of computers, depending on the command-line switches used. |
| |
| The following machine modes are available for use with MMX built-in functions |
| (@pxref{Vector Extensions}): @code{V2SI} for a vector of two 32-bit integers, |
| @code{V4HI} for a vector of four 16-bit integers, and @code{V8QI} for a |
| vector of eight 8-bit integers. Some of the built-in functions operate on |
| MMX registers as a whole 64-bit entity, these use @code{DI} as their mode. |
| |
| If 3Dnow extensions are enabled, @code{V2SF} is used as a mode for a vector |
| of two 32-bit floating point values. |
| |
| If SSE extensions are enabled, @code{V4SF} is used for a vector of four 32-bit |
| floating point values. Some instructions use a vector of four 32-bit |
| integers, these use @code{V4SI}. Finally, some instructions operate on an |
| entire vector register, interpreting it as a 128-bit integer, these use mode |
| @code{TI}. |
| |
| The following built-in functions are made available by @option{-mmmx}. |
| All of them generate the machine instruction that is part of the name. |
| |
| @example |
| v8qi __builtin_ia32_paddb (v8qi, v8qi) |
| v4hi __builtin_ia32_paddw (v4hi, v4hi) |
| v2si __builtin_ia32_paddd (v2si, v2si) |
| v8qi __builtin_ia32_psubb (v8qi, v8qi) |
| v4hi __builtin_ia32_psubw (v4hi, v4hi) |
| v2si __builtin_ia32_psubd (v2si, v2si) |
| v8qi __builtin_ia32_paddsb (v8qi, v8qi) |
| v4hi __builtin_ia32_paddsw (v4hi, v4hi) |
| v8qi __builtin_ia32_psubsb (v8qi, v8qi) |
| v4hi __builtin_ia32_psubsw (v4hi, v4hi) |
| v8qi __builtin_ia32_paddusb (v8qi, v8qi) |
| v4hi __builtin_ia32_paddusw (v4hi, v4hi) |
| v8qi __builtin_ia32_psubusb (v8qi, v8qi) |
| v4hi __builtin_ia32_psubusw (v4hi, v4hi) |
| v4hi __builtin_ia32_pmullw (v4hi, v4hi) |
| v4hi __builtin_ia32_pmulhw (v4hi, v4hi) |
| di __builtin_ia32_pand (di, di) |
| di __builtin_ia32_pandn (di,di) |
| di __builtin_ia32_por (di, di) |
| di __builtin_ia32_pxor (di, di) |
| v8qi __builtin_ia32_pcmpeqb (v8qi, v8qi) |
| v4hi __builtin_ia32_pcmpeqw (v4hi, v4hi) |
| v2si __builtin_ia32_pcmpeqd (v2si, v2si) |
| v8qi __builtin_ia32_pcmpgtb (v8qi, v8qi) |
| v4hi __builtin_ia32_pcmpgtw (v4hi, v4hi) |
| v2si __builtin_ia32_pcmpgtd (v2si, v2si) |
| v8qi __builtin_ia32_punpckhbw (v8qi, v8qi) |
| v4hi __builtin_ia32_punpckhwd (v4hi, v4hi) |
| v2si __builtin_ia32_punpckhdq (v2si, v2si) |
| v8qi __builtin_ia32_punpcklbw (v8qi, v8qi) |
| v4hi __builtin_ia32_punpcklwd (v4hi, v4hi) |
| v2si __builtin_ia32_punpckldq (v2si, v2si) |
| v8qi __builtin_ia32_packsswb (v4hi, v4hi) |
| v4hi __builtin_ia32_packssdw (v2si, v2si) |
| v8qi __builtin_ia32_packuswb (v4hi, v4hi) |
| @end example |
| |
| The following built-in functions are made available either with |
| @option{-msse}, or with a combination of @option{-m3dnow} and |
| @option{-march=athlon}. All of them generate the machine |
| instruction that is part of the name. |
| |
| @example |
| v4hi __builtin_ia32_pmulhuw (v4hi, v4hi) |
| v8qi __builtin_ia32_pavgb (v8qi, v8qi) |
| v4hi __builtin_ia32_pavgw (v4hi, v4hi) |
| v4hi __builtin_ia32_psadbw (v8qi, v8qi) |
| v8qi __builtin_ia32_pmaxub (v8qi, v8qi) |
| v4hi __builtin_ia32_pmaxsw (v4hi, v4hi) |
| v8qi __builtin_ia32_pminub (v8qi, v8qi) |
| v4hi __builtin_ia32_pminsw (v4hi, v4hi) |
| int __builtin_ia32_pextrw (v4hi, int) |
| v4hi __builtin_ia32_pinsrw (v4hi, int, int) |
| int __builtin_ia32_pmovmskb (v8qi) |
| void __builtin_ia32_maskmovq (v8qi, v8qi, char *) |
| void __builtin_ia32_movntq (di *, di) |
| void __builtin_ia32_sfence (void) |
| @end example |
| |
| The following built-in functions are available when @option{-msse} is used. |
| All of them generate the machine instruction that is part of the name. |
| |
| @example |
| int __builtin_ia32_comieq (v4sf, v4sf) |
| int __builtin_ia32_comineq (v4sf, v4sf) |
| int __builtin_ia32_comilt (v4sf, v4sf) |
| int __builtin_ia32_comile (v4sf, v4sf) |
| int __builtin_ia32_comigt (v4sf, v4sf) |
| int __builtin_ia32_comige (v4sf, v4sf) |
| int __builtin_ia32_ucomieq (v4sf, v4sf) |
| int __builtin_ia32_ucomineq (v4sf, v4sf) |
| int __builtin_ia32_ucomilt (v4sf, v4sf) |
| int __builtin_ia32_ucomile (v4sf, v4sf) |
| int __builtin_ia32_ucomigt (v4sf, v4sf) |
| int __builtin_ia32_ucomige (v4sf, v4sf) |
| v4sf __builtin_ia32_addps (v4sf, v4sf) |
| v4sf __builtin_ia32_subps (v4sf, v4sf) |
| v4sf __builtin_ia32_mulps (v4sf, v4sf) |
| v4sf __builtin_ia32_divps (v4sf, v4sf) |
| v4sf __builtin_ia32_addss (v4sf, v4sf) |
| v4sf __builtin_ia32_subss (v4sf, v4sf) |
| v4sf __builtin_ia32_mulss (v4sf, v4sf) |
| v4sf __builtin_ia32_divss (v4sf, v4sf) |
| v4si __builtin_ia32_cmpeqps (v4sf, v4sf) |
| v4si __builtin_ia32_cmpltps (v4sf, v4sf) |
| v4si __builtin_ia32_cmpleps (v4sf, v4sf) |
| v4si __builtin_ia32_cmpgtps (v4sf, v4sf) |
| v4si __builtin_ia32_cmpgeps (v4sf, v4sf) |
| v4si __builtin_ia32_cmpunordps (v4sf, v4sf) |
| v4si __builtin_ia32_cmpneqps (v4sf, v4sf) |
| v4si __builtin_ia32_cmpnltps (v4sf, v4sf) |
| v4si __builtin_ia32_cmpnleps (v4sf, v4sf) |
| v4si __builtin_ia32_cmpngtps (v4sf, v4sf) |
| v4si __builtin_ia32_cmpngeps (v4sf, v4sf) |
| v4si __builtin_ia32_cmpordps (v4sf, v4sf) |
| v4si __builtin_ia32_cmpeqss (v4sf, v4sf) |
| v4si __builtin_ia32_cmpltss (v4sf, v4sf) |
| v4si __builtin_ia32_cmpless (v4sf, v4sf) |
| v4si __builtin_ia32_cmpgtss (v4sf, v4sf) |
| v4si __builtin_ia32_cmpgess (v4sf, v4sf) |
| v4si __builtin_ia32_cmpunordss (v4sf, v4sf) |
| v4si __builtin_ia32_cmpneqss (v4sf, v4sf) |
| v4si __builtin_ia32_cmpnlts (v4sf, v4sf) |
| v4si __builtin_ia32_cmpnless (v4sf, v4sf) |
| v4si __builtin_ia32_cmpngtss (v4sf, v4sf) |
| v4si __builtin_ia32_cmpngess (v4sf, v4sf) |
| v4si __builtin_ia32_cmpordss (v4sf, v4sf) |
| v4sf __builtin_ia32_maxps (v4sf, v4sf) |
| v4sf __builtin_ia32_maxss (v4sf, v4sf) |
| v4sf __builtin_ia32_minps (v4sf, v4sf) |
| v4sf __builtin_ia32_minss (v4sf, v4sf) |
| v4sf __builtin_ia32_andps (v4sf, v4sf) |
| v4sf __builtin_ia32_andnps (v4sf, v4sf) |
| v4sf __builtin_ia32_orps (v4sf, v4sf) |
| v4sf __builtin_ia32_xorps (v4sf, v4sf) |
| v4sf __builtin_ia32_movss (v4sf, v4sf) |
| v4sf __builtin_ia32_movhlps (v4sf, v4sf) |
| v4sf __builtin_ia32_movlhps (v4sf, v4sf) |
| v4sf __builtin_ia32_unpckhps (v4sf, v4sf) |
| v4sf __builtin_ia32_unpcklps (v4sf, v4sf) |
| v4sf __builtin_ia32_cvtpi2ps (v4sf, v2si) |
| v4sf __builtin_ia32_cvtsi2ss (v4sf, int) |
| v2si __builtin_ia32_cvtps2pi (v4sf) |
| int __builtin_ia32_cvtss2si (v4sf) |
| v2si __builtin_ia32_cvttps2pi (v4sf) |
| int __builtin_ia32_cvttss2si (v4sf) |
| v4sf __builtin_ia32_rcpps (v4sf) |
| v4sf __builtin_ia32_rsqrtps (v4sf) |
| v4sf __builtin_ia32_sqrtps (v4sf) |
| v4sf __builtin_ia32_rcpss (v4sf) |
| v4sf __builtin_ia32_rsqrtss (v4sf) |
| v4sf __builtin_ia32_sqrtss (v4sf) |
| v4sf __builtin_ia32_shufps (v4sf, v4sf, int) |
| void __builtin_ia32_movntps (float *, v4sf) |
| int __builtin_ia32_movmskps (v4sf) |
| @end example |
| |
| The following built-in functions are available when @option{-msse} is used. |
| |
| @table @code |
| @item v4sf __builtin_ia32_loadaps (float *) |
| Generates the @code{movaps} machine instruction as a load from memory. |
| @item void __builtin_ia32_storeaps (float *, v4sf) |
| Generates the @code{movaps} machine instruction as a store to memory. |
| @item v4sf __builtin_ia32_loadups (float *) |
| Generates the @code{movups} machine instruction as a load from memory. |
| @item void __builtin_ia32_storeups (float *, v4sf) |
| Generates the @code{movups} machine instruction as a store to memory. |
| @item v4sf __builtin_ia32_loadsss (float *) |
| Generates the @code{movss} machine instruction as a load from memory. |
| @item void __builtin_ia32_storess (float *, v4sf) |
| Generates the @code{movss} machine instruction as a store to memory. |
| @item v4sf __builtin_ia32_loadhps (v4sf, v2si *) |
| Generates the @code{movhps} machine instruction as a load from memory. |
| @item v4sf __builtin_ia32_loadlps (v4sf, v2si *) |
| Generates the @code{movlps} machine instruction as a load from memory |
| @item void __builtin_ia32_storehps (v4sf, v2si *) |
| Generates the @code{movhps} machine instruction as a store to memory. |
| @item void __builtin_ia32_storelps (v4sf, v2si *) |
| Generates the @code{movlps} machine instruction as a store to memory. |
| @end table |
| |
| The following built-in functions are available when @option{-m3dnow} is used. |
| All of them generate the machine instruction that is part of the name. |
| |
| @example |
| void __builtin_ia32_femms (void) |
| v8qi __builtin_ia32_pavgusb (v8qi, v8qi) |
| v2si __builtin_ia32_pf2id (v2sf) |
| v2sf __builtin_ia32_pfacc (v2sf, v2sf) |
| v2sf __builtin_ia32_pfadd (v2sf, v2sf) |
| v2si __builtin_ia32_pfcmpeq (v2sf, v2sf) |
| v2si __builtin_ia32_pfcmpge (v2sf, v2sf) |
| v2si __builtin_ia32_pfcmpgt (v2sf, v2sf) |
| v2sf __builtin_ia32_pfmax (v2sf, v2sf) |
| v2sf __builtin_ia32_pfmin (v2sf, v2sf) |
| v2sf __builtin_ia32_pfmul (v2sf, v2sf) |
| v2sf __builtin_ia32_pfrcp (v2sf) |
| v2sf __builtin_ia32_pfrcpit1 (v2sf, v2sf) |
| v2sf __builtin_ia32_pfrcpit2 (v2sf, v2sf) |
| v2sf __builtin_ia32_pfrsqrt (v2sf) |
| v2sf __builtin_ia32_pfrsqrtit1 (v2sf, v2sf) |
| v2sf __builtin_ia32_pfsub (v2sf, v2sf) |
| v2sf __builtin_ia32_pfsubr (v2sf, v2sf) |
| v2sf __builtin_ia32_pi2fd (v2si) |
| v4hi __builtin_ia32_pmulhrw (v4hi, v4hi) |
| @end example |
| |
| The following built-in functions are available when both @option{-m3dnow} |
| and @option{-march=athlon} are used. All of them generate the machine |
| instruction that is part of the name. |
| |
| @example |
| v2si __builtin_ia32_pf2iw (v2sf) |
| v2sf __builtin_ia32_pfnacc (v2sf, v2sf) |
| v2sf __builtin_ia32_pfpnacc (v2sf, v2sf) |
| v2sf __builtin_ia32_pi2fw (v2si) |
| v2sf __builtin_ia32_pswapdsf (v2sf) |
| v2si __builtin_ia32_pswapdsi (v2si) |
| @end example |
| |
| @node PowerPC AltiVec Built-in Functions |
| @subsection PowerPC AltiVec Built-in Functions |
| |
| These built-in functions are available for the PowerPC family |
| of computers, depending on the command-line switches used. |
| |
| The following machine modes are available for use with AltiVec built-in |
| functions (@pxref{Vector Extensions}): @code{V4SI} for a vector of four |
| 32-bit integers, @code{V4SF} for a vector of four 32-bit floating point |
| numbers, @code{V8HI} for a vector of eight 16-bit integers, and |
| @code{V16QI} for a vector of sixteen 8-bit integers. |
| |
| The following functions are made available by including |
| @code{<altivec.h>} and using @option{-maltivec} and |
| @option{-mabi=altivec}. The functions implement the functionality |
| described in Motorola's AltiVec Programming Interface Manual. |
| |
| @emph{Note:} Only the @code{<altivec.h>} interface is supported. |
| Internally, GCC uses built-in functions to achieve the functionality in |
| the aforementioned header file, but they are not supported and are |
| subject to change without notice. |
| |
| @smallexample |
| vector signed char vec_abs (vector signed char, vector signed char); |
| vector signed short vec_abs (vector signed short, vector signed short); |
| vector signed int vec_abs (vector signed int, vector signed int); |
| vector signed float vec_abs (vector signed float, vector signed float); |
| |
| vector signed char vec_abss (vector signed char, vector signed char); |
| vector signed short vec_abss (vector signed short, vector signed short); |
| |
| vector signed char vec_add (vector signed char, vector signed char); |
| vector unsigned char vec_add (vector signed char, vector unsigned char); |
| |
| vector unsigned char vec_add (vector unsigned char, vector signed char); |
| |
| vector unsigned char vec_add (vector unsigned char, |
| vector unsigned char); |
| vector signed short vec_add (vector signed short, vector signed short); |
| vector unsigned short vec_add (vector signed short, |
| vector unsigned short); |
| vector unsigned short vec_add (vector unsigned short, |
| vector signed short); |
| vector unsigned short vec_add (vector unsigned short, |
| vector unsigned short); |
| vector signed int vec_add (vector signed int, vector signed int); |
| vector unsigned int vec_add (vector signed int, vector unsigned int); |
| vector unsigned int vec_add (vector unsigned int, vector signed int); |
| vector unsigned int vec_add (vector unsigned int, vector unsigned int); |
| vector float vec_add (vector float, vector float); |
| |
| vector unsigned int vec_addc (vector unsigned int, vector unsigned int); |
| |
| vector unsigned char vec_adds (vector signed char, |
| vector unsigned char); |
| vector unsigned char vec_adds (vector unsigned char, |
| vector signed char); |
| vector unsigned char vec_adds (vector unsigned char, |
| vector unsigned char); |
| vector signed char vec_adds (vector signed char, vector signed char); |
| vector unsigned short vec_adds (vector signed short, |
| vector unsigned short); |
| vector unsigned short vec_adds (vector unsigned short, |
| vector signed short); |
| vector unsigned short vec_adds (vector unsigned short, |
| vector unsigned short); |
| vector signed short vec_adds (vector signed short, vector signed short); |
| |
| vector unsigned int vec_adds (vector signed int, vector unsigned int); |
| vector unsigned int vec_adds (vector unsigned int, vector signed int); |
| vector unsigned int vec_adds (vector unsigned int, vector unsigned int); |
| |
| vector signed int vec_adds (vector signed int, vector signed int); |
| |
| vector float vec_and (vector float, vector float); |
| vector float vec_and (vector float, vector signed int); |
| vector float vec_and (vector signed int, vector float); |
| vector signed int vec_and (vector signed int, vector signed int); |
| vector unsigned int vec_and (vector signed int, vector unsigned int); |
| vector unsigned int vec_and (vector unsigned int, vector signed int); |
| vector unsigned int vec_and (vector unsigned int, vector unsigned int); |
| vector signed short vec_and (vector signed short, vector signed short); |
| vector unsigned short vec_and (vector signed short, |
| vector unsigned short); |
| vector unsigned short vec_and (vector unsigned short, |
| vector signed short); |
| vector unsigned short vec_and (vector unsigned short, |
| vector unsigned short); |
| vector signed char vec_and (vector signed char, vector signed char); |
| vector unsigned char vec_and (vector signed char, vector unsigned char); |
| |
| vector unsigned char vec_and (vector unsigned char, vector signed char); |
| |
| vector unsigned char vec_and (vector unsigned char, |
| vector unsigned char); |
| |
| vector float vec_andc (vector float, vector float); |
| vector float vec_andc (vector float, vector signed int); |
| vector float vec_andc (vector signed int, vector float); |
| vector signed int vec_andc (vector signed int, vector signed int); |
| vector unsigned int vec_andc (vector signed int, vector unsigned int); |
| vector unsigned int vec_andc (vector unsigned int, vector signed int); |
| vector unsigned int vec_andc (vector unsigned int, vector unsigned int); |
| |
| vector signed short vec_andc (vector signed short, vector signed short); |
| |
| vector unsigned short vec_andc (vector signed short, |
| vector unsigned short); |
| vector unsigned short vec_andc (vector unsigned short, |
| vector signed short); |
| vector unsigned short vec_andc (vector unsigned short, |
| vector unsigned short); |
| vector signed char vec_andc (vector signed char, vector signed char); |
| vector unsigned char vec_andc (vector signed char, |
| vector unsigned char); |
| vector unsigned char vec_andc (vector unsigned char, |
| vector signed char); |
| vector unsigned char vec_andc (vector unsigned char, |
| vector unsigned char); |
| |
| vector unsigned char vec_avg (vector unsigned char, |
| vector unsigned char); |
| vector signed char vec_avg (vector signed char, vector signed char); |
| vector unsigned short vec_avg (vector unsigned short, |
| vector unsigned short); |
| vector signed short vec_avg (vector signed short, vector signed short); |
| vector unsigned int vec_avg (vector unsigned int, vector unsigned int); |
| vector signed int vec_avg (vector signed int, vector signed int); |
| |
| vector float vec_ceil (vector float); |
| |
| vector signed int vec_cmpb (vector float, vector float); |
| |
| vector signed char vec_cmpeq (vector signed char, vector signed char); |
| vector signed char vec_cmpeq (vector unsigned char, |
| vector unsigned char); |
| vector signed short vec_cmpeq (vector signed short, |
| vector signed short); |
| vector signed short vec_cmpeq (vector unsigned short, |
| vector unsigned short); |
| vector signed int vec_cmpeq (vector signed int, vector signed int); |
| vector signed int vec_cmpeq (vector unsigned int, vector unsigned int); |
| vector signed int vec_cmpeq (vector float, vector float); |
| |
| vector signed int vec_cmpge (vector float, vector float); |
| |
| vector signed char vec_cmpgt (vector unsigned char, |
| vector unsigned char); |
| vector signed char vec_cmpgt (vector signed char, vector signed char); |
| vector signed short vec_cmpgt (vector unsigned short, |
| vector unsigned short); |
| vector signed short vec_cmpgt (vector signed short, |
| vector signed short); |
| vector signed int vec_cmpgt (vector unsigned int, vector unsigned int); |
| vector signed int vec_cmpgt (vector signed int, vector signed int); |
| vector signed int vec_cmpgt (vector float, vector float); |
| |
| vector signed int vec_cmple (vector float, vector float); |
| |
| vector signed char vec_cmplt (vector unsigned char, |
| vector unsigned char); |
| vector signed char vec_cmplt (vector signed char, vector signed char); |
| vector signed short vec_cmplt (vector unsigned short, |
| vector unsigned short); |
| vector signed short vec_cmplt (vector signed short, |
| vector signed short); |
| vector signed int vec_cmplt (vector unsigned int, vector unsigned int); |
| vector signed int vec_cmplt (vector signed int, vector signed int); |
| vector signed int vec_cmplt (vector float, vector float); |
| |
| vector float vec_ctf (vector unsigned int, const char); |
| vector float vec_ctf (vector signed int, const char); |
| |
| vector signed int vec_cts (vector float, const char); |
| |
| vector unsigned int vec_ctu (vector float, const char); |
| |
| void vec_dss (const char); |
| |
| void vec_dssall (void); |
| |
| void vec_dst (void *, int, const char); |
| |
| void vec_dstst (void *, int, const char); |
| |
| void vec_dststt (void *, int, const char); |
| |
| void vec_dstt (void *, int, const char); |
| |
| vector float vec_expte (vector float, vector float); |
| |
| vector float vec_floor (vector float, vector float); |
| |
| vector float vec_ld (int, vector float *); |
| vector float vec_ld (int, float *): |
| vector signed int vec_ld (int, int *); |
| vector signed int vec_ld (int, vector signed int *); |
| vector unsigned int vec_ld (int, vector unsigned int *); |
| vector unsigned int vec_ld (int, unsigned int *); |
| vector signed short vec_ld (int, short *, vector signed short *); |
| vector unsigned short vec_ld (int, unsigned short *, |
| vector unsigned short *); |
| vector signed char vec_ld (int, signed char *); |
| vector signed char vec_ld (int, vector signed char *); |
| vector unsigned char vec_ld (int, unsigned char *); |
| vector unsigned char vec_ld (int, vector unsigned char *); |
| |
| vector signed char vec_lde (int, signed char *); |
| vector unsigned char vec_lde (int, unsigned char *); |
| vector signed short vec_lde (int, short *); |
| vector unsigned short vec_lde (int, unsigned short *); |
| vector float vec_lde (int, float *); |
| vector signed int vec_lde (int, int *); |
| vector unsigned int vec_lde (int, unsigned int *); |
| |
| void float vec_ldl (int, float *); |
| void float vec_ldl (int, vector float *); |
| void signed int vec_ldl (int, vector signed int *); |
| void signed int vec_ldl (int, int *); |
| void unsigned int vec_ldl (int, unsigned int *); |
| void unsigned int vec_ldl (int, vector unsigned int *); |
| void signed short vec_ldl (int, vector signed short *); |
| void signed short vec_ldl (int, short *); |
| void unsigned short vec_ldl (int, vector unsigned short *); |
| void unsigned short vec_ldl (int, unsigned short *); |
| void signed char vec_ldl (int, vector signed char *); |
| void signed char vec_ldl (int, signed char *); |
| void unsigned char vec_ldl (int, vector unsigned char *); |
| void unsigned char vec_ldl (int, unsigned char *); |
| |
| vector float vec_loge (vector float); |
| |
| vector unsigned char vec_lvsl (int, void *, int *); |
| |
| vector unsigned char vec_lvsr (int, void *, int *); |
| |
| vector float vec_madd (vector float, vector float, vector float); |
| |
| vector signed short vec_madds (vector signed short, vector signed short, |
| vector signed short); |
| |
| vector unsigned char vec_max (vector signed char, vector unsigned char); |
| |
| vector unsigned char vec_max (vector unsigned char, vector signed char); |
| |
| vector unsigned char vec_max (vector unsigned char, |
| vector unsigned char); |
| vector signed char vec_max (vector signed char, vector signed char); |
| vector unsigned short vec_max (vector signed short, |
| vector unsigned short); |
| vector unsigned short vec_max (vector unsigned short, |
| vector signed short); |
| vector unsigned short vec_max (vector unsigned short, |
| vector unsigned short); |
| vector signed short vec_max (vector signed short, vector signed short); |
| vector unsigned int vec_max (vector signed int, vector unsigned int); |
| vector unsigned int vec_max (vector unsigned int, vector signed int); |
| vector unsigned int vec_max (vector unsigned int, vector unsigned int); |
| vector signed int vec_max (vector signed int, vector signed int); |
| vector float vec_max (vector float, vector float); |
| |
| vector signed char vec_mergeh (vector signed char, vector signed char); |
| vector unsigned char vec_mergeh (vector unsigned char, |
| vector unsigned char); |
| vector signed short vec_mergeh (vector signed short, |
| vector signed short); |
| vector unsigned short vec_mergeh (vector unsigned short, |
| vector unsigned short); |
| vector float vec_mergeh (vector float, vector float); |
| vector signed int vec_mergeh (vector signed int, vector signed int); |
| vector unsigned int vec_mergeh (vector unsigned int, |
| vector unsigned int); |
| |
| vector signed char vec_mergel (vector signed char, vector signed char); |
| vector unsigned char vec_mergel (vector unsigned char, |
| vector unsigned char); |
| vector signed short vec_mergel (vector signed short, |
| vector signed short); |
| vector unsigned short vec_mergel (vector unsigned short, |
| vector unsigned short); |
| vector float vec_mergel (vector float, vector float); |
| vector signed int vec_mergel (vector signed int, vector signed int); |
| vector unsigned int vec_mergel (vector unsigned int, |
| vector unsigned int); |
| |
| vector unsigned short vec_mfvscr (void); |
| |
| vector unsigned char vec_min (vector signed char, vector unsigned char); |
| |
| vector unsigned char vec_min (vector unsigned char, vector signed char); |
| |
| vector unsigned char vec_min (vector unsigned char, |
| vector unsigned char); |
| vector signed char vec_min (vector signed char, vector signed char); |
| vector unsigned short vec_min (vector signed short, |
| vector unsigned short); |
| vector unsigned short vec_min (vector unsigned short, |
| vector signed short); |
| vector unsigned short vec_min (vector unsigned short, |
| vector unsigned short); |
| vector signed short vec_min (vector signed short, vector signed short); |
| vector unsigned int vec_min (vector signed int, vector unsigned int); |
| vector unsigned int vec_min (vector unsigned int, vector signed int); |
| vector unsigned int vec_min (vector unsigned int, vector unsigned int); |
| vector signed int vec_min (vector signed int, vector signed int); |
| vector float vec_min (vector float, vector float); |
| |
| vector signed short vec_mladd (vector signed short, vector signed short, |
| vector signed short); |
| vector signed short vec_mladd (vector signed short, |
| vector unsigned short, |
| vector unsigned short); |
| vector signed short vec_mladd (vector unsigned short, |
| vector signed short, |
| vector signed short); |
| vector unsigned short vec_mladd (vector unsigned short, |
| vector unsigned short, |
| vector unsigned short); |
| |
| vector signed short vec_mradds (vector signed short, |
| vector signed short, |
| vector signed short); |
| |
| vector unsigned int vec_msum (vector unsigned char, |
| vector unsigned char, |
| vector unsigned int); |
| vector signed int vec_msum (vector signed char, vector unsigned char, |
| vector signed int); |
| vector unsigned int vec_msum (vector unsigned short, |
| vector unsigned short, |
| vector unsigned int); |
| vector signed int vec_msum (vector signed short, vector signed short, |
| vector signed int); |
| |
| vector unsigned int vec_msums (vector unsigned short, |
| vector unsigned short, |
| vector unsigned int); |
| vector signed int vec_msums (vector signed short, vector signed short, |
| vector signed int); |
| |
| void vec_mtvscr (vector signed int); |
| void vec_mtvscr (vector unsigned int); |
| void vec_mtvscr (vector signed short); |
| void vec_mtvscr (vector unsigned short); |
| void vec_mtvscr (vector signed char); |
| void vec_mtvscr (vector unsigned char); |
| |
| vector unsigned short vec_mule (vector unsigned char, |
| vector unsigned char); |
| vector signed short vec_mule (vector signed char, vector signed char); |
| vector unsigned int vec_mule (vector unsigned short, |
| vector unsigned short); |
| vector signed int vec_mule (vector signed short, vector signed short); |
| |
| vector unsigned short vec_mulo (vector unsigned char, |
| vector unsigned char); |
| vector signed short vec_mulo (vector signed char, vector signed char); |
| vector unsigned int vec_mulo (vector unsigned short, |
| vector unsigned short); |
| vector signed int vec_mulo (vector signed short, vector signed short); |
| |
| vector float vec_nmsub (vector float, vector float, vector float); |
| |
| vector float vec_nor (vector float, vector float); |
| vector signed int vec_nor (vector signed int, vector signed int); |
| vector unsigned int vec_nor (vector unsigned int, vector unsigned int); |
| vector signed short vec_nor (vector signed short, vector signed short); |
| vector unsigned short vec_nor (vector unsigned short, |
| vector unsigned short); |
| vector signed char vec_nor (vector signed char, vector signed char); |
| vector unsigned char vec_nor (vector unsigned char, |
| vector unsigned char); |
| |
| vector float vec_or (vector float, vector float); |
| vector float vec_or (vector float, vector signed int); |
| vector float vec_or (vector signed int, vector float); |
| vector signed int vec_or (vector signed int, vector signed int); |
| vector unsigned int vec_or (vector signed int, vector unsigned int); |
| vector unsigned int vec_or (vector unsigned int, vector signed int); |
| vector unsigned int vec_or (vector unsigned int, vector unsigned int); |
| vector signed short vec_or (vector signed short, vector signed short); |
| vector unsigned short vec_or (vector signed short, |
| vector unsigned short); |
| vector unsigned short vec_or (vector unsigned short, |
| vector signed short); |
| vector unsigned short vec_or (vector unsigned short, |
| vector unsigned short); |
| vector signed char vec_or (vector signed char, vector signed char); |
| vector unsigned char vec_or (vector signed char, vector unsigned char); |
| vector unsigned char vec_or (vector unsigned char, vector signed char); |
| vector unsigned char vec_or (vector unsigned char, |
| vector unsigned char); |
| |
| vector signed char vec_pack (vector signed short, vector signed short); |
| vector unsigned char vec_pack (vector unsigned short, |
| vector unsigned short); |
| vector signed short vec_pack (vector signed int, vector signed int); |
| vector unsigned short vec_pack (vector unsigned int, |
| vector unsigned int); |
| |
| vector signed short vec_packpx (vector unsigned int, |
| vector unsigned int); |
| |
| vector unsigned char vec_packs (vector unsigned short, |
| vector unsigned short); |
| vector signed char vec_packs (vector signed short, vector signed short); |
| |
| vector unsigned short vec_packs (vector unsigned int, |
| vector unsigned int); |
| vector signed short vec_packs (vector signed int, vector signed int); |
| |
| vector unsigned char vec_packsu (vector unsigned short, |
| vector unsigned short); |
| vector unsigned char vec_packsu (vector signed short, |
| vector signed short); |
| vector unsigned short vec_packsu (vector unsigned int, |
| vector unsigned int); |
| vector unsigned short vec_packsu (vector signed int, vector signed int); |
| |
| vector float vec_perm (vector float, vector float, |
| vector unsigned char); |
| vector signed int vec_perm (vector signed int, vector signed int, |
| vector unsigned char); |
| vector unsigned int vec_perm (vector unsigned int, vector unsigned int, |
| vector unsigned char); |
| vector signed short vec_perm (vector signed short, vector signed short, |
| vector unsigned char); |
| vector unsigned short vec_perm (vector unsigned short, |
| vector unsigned short, |
| vector unsigned char); |
| vector signed char vec_perm (vector signed char, vector signed char, |
| vector unsigned char); |
| vector unsigned char vec_perm (vector unsigned char, |
| vector unsigned char, |
| vector unsigned char); |
| |
| vector float vec_re (vector float); |
| |
| vector signed char vec_rl (vector signed char, vector unsigned char); |
| vector unsigned char vec_rl (vector unsigned char, |
| vector unsigned char); |
| vector signed short vec_rl (vector signed short, vector unsigned short); |
| |
| vector unsigned short vec_rl (vector unsigned short, |
| vector unsigned short); |
| vector signed int vec_rl (vector signed int, vector unsigned int); |
| vector unsigned int vec_rl (vector unsigned int, vector unsigned int); |
| |
| vector float vec_round (vector float); |
| |
| vector float vec_rsqrte (vector float); |
| |
| vector float vec_sel (vector float, vector float, vector signed int); |
| vector float vec_sel (vector float, vector float, vector unsigned int); |
| vector signed int vec_sel (vector signed int, vector signed int, |
| vector signed int); |
| vector signed int vec_sel (vector signed int, vector signed int, |
| vector unsigned int); |
| vector unsigned int vec_sel (vector unsigned int, vector unsigned int, |
| vector signed int); |
| vector unsigned int vec_sel (vector unsigned int, vector unsigned int, |
| vector unsigned int); |
| vector signed short vec_sel (vector signed short, vector signed short, |
| vector signed short); |
| vector signed short vec_sel (vector signed short, vector signed short, |
| vector unsigned short); |
| vector unsigned short vec_sel (vector unsigned short, |
| vector unsigned short, |
| vector signed short); |
| vector unsigned short vec_sel (vector unsigned short, |
| vector unsigned short, |
| vector unsigned short); |
| vector signed char vec_sel (vector signed char, vector signed char, |
| vector signed char); |
| vector signed char vec_sel (vector signed char, vector signed char, |
| vector unsigned char); |
| vector unsigned char vec_sel (vector unsigned char, |
| vector unsigned char, |
| vector signed char); |
| vector unsigned char vec_sel (vector unsigned char, |
| vector unsigned char, |
| vector unsigned char); |
| |
| vector signed char vec_sl (vector signed char, vector unsigned char); |
| vector unsigned char vec_sl (vector unsigned char, |
| vector unsigned char); |
| vector signed short vec_sl (vector signed short, vector unsigned short); |
| |
| vector unsigned short vec_sl (vector unsigned short, |
| vector unsigned short); |
| vector signed int vec_sl (vector signed int, vector unsigned int); |
| vector unsigned int vec_sl (vector unsigned int, vector unsigned int); |
| |
| vector float vec_sld (vector float, vector float, const char); |
| vector signed int vec_sld (vector signed int, vector signed int, |
| const char); |
| vector unsigned int vec_sld (vector unsigned int, vector unsigned int, |
| const char); |
| vector signed short vec_sld (vector signed short, vector signed short, |
| const char); |
| vector unsigned short vec_sld (vector unsigned short, |
| vector unsigned short, const char); |
| vector signed char vec_sld (vector signed char, vector signed char, |
| const char); |
| vector unsigned char vec_sld (vector unsigned char, |
| vector unsigned char, |
| const char); |
| |
| vector signed int vec_sll (vector signed int, vector unsigned int); |
| vector signed int vec_sll (vector signed int, vector unsigned short); |
| vector signed int vec_sll (vector signed int, vector unsigned char); |
| vector unsigned int vec_sll (vector unsigned int, vector unsigned int); |
| vector unsigned int vec_sll (vector unsigned int, |
| vector unsigned short); |
| vector unsigned int vec_sll (vector unsigned int, vector unsigned char); |
| |
| vector signed short vec_sll (vector signed short, vector unsigned int); |
| vector signed short vec_sll (vector signed short, |
| vector unsigned short); |
| vector signed short vec_sll (vector signed short, vector unsigned char); |
| |
| vector unsigned short vec_sll (vector unsigned short, |
| vector unsigned int); |
| vector unsigned short vec_sll (vector unsigned short, |
| vector unsigned short); |
| vector unsigned short vec_sll (vector unsigned short, |
| vector unsigned char); |
| vector signed char vec_sll (vector signed char, vector unsigned int); |
| vector signed char vec_sll (vector signed char, vector unsigned short); |
| vector signed char vec_sll (vector signed char, vector unsigned char); |
| vector unsigned char vec_sll (vector unsigned char, |
| vector unsigned int); |
| vector unsigned char vec_sll (vector unsigned char, |
| vector unsigned short); |
| vector unsigned char vec_sll (vector unsigned char, |
| vector unsigned char); |
| |
| vector float vec_slo (vector float, vector signed char); |
| vector float vec_slo (vector float, vector unsigned char); |
| vector signed int vec_slo (vector signed int, vector signed char); |
| vector signed int vec_slo (vector signed int, vector unsigned char); |
| vector unsigned int vec_slo (vector unsigned int, vector signed char); |
| vector unsigned int vec_slo (vector unsigned int, vector unsigned char); |
| |
| vector signed short vec_slo (vector signed short, vector signed char); |
| vector signed short vec_slo (vector signed short, vector unsigned char); |
| |
| vector unsigned short vec_slo (vector unsigned short, |
| vector signed char); |
| vector unsigned short vec_slo (vector unsigned short, |
| vector unsigned char); |
| vector signed char vec_slo (vector signed char, vector signed char); |
| vector signed char vec_slo (vector signed char, vector unsigned char); |
| vector unsigned char vec_slo (vector unsigned char, vector signed char); |
| |
| vector unsigned char vec_slo (vector unsigned char, |
| vector unsigned char); |
| |
| vector signed char vec_splat (vector signed char, const char); |
| vector unsigned char vec_splat (vector unsigned char, const char); |
| vector signed short vec_splat (vector signed short, const char); |
| vector unsigned short vec_splat (vector unsigned short, const char); |
| vector float vec_splat (vector float, const char); |
| vector signed int vec_splat (vector signed int, const char); |
| vector unsigned int vec_splat (vector unsigned int, const char); |
| |
| vector signed char vec_splat_s8 (const char); |
| |
| vector signed short vec_splat_s16 (const char); |
| |
| vector signed int vec_splat_s32 (const char); |
| |
| vector unsigned char vec_splat_u8 (const char); |
| |
| vector unsigned short vec_splat_u16 (const char); |
| |
| vector unsigned int vec_splat_u32 (const char); |
| |
| vector signed char vec_sr (vector signed char, vector unsigned char); |
| vector unsigned char vec_sr (vector unsigned char, |
| vector unsigned char); |
| vector signed short vec_sr (vector signed short, vector unsigned short); |
| |
| vector unsigned short vec_sr (vector unsigned short, |
| vector unsigned short); |
| vector signed int vec_sr (vector signed int, vector unsigned int); |
| vector unsigned int vec_sr (vector unsigned int, vector unsigned int); |
| |
| vector signed char vec_sra (vector signed char, vector unsigned char); |
| vector unsigned char vec_sra (vector unsigned char, |
| vector unsigned char); |
| vector signed short vec_sra (vector signed short, |
| vector unsigned short); |
| vector unsigned short vec_sra (vector unsigned short, |
| vector unsigned short); |
| vector signed int vec_sra (vector signed int, vector unsigned int); |
| vector unsigned int vec_sra (vector unsigned int, vector unsigned int); |
| |
| vector signed int vec_srl (vector signed int, vector unsigned int); |
| vector signed int vec_srl (vector signed int, vector unsigned short); |
| vector signed int vec_srl (vector signed int, vector unsigned char); |
| vector unsigned int vec_srl (vector unsigned int, vector unsigned int); |
| vector unsigned int vec_srl (vector unsigned int, |
| vector unsigned short); |
| vector unsigned int vec_srl (vector unsigned int, vector unsigned char); |
| |
| vector signed short vec_srl (vector signed short, vector unsigned int); |
| vector signed short vec_srl (vector signed short, |
| vector unsigned short); |
| vector signed short vec_srl (vector signed short, vector unsigned char); |
| |
| vector unsigned short vec_srl (vector unsigned short, |
| vector unsigned int); |
| vector unsigned short vec_srl (vector unsigned short, |
| vector unsigned short); |
| vector unsigned short vec_srl (vector unsigned short, |
| vector unsigned char); |
| vector signed char vec_srl (vector signed char, vector unsigned int); |
| vector signed char vec_srl (vector signed char, vector unsigned short); |
| vector signed char vec_srl (vector signed char, vector unsigned char); |
| vector unsigned char vec_srl (vector unsigned char, |
| vector unsigned int); |
| vector unsigned char vec_srl (vector unsigned char, |
| vector unsigned short); |
| vector unsigned char vec_srl (vector unsigned char, |
| vector unsigned char); |
| |
| vector float vec_sro (vector float, vector signed char); |
| vector float vec_sro (vector float, vector unsigned char); |
| vector signed int vec_sro (vector signed int, vector signed char); |
| vector signed int vec_sro (vector signed int, vector unsigned char); |
| vector unsigned int vec_sro (vector unsigned int, vector signed char); |
| vector unsigned int vec_sro (vector unsigned int, vector unsigned char); |
| |
| vector signed short vec_sro (vector signed short, vector signed char); |
| vector signed short vec_sro (vector signed short, vector unsigned char); |
| |
| vector unsigned short vec_sro (vector unsigned short, |
| vector signed char); |
| vector unsigned short vec_sro (vector unsigned short, |
| vector unsigned char); |
| vector signed char vec_sro (vector signed char, vector signed char); |
| vector signed char vec_sro (vector signed char, vector unsigned char); |
| vector unsigned char vec_sro (vector unsigned char, vector signed char); |
| |
| vector unsigned char vec_sro (vector unsigned char, |
| vector unsigned char); |
| |
| void vec_st (vector float, int, float *); |
| void vec_st (vector float, int, vector float *); |
| void vec_st (vector signed int, int, int *); |
| void vec_st (vector signed int, int, unsigned int *); |
| void vec_st (vector unsigned int, int, unsigned int *); |
| void vec_st (vector unsigned int, int, vector unsigned int *); |
| void vec_st (vector signed short, int, short *); |
| void vec_st (vector signed short, int, vector unsigned short *); |
| void vec_st (vector signed short, int, vector signed short *); |
| void vec_st (vector unsigned short, int, unsigned short *); |
| void vec_st (vector unsigned short, int, vector unsigned short *); |
| void vec_st (vector signed char, int, signed char *); |
| void vec_st (vector signed char, int, unsigned char *); |
| void vec_st (vector signed char, int, vector signed char *); |
| void vec_st (vector unsigned char, int, unsigned char *); |
| void vec_st (vector unsigned char, int, vector unsigned char *); |
| |
| void vec_ste (vector signed char, int, unsigned char *); |
| void vec_ste (vector signed char, int, signed char *); |
| void vec_ste (vector unsigned char, int, unsigned char *); |
| void vec_ste (vector signed short, int, short *); |
| void vec_ste (vector signed short, int, unsigned short *); |
| void vec_ste (vector unsigned short, int, void *); |
| void vec_ste (vector signed int, int, unsigned int *); |
| void vec_ste (vector signed int, int, int *); |
| void vec_ste (vector unsigned int, int, unsigned int *); |
| void vec_ste (vector float, int, float *); |
| |
| void vec_stl (vector float, int, vector float *); |
| void vec_stl (vector float, int, float *); |
| void vec_stl (vector signed int, int, vector signed int *); |
| void vec_stl (vector signed int, int, int *); |
| void vec_stl (vector signed int, int, unsigned int *); |
| void vec_stl (vector unsigned int, int, vector unsigned int *); |
| void vec_stl (vector unsigned int, int, unsigned int *); |
| void vec_stl (vector signed short, int, short *); |
| void vec_stl (vector signed short, int, unsigned short *); |
| void vec_stl (vector signed short, int, vector signed short *); |
| void vec_stl (vector unsigned short, int, unsigned short *); |
| void vec_stl (vector unsigned short, int, vector signed short *); |
| void vec_stl (vector signed char, int, signed char *); |
| void vec_stl (vector signed char, int, unsigned char *); |
| void vec_stl (vector signed char, int, vector signed char *); |
| void vec_stl (vector unsigned char, int, unsigned char *); |
| void vec_stl (vector unsigned char, int, vector unsigned char *); |
| |
| vector signed char vec_sub (vector signed char, vector signed char); |
| vector unsigned char vec_sub (vector signed char, vector unsigned char); |
| |
| vector unsigned char vec_sub (vector unsigned char, vector signed char); |
| |
| vector unsigned char vec_sub (vector unsigned char, |
| vector unsigned char); |
| vector signed short vec_sub (vector signed short, vector signed short); |
| vector unsigned short vec_sub (vector signed short, |
| vector unsigned short); |
| vector unsigned short vec_sub (vector unsigned short, |
| vector signed short); |
| vector unsigned short vec_sub (vector unsigned short, |
| vector unsigned short); |
| vector signed int vec_sub (vector signed int, vector signed int); |
| vector unsigned int vec_sub (vector signed int, vector unsigned int); |
| vector unsigned int vec_sub (vector unsigned int, vector signed int); |
| vector unsigned int vec_sub (vector unsigned int, vector unsigned int); |
| vector float vec_sub (vector float, vector float); |
| |
| vector unsigned int vec_subc (vector unsigned int, vector unsigned int); |
| |
| vector unsigned char vec_subs (vector signed char, |
| vector unsigned char); |
| vector unsigned char vec_subs (vector unsigned char, |
| vector signed char); |
| vector unsigned char vec_subs (vector unsigned char, |
| vector unsigned char); |
| vector signed char vec_subs (vector signed char, vector signed char); |
| vector unsigned short vec_subs (vector signed short, |
| vector unsigned short); |
| vector unsigned short vec_subs (vector unsigned short, |
| vector signed short); |
| vector unsigned short vec_subs (vector unsigned short, |
| vector unsigned short); |
| vector signed short vec_subs (vector signed short, vector signed short); |
| |
| vector unsigned int vec_subs (vector signed int, vector unsigned int); |
| vector unsigned int vec_subs (vector unsigned int, vector signed int); |
| vector unsigned int vec_subs (vector unsigned int, vector unsigned int); |
| |
| vector signed int vec_subs (vector signed int, vector signed int); |
| |
| vector unsigned int vec_sum4s (vector unsigned char, |
| vector unsigned int); |
| vector signed int vec_sum4s (vector signed char, vector signed int); |
| vector signed int vec_sum4s (vector signed short, vector signed int); |
| |
| vector signed int vec_sum2s (vector signed int, vector signed int); |
| |
| vector signed int vec_sums (vector signed int, vector signed int); |
| |
| vector float vec_trunc (vector float); |
| |
| vector signed short vec_unpackh (vector signed char); |
| vector unsigned int vec_unpackh (vector signed short); |
| vector signed int vec_unpackh (vector signed short); |
| |
| vector signed short vec_unpackl (vector signed char); |
| vector unsigned int vec_unpackl (vector signed short); |
| vector signed int vec_unpackl (vector signed short); |
| |
| vector float vec_xor (vector float, vector float); |
| vector float vec_xor (vector float, vector signed int); |
| vector float vec_xor (vector signed int, vector float); |
| vector signed int vec_xor (vector signed int, vector signed int); |
| vector unsigned int vec_xor (vector signed int, vector unsigned int); |
| vector unsigned int vec_xor (vector unsigned int, vector signed int); |
| vector unsigned int vec_xor (vector unsigned int, vector unsigned int); |
| vector signed short vec_xor (vector signed short, vector signed short); |
| vector unsigned short vec_xor (vector signed short, |
| vector unsigned short); |
| vector unsigned short vec_xor (vector unsigned short, |
| vector signed short); |
| vector unsigned short vec_xor (vector unsigned short, |
| vector unsigned short); |
| vector signed char vec_xor (vector signed char, vector signed char); |
| vector unsigned char vec_xor (vector signed char, vector unsigned char); |
| |
| vector unsigned char vec_xor (vector unsigned char, vector signed char); |
| |
| vector unsigned char vec_xor (vector unsigned char, |
| vector unsigned char); |
| |
| vector signed int vec_all_eq (vector signed char, vector unsigned char); |
| |
| vector signed int vec_all_eq (vector signed char, vector signed char); |
| vector signed int vec_all_eq (vector unsigned char, vector signed char); |
| |
| vector signed int vec_all_eq (vector unsigned char, |
| vector unsigned char); |
| vector signed int vec_all_eq (vector signed short, |
| vector unsigned short); |
| vector signed int vec_all_eq (vector signed short, vector signed short); |
| |
| vector signed int vec_all_eq (vector unsigned short, |
| vector signed short); |
| vector signed int vec_all_eq (vector unsigned short, |
| vector unsigned short); |
| vector signed int vec_all_eq (vector signed int, vector unsigned int); |
| vector signed int vec_all_eq (vector signed int, vector signed int); |
| vector signed int vec_all_eq (vector unsigned int, vector signed int); |
| vector signed int vec_all_eq (vector unsigned int, vector unsigned int); |
| |
| vector signed int vec_all_eq (vector float, vector float); |
| |
| vector signed int vec_all_ge (vector signed char, vector unsigned char); |
| |
| vector signed int vec_all_ge (vector unsigned char, vector signed char); |
| |
| vector signed int vec_all_ge (vector unsigned char, |
| vector unsigned char); |
| vector signed int vec_all_ge (vector signed char, vector signed char); |
| vector signed int vec_all_ge (vector signed short, |
| vector unsigned short); |
| vector signed int vec_all_ge (vector unsigned short, |
| vector signed short); |
| vector signed int vec_all_ge (vector unsigned short, |
| vector unsigned short); |
| vector signed int vec_all_ge (vector signed short, vector signed short); |
| |
| vector signed int vec_all_ge (vector signed int, vector unsigned int); |
| vector signed int vec_all_ge (vector unsigned int, vector signed int); |
| vector signed int vec_all_ge (vector unsigned int, vector unsigned int); |
| |
| vector signed int vec_all_ge (vector signed int, vector signed int); |
| vector signed int vec_all_ge (vector float, vector float); |
| |
| vector signed int vec_all_gt (vector signed char, vector unsigned char); |
| |
| vector signed int vec_all_gt (vector unsigned char, vector signed char); |
| |
| vector signed int vec_all_gt (vector unsigned char, |
| vector unsigned char); |
| vector signed int vec_all_gt (vector signed char, vector signed char); |
| vector signed int vec_all_gt (vector signed short, |
| vector unsigned short); |
| vector signed int vec_all_gt (vector unsigned short, |
| vector signed short); |
| vector signed int vec_all_gt (vector unsigned short, |
| vector unsigned short); |
| vector signed int vec_all_gt (vector signed short, vector signed short); |
| |
| vector signed int vec_all_gt (vector signed int, vector unsigned int); |
| vector signed int vec_all_gt (vector unsigned int, vector signed int); |
| vector signed int vec_all_gt (vector unsigned int, vector unsigned int); |
| |
| vector signed int vec_all_gt (vector signed int, vector signed int); |
| vector signed int vec_all_gt (vector float, vector float); |
| |
| vector signed int vec_all_in (vector float, vector float); |
| |
| vector signed int vec_all_le (vector signed char, vector unsigned char); |
| |
| vector signed int vec_all_le (vector unsigned char, vector signed char); |
| |
| vector signed int vec_all_le (vector unsigned char, |
| vector unsigned char); |
| vector signed int vec_all_le (vector signed char, vector signed char); |
| vector signed int vec_all_le (vector signed short, |
| vector unsigned short); |
| vector signed int vec_all_le (vector unsigned short, |
| vector signed short); |
| vector signed int vec_all_le (vector unsigned short, |
| vector unsigned short); |
| vector signed int vec_all_le (vector signed short, vector signed short); |
| |
| vector signed int vec_all_le (vector signed int, vector unsigned int); |
| vector signed int vec_all_le (vector unsigned int, vector signed int); |
| vector signed int vec_all_le (vector unsigned int, vector unsigned int); |
| |
| vector signed int vec_all_le (vector signed int, vector signed int); |
| vector signed int vec_all_le (vector float, vector float); |
| |
| vector signed int vec_all_lt (vector signed char, vector unsigned char); |
| |
| vector signed int vec_all_lt (vector unsigned char, vector signed char); |
| |
| vector signed int vec_all_lt (vector unsigned char, |
| vector unsigned char); |
| vector signed int vec_all_lt (vector signed char, vector signed char); |
| vector signed int vec_all_lt (vector signed short, |
| vector unsigned short); |
| vector signed int vec_all_lt (vector unsigned short, |
| vector signed short); |
| vector signed int vec_all_lt (vector unsigned short, |
| vector unsigned short); |
| vector signed int vec_all_lt (vector signed short, vector signed short); |
| |
| vector signed int vec_all_lt (vector signed int, vector unsigned int); |
| vector signed int vec_all_lt (vector unsigned int, vector signed int); |
| vector signed int vec_all_lt (vector unsigned int, vector unsigned int); |
| |
| vector signed int vec_all_lt (vector signed int, vector signed int); |
| vector signed int vec_all_lt (vector float, vector float); |
| |
| vector signed int vec_all_nan (vector float); |
| |
| vector signed int vec_all_ne (vector signed char, vector unsigned char); |
| |
| vector signed int vec_all_ne (vector signed char, vector signed char); |
| vector signed int vec_all_ne (vector unsigned char, vector signed char); |
| |
| vector signed int vec_all_ne (vector unsigned char, |
| vector unsigned char); |
| vector signed int vec_all_ne (vector signed short, |
| vector unsigned short); |
| vector signed int vec_all_ne (vector signed short, vector signed short); |
| |
| vector signed int vec_all_ne (vector unsigned short, |
| vector signed short); |
| vector signed int vec_all_ne (vector unsigned short, |
| vector unsigned short); |
| vector signed int vec_all_ne (vector signed int, vector unsigned int); |
| vector signed int vec_all_ne (vector signed int, vector signed int); |
| vector signed int vec_all_ne (vector unsigned int, vector signed int); |
| vector signed int vec_all_ne (vector unsigned int, vector unsigned int); |
| |
| vector signed int vec_all_ne (vector float, vector float); |
| |
| vector signed int vec_all_nge (vector float, vector float); |
| |
| vector signed int vec_all_ngt (vector float, vector float); |
| |
| vector signed int vec_all_nle (vector float, vector float); |
| |
| vector signed int vec_all_nlt (vector float, vector float); |
| |
| vector signed int vec_all_numeric (vector float); |
| |
| vector signed int vec_any_eq (vector signed char, vector unsigned char); |
| |
| vector signed int vec_any_eq (vector signed char, vector signed char); |
| vector signed int vec_any_eq (vector unsigned char, vector signed char); |
| |
| vector signed int vec_any_eq (vector unsigned char, |
| vector unsigned char); |
| vector signed int vec_any_eq (vector signed short, |
| vector unsigned short); |
| vector signed int vec_any_eq (vector signed short, vector signed short); |
| |
| vector signed int vec_any_eq (vector unsigned short, |
| vector signed short); |
| vector signed int vec_any_eq (vector unsigned short, |
| vector unsigned short); |
| vector signed int vec_any_eq (vector signed int, vector unsigned int); |
| vector signed int vec_any_eq (vector signed int, vector signed int); |
| vector signed int vec_any_eq (vector unsigned int, vector signed int); |
| vector signed int vec_any_eq (vector unsigned int, vector unsigned int); |
| |
| vector signed int vec_any_eq (vector float, vector float); |
| |
| vector signed int vec_any_ge (vector signed char, vector unsigned char); |
| |
| vector signed int vec_any_ge (vector unsigned char, vector signed char); |
| |
| vector signed int vec_any_ge (vector unsigned char, |
| vector unsigned char); |
| vector signed int vec_any_ge (vector signed char, vector signed char); |
| vector signed int vec_any_ge (vector signed short, |
| vector unsigned short); |
| vector signed int vec_any_ge (vector unsigned short, |
| vector signed short); |
| vector signed int vec_any_ge (vector unsigned short, |
| vector unsigned short); |
| vector signed int vec_any_ge (vector signed short, vector signed short); |
| |
| vector signed int vec_any_ge (vector signed int, vector unsigned int); |
| vector signed int vec_any_ge (vector unsigned int, vector signed int); |
| vector signed int vec_any_ge (vector unsigned int, vector unsigned int); |
| |
| vector signed int vec_any_ge (vector signed int, vector signed int); |
| vector signed int vec_any_ge (vector float, vector float); |
| |
| vector signed int vec_any_gt (vector signed char, vector unsigned char); |
| |
| vector signed int vec_any_gt (vector unsigned char, vector signed char); |
| |
| vector signed int vec_any_gt (vector unsigned char, |
| vector unsigned char); |
| vector signed int vec_any_gt (vector signed char, vector signed char); |
| vector signed int vec_any_gt (vector signed short, |
| vector unsigned short); |
| vector signed int vec_any_gt (vector unsigned short, |
| vector signed short); |
| vector signed int vec_any_gt (vector unsigned short, |
| vector unsigned short); |
| vector signed int vec_any_gt (vector signed short, vector signed short); |
| |
| vector signed int vec_any_gt (vector signed int, vector unsigned int); |
| vector signed int vec_any_gt (vector unsigned int, vector signed int); |
| vector signed int vec_any_gt (vector unsigned int, vector unsigned int); |
| |
| vector signed int vec_any_gt (vector signed int, vector signed int); |
| vector signed int vec_any_gt (vector float, vector float); |
| |
| vector signed int vec_any_le (vector signed char, vector unsigned char); |
| |
| vector signed int vec_any_le (vector unsigned char, vector signed char); |
| |
| vector signed int vec_any_le (vector unsigned char, |
| vector unsigned char); |
| vector signed int vec_any_le (vector signed char, vector signed char); |
| vector signed int vec_any_le (vector signed short, |
| vector unsigned short); |
| vector signed int vec_any_le (vector unsigned short, |
| vector signed short); |
| vector signed int vec_any_le (vector unsigned short, |
| vector unsigned short); |
| vector signed int vec_any_le (vector signed short, vector signed short); |
| |
| vector signed int vec_any_le (vector signed int, vector unsigned int); |
| vector signed int vec_any_le (vector unsigned int, vector signed int); |
| vector signed int vec_any_le (vector unsigned int, vector unsigned int); |
| |
| vector signed int vec_any_le (vector signed int, vector signed int); |
| vector signed int vec_any_le (vector float, vector float); |
| |
| vector signed int vec_any_lt (vector signed char, vector unsigned char); |
| |
| vector signed int vec_any_lt (vector unsigned char, vector signed char); |
| |
| vector signed int vec_any_lt (vector unsigned char, |
| vector unsigned char); |
| vector signed int vec_any_lt (vector signed char, vector signed char); |
| vector signed int vec_any_lt (vector signed short, |
| vector unsigned short); |
| vector signed int vec_any_lt (vector unsigned short, |
| vector signed short); |
| vector signed int vec_any_lt (vector unsigned short, |
| vector unsigned short); |
| vector signed int vec_any_lt (vector signed short, vector signed short); |
| |
| vector signed int vec_any_lt (vector signed int, vector unsigned int); |
| vector signed int vec_any_lt (vector unsigned int, vector signed int); |
| vector signed int vec_any_lt (vector unsigned int, vector unsigned int); |
| |
| vector signed int vec_any_lt (vector signed int, vector signed int); |
| vector signed int vec_any_lt (vector float, vector float); |
| |
| vector signed int vec_any_nan (vector float); |
| |
| vector signed int vec_any_ne (vector signed char, vector unsigned char); |
| |
| vector signed int vec_any_ne (vector signed char, vector signed char); |
| vector signed int vec_any_ne (vector unsigned char, vector signed char); |
| |
| vector signed int vec_any_ne (vector unsigned char, |
| vector unsigned char); |
| vector signed int vec_any_ne (vector signed short, |
| vector unsigned short); |
| vector signed int vec_any_ne (vector signed short, vector signed short); |
| |
| vector signed int vec_any_ne (vector unsigned short, |
| vector signed short); |
| vector signed int vec_any_ne (vector unsigned short, |
| vector unsigned short); |
| vector signed int vec_any_ne (vector signed int, vector unsigned int); |
| vector signed int vec_any_ne (vector signed int, vector signed int); |
| vector signed int vec_any_ne (vector unsigned int, vector signed int); |
| vector signed int vec_any_ne (vector unsigned int, vector unsigned int); |
| |
| vector signed int vec_any_ne (vector float, vector float); |
| |
| vector signed int vec_any_nge (vector float, vector float); |
| |
| vector signed int vec_any_ngt (vector float, vector float); |
| |
| vector signed int vec_any_nle (vector float, vector float); |
| |
| vector signed int vec_any_nlt (vector float, vector float); |
| |
| vector signed int vec_any_numeric (vector float); |
| |
| vector signed int vec_any_out (vector float, vector float); |
| @end smallexample |
| |
| @node Pragmas |
| @section Pragmas Accepted by GCC |
| @cindex pragmas |
| @cindex #pragma |
| |
| GCC supports several types of pragmas, primarily in order to compile |
| code originally written for other compilers. Note that in general |
| we do not recommend the use of pragmas; @xref{Function Attributes}, |
| for further explanation. |
| |
| @menu |
| * ARM Pragmas:: |
| * Darwin Pragmas:: |
| @end menu |
| |
| @node ARM Pragmas |
| @subsection ARM Pragmas |
| |
| The ARM target defines pragmas for controlling the default addition of |
| @code{long_call} and @code{short_call} attributes to functions. |
| @xref{Function Attributes}, for information about the effects of these |
| attributes. |
| |
| @table @code |
| @item long_calls |
| @cindex pragma, long_calls |
| Set all subsequent functions to have the @code{long_call} attribute. |
| |
| @item no_long_calls |
| @cindex pragma, no_long_calls |
| Set all subsequent functions to have the @code{short_call} attribute. |
| |
| @item long_calls_off |
| @cindex pragma, long_calls_off |
| Do not affect the @code{long_call} or @code{short_call} attributes of |
| subsequent functions. |
| @end table |
| |
| @c Describe c4x pragmas here. |
| @c Describe h8300 pragmas here. |
| @c Describe i370 pragmas here. |
| @c Describe i960 pragmas here. |
| @c Describe sh pragmas here. |
| @c Describe v850 pragmas here. |
| |
| @node Darwin Pragmas |
| @subsection Darwin Pragmas |
| |
| The following pragmas are available for all architectures running the |
| Darwin operating system. These are useful for compatibility with other |
| MacOS compilers. |
| |
| @table @code |
| @item mark @var{tokens}@dots{} |
| @cindex pragma, mark |
| This pragma is accepted, but has no effect. |
| |
| @item options align=@var{alignment} |
| @cindex pragma, options align |
| This pragma sets the alignment of fields in structures. The values of |
| @var{alignment} may be @code{mac68k}, to emulate m68k alignment, or |
| @code{power}, to emulate PowerPC alignment. Uses of this pragma nest |
| properly; to restore the previous setting, use @code{reset} for the |
| @var{alignment}. |
| |
| @item segment @var{tokens}@dots{} |
| @cindex pragma, segment |
| This pragma is accepted, but has no effect. |
| |
| @item unused (@var{var} [, @var{var}]@dots{}) |
| @cindex pragma, unused |
| This pragma declares variables to be possibly unused. GCC will not |
| produce warnings for the listed variables. The effect is similar to |
| that of the @code{unused} attribute, except that this pragma may appear |
| anywhere within the variables' scopes. |
| @end table |
| |
| @node Unnamed Fields |
| @section Unnamed struct/union fields within structs/unions. |
| @cindex struct |
| @cindex union |
| |
| For compatibility with other compilers, GCC allows you to define |
| a structure or union that contains, as fields, structures and unions |
| without names. For example: |
| |
| @example |
| struct @{ |
| int a; |
| union @{ |
| int b; |
| float c; |
| @}; |
| int d; |
| @} foo; |
| @end example |
| |
| In this example, the user would be able to access members of the unnamed |
| union with code like @samp{foo.b}. Note that only unnamed structs and |
| unions are allowed, you may not have, for example, an unnamed |
| @code{int}. |
| |
| You must never create such structures that cause ambiguous field definitions. |
| For example, this structure: |
| |
| @example |
| struct @{ |
| int a; |
| struct @{ |
| int a; |
| @}; |
| @} foo; |
| @end example |
| |
| It is ambiguous which @code{a} is being referred to with @samp{foo.a}. |
| Such constructs are not supported and must be avoided. In the future, |
| such constructs may be detected and treated as compilation errors. |
| |
| @node C++ Extensions |
| @chapter Extensions to the C++ Language |
| @cindex extensions, C++ language |
| @cindex C++ language extensions |
| |
| The GNU compiler provides these extensions to the C++ language (and you |
| can also use most of the C language extensions in your C++ programs). If you |
| want to write code that checks whether these features are available, you can |
| test for the GNU compiler the same way as for C programs: check for a |
| predefined macro @code{__GNUC__}. You can also use @code{__GNUG__} to |
| test specifically for GNU C++ (@pxref{Standard Predefined,,Standard |
| Predefined Macros,cpp.info,The C Preprocessor}). |
| |
| @menu |
| * Min and Max:: C++ Minimum and maximum operators. |
| * Volatiles:: What constitutes an access to a volatile object. |
| * Restricted Pointers:: C99 restricted pointers and references. |
| * Vague Linkage:: Where G++ puts inlines, vtables and such. |
| * C++ Interface:: You can use a single C++ header file for both |
| declarations and definitions. |
| * Template Instantiation:: Methods for ensuring that exactly one copy of |
| each needed template instantiation is emitted. |
| * Bound member functions:: You can extract a function pointer to the |
| method denoted by a @samp{->*} or @samp{.*} expression. |
| * C++ Attributes:: Variable, function, and type attributes for C++ only. |
| * Java Exceptions:: Tweaking exception handling to work with Java. |
| * Deprecated Features:: Things might disappear from g++. |
| * Backwards Compatibility:: Compatibilities with earlier definitions of C++. |
| @end menu |
| |
| @node Min and Max |
| @section Minimum and Maximum Operators in C++ |
| |
| It is very convenient to have operators which return the ``minimum'' or the |
| ``maximum'' of two arguments. In GNU C++ (but not in GNU C), |
| |
| @table @code |
| @item @var{a} <? @var{b} |
| @findex <? |
| @cindex minimum operator |
| is the @dfn{minimum}, returning the smaller of the numeric values |
| @var{a} and @var{b}; |
| |
| @item @var{a} >? @var{b} |
| @findex >? |
| @cindex maximum operator |
| is the @dfn{maximum}, returning the larger of the numeric values @var{a} |
| and @var{b}. |
| @end table |
| |
| These operations are not primitive in ordinary C++, since you can |
| use a macro to return the minimum of two things in C++, as in the |
| following example. |
| |
| @example |
| #define MIN(X,Y) ((X) < (Y) ? : (X) : (Y)) |
| @end example |
| |
| @noindent |
| You might then use @w{@samp{int min = MIN (i, j);}} to set @var{min} to |
| the minimum value of variables @var{i} and @var{j}. |
| |
| However, side effects in @code{X} or @code{Y} may cause unintended |
| behavior. For example, @code{MIN (i++, j++)} will fail, incrementing |
| the smaller counter twice. A GNU C extension allows you to write safe |
| macros that avoid this kind of problem (@pxref{Naming Types,,Naming an |
| Expression's Type}). However, writing @code{MIN} and @code{MAX} as |
| macros also forces you to use function-call notation for a |
| fundamental arithmetic operation. Using GNU C++ extensions, you can |
| write @w{@samp{int min = i <? j;}} instead. |
| |
| Since @code{<?} and @code{>?} are built into the compiler, they properly |
| handle expressions with side-effects; @w{@samp{int min = i++ <? j++;}} |
| works correctly. |
| |
| @node Volatiles |
| @section When is a Volatile Object Accessed? |
| @cindex accessing volatiles |
| @cindex volatile read |
| @cindex volatile write |
| @cindex volatile access |
| |
| Both the C and C++ standard have the concept of volatile objects. These |
| are normally accessed by pointers and used for accessing hardware. The |
| standards encourage compilers to refrain from optimizations |
| concerning accesses to volatile objects that it might perform on |
| non-volatile objects. The C standard leaves it implementation defined |
| as to what constitutes a volatile access. The C++ standard omits to |
| specify this, except to say that C++ should behave in a similar manner |
| to C with respect to volatiles, where possible. The minimum either |
| standard specifies is that at a sequence point all previous accesses to |
| volatile objects have stabilized and no subsequent accesses have |
| occurred. Thus an implementation is free to reorder and combine |
| volatile accesses which occur between sequence points, but cannot do so |
| for accesses across a sequence point. The use of volatiles does not |
| allow you to violate the restriction on updating objects multiple times |
| within a sequence point. |
| |
| In most expressions, it is intuitively obvious what is a read and what is |
| a write. For instance |
| |
| @example |
| volatile int *dst = @var{somevalue}; |
| volatile int *src = @var{someothervalue}; |
| *dst = *src; |
| @end example |
| |
| @noindent |
| will cause a read of the volatile object pointed to by @var{src} and stores the |
| value into the volatile object pointed to by @var{dst}. There is no |
| guarantee that these reads and writes are atomic, especially for objects |
| larger than @code{int}. |
| |
| Less obvious expressions are where something which looks like an access |
| is used in a void context. An example would be, |
| |
| @example |
| volatile int *src = @var{somevalue}; |
| *src; |
| @end example |
| |
| With C, such expressions are rvalues, and as rvalues cause a read of |
| the object, GCC interprets this as a read of the volatile being pointed |
| to. The C++ standard specifies that such expressions do not undergo |
| lvalue to rvalue conversion, and that the type of the dereferenced |
| object may be incomplete. The C++ standard does not specify explicitly |
| that it is this lvalue to rvalue conversion which is responsible for |
| causing an access. However, there is reason to believe that it is, |
| because otherwise certain simple expressions become undefined. However, |
| because it would surprise most programmers, G++ treats dereferencing a |
| pointer to volatile object of complete type in a void context as a read |
| of the object. When the object has incomplete type, G++ issues a |
| warning. |
| |
| @example |
| struct S; |
| struct T @{int m;@}; |
| volatile S *ptr1 = @var{somevalue}; |
| volatile T *ptr2 = @var{somevalue}; |
| *ptr1; |
| *ptr2; |
| @end example |
| |
| In this example, a warning is issued for @code{*ptr1}, and @code{*ptr2} |
| causes a read of the object pointed to. If you wish to force an error on |
| the first case, you must force a conversion to rvalue with, for instance |
| a static cast, @code{static_cast<S>(*ptr1)}. |
| |
| When using a reference to volatile, G++ does not treat equivalent |
| expressions as accesses to volatiles, but instead issues a warning that |
| no volatile is accessed. The rationale for this is that otherwise it |
| becomes difficult to determine where volatile access occur, and not |
| possible to ignore the return value from functions returning volatile |
| references. Again, if you wish to force a read, cast the reference to |
| an rvalue. |
| |
| @node Restricted Pointers |
| @section Restricting Pointer Aliasing |
| @cindex restricted pointers |
| @cindex restricted references |
| @cindex restricted this pointer |
| |
| As with gcc, g++ understands the C99 feature of restricted pointers, |
| specified with the @code{__restrict__}, or @code{__restrict} type |
| qualifier. Because you cannot compile C++ by specifying the @option{-std=c99} |
| language flag, @code{restrict} is not a keyword in C++. |
| |
| In addition to allowing restricted pointers, you can specify restricted |
| references, which indicate that the reference is not aliased in the local |
| context. |
| |
| @example |
| void fn (int *__restrict__ rptr, int &__restrict__ rref) |
| @{ |
| @dots{} |
| @} |
| @end example |
| |
| @noindent |
| In the body of @code{fn}, @var{rptr} points to an unaliased integer and |
| @var{rref} refers to a (different) unaliased integer. |
| |
| You may also specify whether a member function's @var{this} pointer is |
| unaliased by using @code{__restrict__} as a member function qualifier. |
| |
| @example |
| void T::fn () __restrict__ |
| @{ |
| @dots{} |
| @} |
| @end example |
| |
| @noindent |
| Within the body of @code{T::fn}, @var{this} will have the effective |
| definition @code{T *__restrict__ const this}. Notice that the |
| interpretation of a @code{__restrict__} member function qualifier is |
| different to that of @code{const} or @code{volatile} qualifier, in that it |
| is applied to the pointer rather than the object. This is consistent with |
| other compilers which implement restricted pointers. |
| |
| As with all outermost parameter qualifiers, @code{__restrict__} is |
| ignored in function definition matching. This means you only need to |
| specify @code{__restrict__} in a function definition, rather than |
| in a function prototype as well. |
| |
| @node Vague Linkage |
| @section Vague Linkage |
| @cindex vague linkage |
| |
| There are several constructs in C++ which require space in the object |
| file but are not clearly tied to a single translation unit. We say that |
| these constructs have ``vague linkage''. Typically such constructs are |
| emitted wherever they are needed, though sometimes we can be more |
| clever. |
| |
| @table @asis |
| @item Inline Functions |
| Inline functions are typically defined in a header file which can be |
| included in many different compilations. Hopefully they can usually be |
| inlined, but sometimes an out-of-line copy is necessary, if the address |
| of the function is taken or if inlining fails. In general, we emit an |
| out-of-line copy in all translation units where one is needed. As an |
| exception, we only emit inline virtual functions with the vtable, since |
| it will always require a copy. |
| |
| Local static variables and string constants used in an inline function |
| are also considered to have vague linkage, since they must be shared |
| between all inlined and out-of-line instances of the function. |
| |
| @item VTables |
| @cindex vtable |
| C++ virtual functions are implemented in most compilers using a lookup |
| table, known as a vtable. The vtable contains pointers to the virtual |
| functions provided by a class, and each object of the class contains a |
| pointer to its vtable (or vtables, in some multiple-inheritance |
| situations). If the class declares any non-inline, non-pure virtual |
| functions, the first one is chosen as the ``key method'' for the class, |
| and the vtable is only emitted in the translation unit where the key |
| method is defined. |
| |
| @emph{Note:} If the chosen key method is later defined as inline, the |
| vtable will still be emitted in every translation unit which defines it. |
| Make sure that any inline virtuals are declared inline in the class |
| body, even if they are not defined there. |
| |
| @item type_info objects |
| @cindex type_info |
| @cindex RTTI |
| C++ requires information about types to be written out in order to |
| implement @samp{dynamic_cast}, @samp{typeid} and exception handling. |
| For polymorphic classes (classes with virtual functions), the type_info |
| object is written out along with the vtable so that @samp{dynamic_cast} |
| can determine the dynamic type of a class object at runtime. For all |
| other types, we write out the type_info object when it is used: when |
| applying @samp{typeid} to an expression, throwing an object, or |
| referring to a type in a catch clause or exception specification. |
| |
| @item Template Instantiations |
| Most everything in this section also applies to template instantiations, |
| but there are other options as well. |
| @xref{Template Instantiation,,Where's the Template?}. |
| |
| @end table |
| |
| When used with GNU ld version 2.8 or later on an ELF system such as |
| Linux/GNU or Solaris 2, or on Microsoft Windows, duplicate copies of |
| these constructs will be discarded at link time. This is known as |
| COMDAT support. |
| |
| On targets that don't support COMDAT, but do support weak symbols, GCC |
| will use them. This way one copy will override all the others, but |
| the unused copies will still take up space in the executable. |
| |
| For targets which do not support either COMDAT or weak symbols, |
| most entities with vague linkage will be emitted as local symbols to |
| avoid duplicate definition errors from the linker. This will not happen |
| for local statics in inlines, however, as having multiple copies will |
| almost certainly break things. |
| |
| @xref{C++ Interface,,Declarations and Definitions in One Header}, for |
| another way to control placement of these constructs. |
| |
| @node C++ Interface |
| @section Declarations and Definitions in One Header |
| |
| @cindex interface and implementation headers, C++ |
| @cindex C++ interface and implementation headers |
| C++ object definitions can be quite complex. In principle, your source |
| code will need two kinds of things for each object that you use across |
| more than one source file. First, you need an @dfn{interface} |
| specification, describing its structure with type declarations and |
| function prototypes. Second, you need the @dfn{implementation} itself. |
| It can be tedious to maintain a separate interface description in a |
| header file, in parallel to the actual implementation. It is also |
| dangerous, since separate interface and implementation definitions may |
| not remain parallel. |
| |
| @cindex pragmas, interface and implementation |
| With GNU C++, you can use a single header file for both purposes. |
| |
| @quotation |
| @emph{Warning:} The mechanism to specify this is in transition. For the |
| nonce, you must use one of two @code{#pragma} commands; in a future |
| release of GNU C++, an alternative mechanism will make these |
| @code{#pragma} commands unnecessary. |
| @end quotation |
| |
| The header file contains the full definitions, but is marked with |
| @samp{#pragma interface} in the source code. This allows the compiler |
| to use the header file only as an interface specification when ordinary |
| source files incorporate it with @code{#include}. In the single source |
| file where the full implementation belongs, you can use either a naming |
| convention or @samp{#pragma implementation} to indicate this alternate |
| use of the header file. |
| |
| @table @code |
| @item #pragma interface |
| @itemx #pragma interface "@var{subdir}/@var{objects}.h" |
| @kindex #pragma interface |
| Use this directive in @emph{header files} that define object classes, to save |
| space in most of the object files that use those classes. Normally, |
| local copies of certain information (backup copies of inline member |
| functions, debugging information, and the internal tables that implement |
| virtual functions) must be kept in each object file that includes class |
| definitions. You can use this pragma to avoid such duplication. When a |
| header file containing @samp{#pragma interface} is included in a |
| compilation, this auxiliary information will not be generated (unless |
| the main input source file itself uses @samp{#pragma implementation}). |
| Instead, the object files will contain references to be resolved at link |
| time. |
| |
| The second form of this directive is useful for the case where you have |
| multiple headers with the same name in different directories. If you |
| use this form, you must specify the same string to @samp{#pragma |
| implementation}. |
| |
| @item #pragma implementation |
| @itemx #pragma implementation "@var{objects}.h" |
| @kindex #pragma implementation |
| Use this pragma in a @emph{main input file}, when you want full output from |
| included header files to be generated (and made globally visible). The |
| included header file, in turn, should use @samp{#pragma interface}. |
| Backup copies of inline member functions, debugging information, and the |
| internal tables used to implement virtual functions are all generated in |
| implementation files. |
| |
| @cindex implied @code{#pragma implementation} |
| @cindex @code{#pragma implementation}, implied |
| @cindex naming convention, implementation headers |
| If you use @samp{#pragma implementation} with no argument, it applies to |
| an include file with the same basename@footnote{A file's @dfn{basename} |
| was the name stripped of all leading path information and of trailing |
| suffixes, such as @samp{.h} or @samp{.C} or @samp{.cc}.} as your source |
| file. For example, in @file{allclass.cc}, giving just |
| @samp{#pragma implementation} |
| by itself is equivalent to @samp{#pragma implementation "allclass.h"}. |
| |
| In versions of GNU C++ prior to 2.6.0 @file{allclass.h} was treated as |
| an implementation file whenever you would include it from |
| @file{allclass.cc} even if you never specified @samp{#pragma |
| implementation}. This was deemed to be more trouble than it was worth, |
| however, and disabled. |
| |
| If you use an explicit @samp{#pragma implementation}, it must appear in |
| your source file @emph{before} you include the affected header files. |
| |
| Use the string argument if you want a single implementation file to |
| include code from multiple header files. (You must also use |
| @samp{#include} to include the header file; @samp{#pragma |
| implementation} only specifies how to use the file---it doesn't actually |
| include it.) |
| |
| There is no way to split up the contents of a single header file into |
| multiple implementation files. |
| @end table |
| |
| @cindex inlining and C++ pragmas |
| @cindex C++ pragmas, effect on inlining |
| @cindex pragmas in C++, effect on inlining |
| @samp{#pragma implementation} and @samp{#pragma interface} also have an |
| effect on function inlining. |
| |
| If you define a class in a header file marked with @samp{#pragma |
| interface}, the effect on a function defined in that class is similar to |
| an explicit @code{extern} declaration---the compiler emits no code at |
| all to define an independent version of the function. Its definition |
| is used only for inlining with its callers. |
| |
| @opindex fno-implement-inlines |
| Conversely, when you include the same header file in a main source file |
| that declares it as @samp{#pragma implementation}, the compiler emits |
| code for the function itself; this defines a version of the function |
| that can be found via pointers (or by callers compiled without |
| inlining). If all calls to the function can be inlined, you can avoid |
| emitting the function by compiling with @option{-fno-implement-inlines}. |
| If any calls were not inlined, you will get linker errors. |
| |
| @node Template Instantiation |
| @section Where's the Template? |
| |
| @cindex template instantiation |
| |
| C++ templates are the first language feature to require more |
| intelligence from the environment than one usually finds on a UNIX |
| system. Somehow the compiler and linker have to make sure that each |
| template instance occurs exactly once in the executable if it is needed, |
| and not at all otherwise. There are two basic approaches to this |
| problem, which I will refer to as the Borland model and the Cfront model. |
| |
| @table @asis |
| @item Borland model |
| Borland C++ solved the template instantiation problem by adding the code |
| equivalent of common blocks to their linker; the compiler emits template |
| instances in each translation unit that uses them, and the linker |
| collapses them together. The advantage of this model is that the linker |
| only has to consider the object files themselves; there is no external |
| complexity to worry about. This disadvantage is that compilation time |
| is increased because the template code is being compiled repeatedly. |
| Code written for this model tends to include definitions of all |
| templates in the header file, since they must be seen to be |
| instantiated. |
| |
| @item Cfront model |
| The AT&T C++ translator, Cfront, solved the template instantiation |
| problem by creating the notion of a template repository, an |
| automatically maintained place where template instances are stored. A |
| more modern version of the repository works as follows: As individual |
| object files are built, the compiler places any template definitions and |
| instantiations encountered in the repository. At link time, the link |
| wrapper adds in the objects in the repository and compiles any needed |
| instances that were not previously emitted. The advantages of this |
| model are more optimal compilation speed and the ability to use the |
| system linker; to implement the Borland model a compiler vendor also |
| needs to replace the linker. The disadvantages are vastly increased |
| complexity, and thus potential for error; for some code this can be |
| just as transparent, but in practice it can been very difficult to build |
| multiple programs in one directory and one program in multiple |
| directories. Code written for this model tends to separate definitions |
| of non-inline member templates into a separate file, which should be |
| compiled separately. |
| @end table |
| |
| When used with GNU ld version 2.8 or later on an ELF system such as |
| Linux/GNU or Solaris 2, or on Microsoft Windows, g++ supports the |
| Borland model. On other systems, g++ implements neither automatic |
| model. |
| |
| A future version of g++ will support a hybrid model whereby the compiler |
| will emit any instantiations for which the template definition is |
| included in the compile, and store template definitions and |
| instantiation context information into the object file for the rest. |
| The link wrapper will extract that information as necessary and invoke |
| the compiler to produce the remaining instantiations. The linker will |
| then combine duplicate instantiations. |
| |
| In the mean time, you have the following options for dealing with |
| template instantiations: |
| |
| @enumerate |
| @item |
| @opindex frepo |
| Compile your template-using code with @option{-frepo}. The compiler will |
| generate files with the extension @samp{.rpo} listing all of the |
| template instantiations used in the corresponding object files which |
| could be instantiated there; the link wrapper, @samp{collect2}, will |
| then update the @samp{.rpo} files to tell the compiler where to place |
| those instantiations and rebuild any affected object files. The |
| link-time overhead is negligible after the first pass, as the compiler |
| will continue to place the instantiations in the same files. |
| |
| This is your best option for application code written for the Borland |
| model, as it will just work. Code written for the Cfront model will |
| need to be modified so that the template definitions are available at |
| one or more points of instantiation; usually this is as simple as adding |
| @code{#include <tmethods.cc>} to the end of each template header. |
| |
| For library code, if you want the library to provide all of the template |
| instantiations it needs, just try to link all of its object files |
| together; the link will fail, but cause the instantiations to be |
| generated as a side effect. Be warned, however, that this may cause |
| conflicts if multiple libraries try to provide the same instantiations. |
| For greater control, use explicit instantiation as described in the next |
| option. |
| |
| @item |
| @opindex fno-implicit-templates |
| Compile your code with @option{-fno-implicit-templates} to disable the |
| implicit generation of template instances, and explicitly instantiate |
| all the ones you use. This approach requires more knowledge of exactly |
| which instances you need than do the others, but it's less |
| mysterious and allows greater control. You can scatter the explicit |
| instantiations throughout your program, perhaps putting them in the |
| translation units where the instances are used or the translation units |
| that define the templates themselves; you can put all of the explicit |
| instantiations you need into one big file; or you can create small files |
| like |
| |
| @example |
| #include "Foo.h" |
| #include "Foo.cc" |
| |
| template class Foo<int>; |
| template ostream& operator << |
| (ostream&, const Foo<int>&); |
| @end example |
| |
| for each of the instances you need, and create a template instantiation |
| library from those. |
| |
| If you are using Cfront-model code, you can probably get away with not |
| using @option{-fno-implicit-templates} when compiling files that don't |
| @samp{#include} the member template definitions. |
| |
| If you use one big file to do the instantiations, you may want to |
| compile it without @option{-fno-implicit-templates} so you get all of the |
| instances required by your explicit instantiations (but not by any |
| other files) without having to specify them as well. |
| |
| g++ has extended the template instantiation syntax outlined in the |
| Working Paper to allow forward declaration of explicit instantiations |
| (with @code{extern}), instantiation of the compiler support data for a |
| template class (i.e.@: the vtable) without instantiating any of its |
| members (with @code{inline}), and instantiation of only the static data |
| members of a template class, without the support data or member |
| functions (with (@code{static}): |
| |
| @example |
| extern template int max (int, int); |
| inline template class Foo<int>; |
| static template class Foo<int>; |
| @end example |
| |
| @item |
| Do nothing. Pretend g++ does implement automatic instantiation |
| management. Code written for the Borland model will work fine, but |
| each translation unit will contain instances of each of the templates it |
| uses. In a large program, this can lead to an unacceptable amount of code |
| duplication. |
| |
| @item |
| @opindex fexternal-templates |
| Add @samp{#pragma interface} to all files containing template |
| definitions. For each of these files, add @samp{#pragma implementation |
| "@var{filename}"} to the top of some @samp{.C} file which |
| @samp{#include}s it. Then compile everything with |
| @option{-fexternal-templates}. The templates will then only be expanded |
| in the translation unit which implements them (i.e.@: has a @samp{#pragma |
| implementation} line for the file where they live); all other files will |
| use external references. If you're lucky, everything should work |
| properly. If you get undefined symbol errors, you need to make sure |
| that each template instance which is used in the program is used in the |
| file which implements that template. If you don't have any use for a |
| particular instance in that file, you can just instantiate it |
| explicitly, using the syntax from the latest C++ working paper: |
| |
| @example |
| template class A<int>; |
| template ostream& operator << (ostream&, const A<int>&); |
| @end example |
| |
| This strategy will work with code written for either model. If you are |
| using code written for the Cfront model, the file containing a class |
| template and the file containing its member templates should be |
| implemented in the same translation unit. |
| |
| @item |
| @opindex falt-external-templates |
| A slight variation on this approach is to use the flag |
| @option{-falt-external-templates} instead. This flag causes template |
| instances to be emitted in the translation unit that implements the |
| header where they are first instantiated, rather than the one which |
| implements the file where the templates are defined. This header must |
| be the same in all translation units, or things are likely to break. |
| |
| @xref{C++ Interface,,Declarations and Definitions in One Header}, for |
| more discussion of these pragmas. |
| @end enumerate |
| |
| @node Bound member functions |
| @section Extracting the function pointer from a bound pointer to member function |
| |
| @cindex pmf |
| @cindex pointer to member function |
| @cindex bound pointer to member function |
| |
| In C++, pointer to member functions (PMFs) are implemented using a wide |
| pointer of sorts to handle all the possible call mechanisms; the PMF |
| needs to store information about how to adjust the @samp{this} pointer, |
| and if the function pointed to is virtual, where to find the vtable, and |
| where in the vtable to look for the member function. If you are using |
| PMFs in an inner loop, you should really reconsider that decision. If |
| that is not an option, you can extract the pointer to the function that |
| would be called for a given object/PMF pair and call it directly inside |
| the inner loop, to save a bit of time. |
| |
| Note that you will still be paying the penalty for the call through a |
| function pointer; on most modern architectures, such a call defeats the |
| branch prediction features of the CPU@. This is also true of normal |
| virtual function calls. |
| |
| The syntax for this extension is |
| |
| @example |
| extern A a; |
| extern int (A::*fp)(); |
| typedef int (*fptr)(A *); |
| |
| fptr p = (fptr)(a.*fp); |
| @end example |
| |
| For PMF constants (i.e.@: expressions of the form @samp{&Klasse::Member}), |
| no object is needed to obtain the address of the function. They can be |
| converted to function pointers directly: |
| |
| @example |
| fptr p1 = (fptr)(&A::foo); |
| @end example |
| |
| @opindex Wno-pmf-conversions |
| You must specify @option{-Wno-pmf-conversions} to use this extension. |
| |
| @node C++ Attributes |
| @section C++-Specific Variable, Function, and Type Attributes |
| |
| Some attributes only make sense for C++ programs. |
| |
| @table @code |
| @item init_priority (@var{priority}) |
| @cindex init_priority attribute |
| |
| |
| In Standard C++, objects defined at namespace scope are guaranteed to be |
| initialized in an order in strict accordance with that of their definitions |
| @emph{in a given translation unit}. No guarantee is made for initializations |
| across translation units. However, GNU C++ allows users to control the |
| order of initialization of objects defined at namespace scope with the |
| @code{init_priority} attribute by specifying a relative @var{priority}, |
| a constant integral expression currently bounded between 101 and 65535 |
| inclusive. Lower numbers indicate a higher priority. |
| |
| In the following example, @code{A} would normally be created before |
| @code{B}, but the @code{init_priority} attribute has reversed that order: |
| |
| @example |
| Some_Class A __attribute__ ((init_priority (2000))); |
| Some_Class B __attribute__ ((init_priority (543))); |
| @end example |
| |
| @noindent |
| Note that the particular values of @var{priority} do not matter; only their |
| relative ordering. |
| |
| @item java_interface |
| @cindex java_interface attribute |
| |
| This type attribute informs C++ that the class is a Java interface. It may |
| only be applied to classes declared within an @code{extern "Java"} block. |
| Calls to methods declared in this interface will be dispatched using GCJ's |
| interface table mechanism, instead of regular virtual table dispatch. |
| |
| @end table |
| |
| @node Java Exceptions |
| @section Java Exceptions |
| |
| The Java language uses a slightly different exception handling model |
| from C++. Normally, GNU C++ will automatically detect when you are |
| writing C++ code that uses Java exceptions, and handle them |
| appropriately. However, if C++ code only needs to execute destructors |
| when Java exceptions are thrown through it, GCC will guess incorrectly. |
| Sample problematic code is: |
| |
| @example |
| struct S @{ ~S(); @}; |
| extern void bar(); // is written in Java, and may throw exceptions |
| void foo() |
| @{ |
| S s; |
| bar(); |
| @} |
| @end example |
| |
| @noindent |
| The usual effect of an incorrect guess is a link failure, complaining of |
| a missing routine called @samp{__gxx_personality_v0}. |
| |
| You can inform the compiler that Java exceptions are to be used in a |
| translation unit, irrespective of what it might think, by writing |
| @samp{@w{#pragma GCC java_exceptions}} at the head of the file. This |
| @samp{#pragma} must appear before any functions that throw or catch |
| exceptions, or run destructors when exceptions are thrown through them. |
| |
| You cannot mix Java and C++ exceptions in the same translation unit. It |
| is believed to be safe to throw a C++ exception from one file through |
| another file compiled for the Java exception model, or vice versa, but |
| there may be bugs in this area. |
| |
| @node Deprecated Features |
| @section Deprecated Features |
| |
| In the past, the GNU C++ compiler was extended to experiment with new |
| features, at a time when the C++ language was still evolving. Now that |
| the C++ standard is complete, some of those features are superseded by |
| superior alternatives. Using the old features might cause a warning in |
| some cases that the feature will be dropped in the future. In other |
| cases, the feature might be gone already. |
| |
| While the list below is not exhaustive, it documents some of the options |
| that are now deprecated: |
| |
| @table @code |
| @item -fexternal-templates |
| @itemx -falt-external-templates |
| These are two of the many ways for g++ to implement template |
| instantiation. @xref{Template Instantiation}. The C++ standard clearly |
| defines how template definitions have to be organized across |
| implementation units. g++ has an implicit instantiation mechanism that |
| should work just fine for standard-conforming code. |
| |
| @item -fstrict-prototype |
| @itemx -fno-strict-prototype |
| Previously it was possible to use an empty prototype parameter list to |
| indicate an unspecified number of parameters (like C), rather than no |
| parameters, as C++ demands. This feature has been removed, except where |
| it is required for backwards compatibility @xref{Backwards Compatibility}. |
| @end table |
| |
| The named return value extension has been deprecated, and is now |
| removed from g++. |
| |
| The use of initializer lists with new expressions has been deprecated, |
| and is now removed from g++. |
| |
| Floating and complex non-type template parameters have been deprecated, |
| and are now removed from g++. |
| |
| The implicit typename extension has been deprecated and will be removed |
| from g++ at some point. In some cases g++ determines that a dependant |
| type such as @code{TPL<T>::X} is a type without needing a |
| @code{typename} keyword, contrary to the standard. |
| |
| @node Backwards Compatibility |
| @section Backwards Compatibility |
| @cindex Backwards Compatibility |
| @cindex ARM [Annotated C++ Reference Manual] |
| |
| Now that there is a definitive ISO standard C++, G++ has a specification |
| to adhere to. The C++ language evolved over time, and features that |
| used to be acceptable in previous drafts of the standard, such as the ARM |
| [Annotated C++ Reference Manual], are no longer accepted. In order to allow |
| compilation of C++ written to such drafts, G++ contains some backwards |
| compatibilities. @emph{All such backwards compatibility features are |
| liable to disappear in future versions of G++.} They should be considered |
| deprecated @xref{Deprecated Features}. |
| |
| @table @code |
| @item For scope |
| If a variable is declared at for scope, it used to remain in scope until |
| the end of the scope which contained the for statement (rather than just |
| within the for scope). G++ retains this, but issues a warning, if such a |
| variable is accessed outside the for scope. |
| |
| @item Implicit C language |
| Old C system header files did not contain an @code{extern "C" @{@dots{}@}} |
| scope to set the language. On such systems, all header files are |
| implicitly scoped inside a C language scope. Also, an empty prototype |
| @code{()} will be treated as an unspecified number of arguments, rather |
| than no arguments, as C++ demands. |
| @end table |