gcc/gcc.info-9

This is Info file gcc.info, produced by Makeinfo version 1.68 from the
input file gcc.texi.

   This file documents the use and the internals of the GNU compiler.

   Published by the Free Software Foundation 59 Temple Place - Suite 330
Boston, MA 02111-1307 USA

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Free Software Foundation, Inc.

   Permission is granted to make and distribute verbatim copies of this
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   Permission is granted to copy and distribute translations of this
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File: gcc.info,  Node: Variable Length,  Next: Macro Varargs,  Prev: Zero Length,  Up: C Extensions

Arrays of Variable Length
=========================

   Variable-length automatic arrays are allowed in GNU C.  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:

     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);
     }

   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.

   You can use the function `alloca' to get an effect much like
variable-length arrays.  The function `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 `alloca' exists until the containing *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
`alloca' in the same function, deallocation of a variable-length array
will also deallocate anything more recently allocated with `alloca'.)

   You can also use variable-length arrays as arguments to functions:

     struct entry
     tester (int len, char data[len][len])
     {
       ...
     }

   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
`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.

     struct entry
     tester (int len; char data[len][len], int len)
     {
       ...
     }

   The `int len' before the semicolon is a "parameter forward
declaration", and it serves the purpose of making the name `len' known
when the declaration of `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.


File: gcc.info,  Node: Macro Varargs,  Next: Subscripting,  Prev: Variable Length,  Up: C Extensions

Macros with Variable Numbers of Arguments
=========================================

   In GNU C, a macro can accept a variable number of arguments, much as
a function can.  The syntax for defining the macro looks much like that
used for a function.  Here is an example:

     #define eprintf(format, args...)  \
      fprintf (stderr, format , ## args)

   Here `args' is a "rest argument": it takes in zero or more
arguments, as many as the call contains.  All of them plus the commas
between them form the value of `args', which is substituted into the
macro body where `args' is used.  Thus, we have this expansion:

     eprintf ("%s:%d: ", input_file_name, line_number)
     ==>
     fprintf (stderr, "%s:%d: " , input_file_name, line_number)

Note that the comma after the string constant comes from the definition
of `eprintf', whereas the last comma comes from the value of `args'.

   The reason for using `##' is to handle the case when `args' matches
no arguments at all.  In this case, `args' has an empty value.  In this
case, the second comma in the definition becomes an embarrassment: if
it got through to the expansion of the macro, we would get something
like this:

     fprintf (stderr, "success!\n" , )

which is invalid C syntax.  `##' gets rid of the comma, so we get the
following instead:

     fprintf (stderr, "success!\n")

   This is a special feature of the GNU C preprocessor: `##' before a
rest argument that is empty discards the preceding sequence of
non-whitespace characters from the macro definition.  (If another macro
argument precedes, none of it is discarded.)

   It might be better to discard the last preprocessor token instead of
the last preceding sequence of non-whitespace characters; in fact, we
may someday change this feature to do so.  We advise you to write the
macro definition so that the preceding sequence of non-whitespace
characters is just a single token, so that the meaning will not change
if we change the definition of this feature.


File: gcc.info,  Node: Subscripting,  Next: Pointer Arith,  Prev: Macro Varargs,  Up: C Extensions

Non-Lvalue Arrays May Have Subscripts
=====================================

   Subscripting is allowed on arrays that are not lvalues, even though
the unary `&' operator is not.  For example, this is valid in GNU C
though not valid in other C dialects:

     struct foo {int a[4];};
     
     struct foo f();
     
     bar (int index)
     {
       return f().a[index];
     }


File: gcc.info,  Node: Pointer Arith,  Next: Initializers,  Prev: Subscripting,  Up: C Extensions

Arithmetic on `void'- and Function-Pointers
===========================================

   In GNU C, addition and subtraction operations are supported on
pointers to `void' and on pointers to functions.  This is done by
treating the size of a `void' or of a function as 1.

   A consequence of this is that `sizeof' is also allowed on `void' and
on function types, and returns 1.

   The option `-Wpointer-arith' requests a warning if these extensions
are used.


File: gcc.info,  Node: Initializers,  Next: Constructors,  Prev: Pointer Arith,  Up: C Extensions

Non-Constant Initializers
=========================

   As in standard C++, 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:

     foo (float f, float g)
     {
       float beat_freqs[2] = { f-g, f+g };
       ...
     }


File: gcc.info,  Node: Constructors,  Next: Labeled Elements,  Prev: Initializers,  Up: C Extensions

Constructor Expressions
=======================

   GNU C supports constructor expressions.  A constructor 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.

   Usually, the specified type is a structure.  Assume that `struct
foo' and `structure' are declared as shown:

     struct foo {int a; char b[2];} structure;

Here is an example of constructing a `struct foo' with a constructor:

     structure = ((struct foo) {x + y, 'a', 0});

This is equivalent to writing the following:

     {
       struct foo temp = {x + y, 'a', 0};
       structure = temp;
     }

   You can also construct an array.  If all the elements of the
constructor are (made up of) simple constant expressions, suitable for
use in initializers, then the constructor is an lvalue and can be
coerced to a pointer to its first element, as shown here:

     char **foo = (char *[]) { "x", "y", "z" };

   Array constructors whose elements are not simple constants are not
very useful, because the constructor is not an lvalue.  There are only
two valid ways to use it: to subscript it, or initialize an array
variable with it.  The former is probably slower than a `switch'
statement, while the latter does the same thing an ordinary C
initializer would do.  Here is an example of subscripting an array
constructor:

     output = ((int[]) { 2, x, 28 }) [input];

   Constructor expressions for scalar types and union types are is also
allowed, but then the constructor expression is equivalent to a cast.


File: gcc.info,  Node: Labeled Elements,  Next: Cast to Union,  Prev: Constructors,  Up: C Extensions

Labeled Elements in Initializers
================================

   Standard C 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 GNU C you can give the elements in any order, specifying the array
indices or structure field names they apply to.  This extension is not
implemented in GNU C++.

   To specify an array index, write `[INDEX]' or `[INDEX] =' before the
element value.  For example,

     int a[6] = { [4] 29, [2] = 15 };

is equivalent to

     int a[6] = { 0, 0, 15, 0, 29, 0 };

The index values must be constant expressions, even if the array being
initialized is automatic.

   To initialize a range of elements to the same value, write `[FIRST
... LAST] = VALUE'.  For example,

     int widths[] = { [0 ... 9] = 1, [10 ... 99] = 2, [100] = 3 };

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 `FIELDNAME:' before the element value.  For example, given the
following structure,

     struct point { int x, y; };

the following initialization

     struct point p = { y: yvalue, x: xvalue };

is equivalent to

     struct point p = { xvalue, yvalue };

   Another syntax which has the same meaning is `.FIELDNAME ='., as
shown here:

     struct point p = { .y = yvalue, .x = xvalue };

   You can also use an element label (with either the colon syntax or
the period-equal syntax) when initializing a union, to specify which
element of the union should be used.  For example,

     union foo { int i; double d; };
     
     union foo f = { d: 4 };

will convert 4 to a `double' to store it in the union using the second
element.  By contrast, casting 4 to type `union foo' would store it
into the union as the integer `i', since it is an integer.  (*Note 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 label applies to the next consecutive element of the
array or structure.  For example,

     int a[6] = { [1] = v1, v2, [4] = v4 };

is equivalent to

     int a[6] = { 0, v1, v2, 0, v4, 0 };

   Labeling the elements of an array initializer is especially useful
when the indices are characters or belong to an `enum' type.  For
example:

     int whitespace[256]
       = { [' '] = 1, ['\t'] = 1, ['\h'] = 1,
           ['\f'] = 1, ['\n'] = 1, ['\r'] = 1 };


File: gcc.info,  Node: Case Ranges,  Next: Function Attributes,  Prev: Cast to Union,  Up: C Extensions

Case Ranges
===========

   You can specify a range of consecutive values in a single `case'
label, like this:

     case LOW ... HIGH:

This has the same effect as the proper number of individual `case'
labels, one for each integer value from LOW to HIGH, inclusive.

   This feature is especially useful for ranges of ASCII character
codes:

     case 'A' ... 'Z':

   *Be careful:* Write spaces around the `...', for otherwise it may be
parsed wrong when you use it with integer values.  For example, write
this:

     case 1 ... 5:

rather than this:

     case 1...5:


File: gcc.info,  Node: Cast to Union,  Next: Case Ranges,  Prev: Labeled Elements,  Up: C Extensions

Cast to a Union Type
====================

   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 `union
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.  (*Note Constructors::.)

   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:

     union foo { int i; double d; };
     int x;
     double y;

both `x' and `y' can be cast to type `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:

     union foo u;
     ...
     u = (union foo) x  ==  u.i = x
     u = (union foo) y  ==  u.d = y

   You can also use the union cast as a function argument:

     void hack (union foo);
     ...
     hack ((union foo) x);


File: gcc.info,  Node: Function Attributes,  Next: Function Prototypes,  Prev: Case Ranges,  Up: C Extensions

Declaring Attributes of Functions
=================================

   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 `__attribute__' allows you to specify special attributes
when making a declaration.  This keyword is followed by an attribute
specification inside double parentheses.  Eight attributes, `noreturn',
`const', `format', `section', `constructor', `destructor', `unused' and
`weak' are currently defined for functions.  Other attributes, including
`section' are supported for variables declarations (*note Variable
Attributes::.) and for types (*note Type Attributes::.).

   You may also specify attributes with `__' 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 `__noreturn__' instead of `noreturn'.

`noreturn'
     A few standard library functions, such as `abort' and `exit',
     cannot return.  GNU CC knows this automatically.  Some programs
     define their own functions that never return.  You can declare them
     `noreturn' to tell the compiler this fact.  For example,

          void fatal () __attribute__ ((noreturn));
          
          void
          fatal (...)
          {
            ... /* Print error message. */ ...
            exit (1);
          }

     The `noreturn' keyword tells the compiler to assume that `fatal'
     cannot return.  It can then optimize without regard to what would
     happen if `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 `noreturn' function.

     It does not make sense for a `noreturn' function to have a return
     type other than `void'.

     The attribute `noreturn' is not implemented in GNU C 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:

          typedef void voidfn ();
          
          volatile voidfn fatal;

`const'
     Many functions do not examine any values except their arguments,
     and have no effects except the return value.  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 `const'.  For
     example,

          int square (int) __attribute__ ((const));

     says that the hypothetical function `square' is safe to call fewer
     times than the program says.

     The attribute `const' is not implemented in GNU C 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:

          typedef int intfn ();
          
          extern const intfn square;

     This approach does not work in GNU C++ from 2.6.0 on, since the
     language specifies that the `const' must be attached to the return
     value.

     Note that a function that has pointer arguments and examines the
     data pointed to must *not* be declared `const'.  Likewise, a
     function that calls a non-`const' function usually must not be
     `const'.  It does not make sense for a `const' function to return
     `void'.

`format (ARCHETYPE, STRING-INDEX, FIRST-TO-CHECK)'
     The `format' attribute specifies that a function takes `printf' or
     `scanf' style arguments which should be type-checked against a
     format string.  For example, the declaration:

          extern int
          my_printf (void *my_object, const char *my_format, ...)
                __attribute__ ((format (printf, 2, 3)));

     causes the compiler to check the arguments in calls to `my_printf'
     for consistency with the `printf' style format string argument
     `my_format'.

     The parameter ARCHETYPE determines how the format string is
     interpreted, and should be either `printf' or `scanf'.  The
     parameter STRING-INDEX specifies which argument is the format
     string argument (starting from 1), while 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 `vprintf'), specify the third parameter as zero.  In this
     case the compiler only checks the format string for consistency.

     In the example above, the format string (`my_format') is the second
     argument of the function `my_print', and the arguments to check
     start with the third argument, so the correct parameters for the
     format attribute are 2 and 3.

     The `format' attribute allows you to identify your own functions
     which take format strings as arguments, so that GNU CC can check
     the calls to these functions for errors.  The compiler always
     checks formats for the ANSI library functions `printf', `fprintf',
     `sprintf', `scanf', `fscanf', `sscanf', `vprintf', `vfprintf' and
     `vsprintf' whenever such warnings are requested (using
     `-Wformat'), so there is no need to modify the header file
     `stdio.h'.

`format_arg (STRING-INDEX)'
     The `format_arg' attribute specifies that a function takes
     `printf' or `scanf' style arguments, modifies it (for example, to
     translate it into another language), and passes it to a `printf'
     or `scanf' style function.  For example, the declaration:

          extern char *
          my_dgettext (char *my_domain, const char *my_format)
                __attribute__ ((format_arg (2)));

     causes the compiler to check the arguments in calls to
     `my_dgettext' whose result is passed to a `printf' or `scanf' type
     function for consistency with the `printf' style format string
     argument `my_format'.

     The parameter STRING-INDEX specifies which argument is the format
     string argument (starting from 1).

     The `format-arg' attribute allows you to identify your own
     functions which modify format strings, so that GNU CC can check the
     calls to `printf' and `scanf' function whose operands are a call
     to one of your own function.  The compiler always treats
     `gettext', `dgettext', and `dcgettext' in this manner.

`section ("section-name")'
     Normally, the compiler places the code it generates in the `text'
     section.  Sometimes, however, you need additional sections, or you
     need certain particular functions to appear in special sections.
     The `section' attribute specifies that a function lives in a
     particular section.  For example, the declaration:

          extern void foobar (void) __attribute__ ((section ("bar")));

     puts the function `foobar' in the `bar' section.

     Some file formats do not support arbitrary sections so the
     `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.

`constructor'
`destructor'
     The `constructor' attribute causes the function to be called
     automatically before execution enters `main ()'.  Similarly, the
     `destructor' attribute causes the function to be called
     automatically after `main ()' has completed or `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.

`unused'
     This attribute, attached to a function, means that the function is
     meant to be possibly unused.  GNU CC will not produce a warning
     for this function.  GNU C++ does not currently support this
     attribute as definitions without parameters are valid in C++.

`weak'
     The `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.

`alias ("target")'
     The `alias' attribute causes the declaration to be emitted as an
     alias for another symbol, which must be specified.  For instance,

          void __f () { /* do something */; }
          void f () __attribute__ ((weak, alias ("__f")));

     declares `f' to be a weak alias for `__f'.  In C++, the mangled
     name for the target must be used.

     Not all target machines support this attribute.

`regparm (NUMBER)'
     On the Intel 386, the `regparm' attribute causes the compiler to
     pass up to 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.

`stdcall'
     On the Intel 386, the `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 `stdcall'
     attribute.

`cdecl'
     On the Intel 386, the `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
     `-mrtd' switch.

     The PowerPC compiler for Windows NT currently ignores the `cdecl'
     attribute.

`longcall'
     On the RS/6000 and PowerPC, the `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.

`dllimport'
     On the PowerPC running Windows NT, the `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 `__imp_' and the function
     name.

`dllexport'
     On the PowerPC running Windows NT, the `dllexport' attribute causes
     the compiler to provide a global pointer to the function pointer,
     so that it can be called with the `dllimport' attribute.  The
     pointer name is formed by combining `__imp_' and the function name.

`exception (EXCEPT-FUNC [, EXCEPT-ARG])'
     On the PowerPC running Windows NT, the `exception' attribute causes
     the compiler to modify the structured exception table entry it
     emits for the declared function.  The string or identifier
     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 EXCEPT-ARG is placed in the
     fourth entry of the structured exception table.

`function_vector'
     Use this option 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 option to work correctly.

`interrupt_handler'
     Use this option on the H8/300 and H8/300H 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.

`eightbit_data'
     Use this option 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 option to work correctly.

`tiny_data'
     Use this option 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.

`interrupt'
     Use this option on the M32R/D 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.

`model (MODEL-NAME)'
     Use this attribute on the M32R/D to set the addressability of an
     object, and the code generated for a function.  The identifier
     MODEL-NAME is one of `small', `medium', or `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 `ld24' instruction), and are
     callable with the `bl' instruction.

     Medium model objects may live anywhere in the 32 bit address space
     (the compiler will generate `seth/add3' instructions to load their
     addresses), and are callable with the `bl' instruction.

     Large model objects may live anywhere in the 32 bit address space
     (the compiler will generate `seth/add3' instructions to load their
     addresses), and may not be reachable with the `bl' instruction
     (the compiler will generate the much slower `seth/add3/jl'
     instruction sequence).

   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.

   Some people object to the `__attribute__' feature, suggesting that
ANSI C's `#pragma' should be used instead.  There are two reasons for
not doing this.

  1. It is impossible to generate `#pragma' commands from a macro.

  2. There is no telling what the same `#pragma' might mean in another
     compiler.

   These two reasons apply to almost any application that might be
proposed for `#pragma'.  It is basically a mistake to use `#pragma' for
*anything*.


File: gcc.info,  Node: Function Prototypes,  Next: C++ Comments,  Prev: Function Attributes,  Up: C Extensions

Prototypes and Old-Style Function Definitions
=============================================

   GNU C extends ANSI C to allow a function prototype to override a
later old-style non-prototype definition.  Consider the following
example:

     /* Use prototypes unless the compiler is old-fashioned.  */
     #ifdef __STDC__
     #define P(x) x
     #else
     #define P(x) ()
     #endif
     
     /* Prototype function declaration.  */
     int isroot P((uid_t));
     
     /* Old-style function definition.  */
     int
     isroot (x)   /* ??? lossage here ??? */
          uid_t x;
     {
       return x == 0;
     }

   Suppose the type `uid_t' happens to be `short'.  ANSI 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 `int', which does not match
the prototype argument type of `short'.

   This restriction of ANSI C makes it hard to write code that is
portable to traditional C compilers, because the programmer does not
know whether the `uid_t' type is `short', `int', or `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:

     int isroot (uid_t);
     
     int
     isroot (uid_t x)
     {
       return x == 0;
     }

   GNU C++ does not support old-style function definitions, so this
extension is irrelevant.


File: gcc.info,  Node: C++ Comments,  Next: Dollar Signs,  Prev: Function Prototypes,  Up: C Extensions

C++ Style Comments
==================

   In GNU C, you may use C++ style comments, which start with `//' and
continue until the end of the line.  Many other C implementations allow
such comments, and they are likely to be in a future C standard.
However, C++ style comments are not recognized if you specify `-ansi'
or `-traditional', since they are incompatible with traditional
constructs like `dividend//*comment*/divisor'.


File: gcc.info,  Node: Dollar Signs,  Next: Character Escapes,  Prev: C++ Comments,  Up: C Extensions

Dollar Signs in Identifier Names
================================

   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.


File: gcc.info,  Node: Character Escapes,  Next: Variable Attributes,  Prev: Dollar Signs,  Up: C Extensions

The Character <ESC> in Constants
================================

   You can use the sequence `\e' in a string or character constant to
stand for the ASCII character <ESC>.


File: gcc.info,  Node: Alignment,  Next: Inline,  Prev: Type Attributes,  Up: C Extensions

Inquiring on Alignment of Types or Variables
============================================

   The keyword `__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 `sizeof'.

   For example, if the target machine requires a `double' value to be
aligned on an 8-byte boundary, then `__alignof__ (double)' is 8.  This
is true on many RISC machines.  On more traditional machine designs,
`__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,
`__alignof__' reports the *recommended* alignment of a type.

   When the operand of `__alignof__' is an lvalue rather than a type,
the value is the largest alignment that the lvalue is known to have.
It may have this alignment as a result of its data type, or because it
is part of a structure and inherits alignment from that structure.  For
example, after this declaration:

     struct foo { int x; char y; } foo1;

the value of `__alignof__ (foo1.y)' is probably 2 or 4, the same as
`__alignof__ (int)', even though the data type of `foo1.y' does not
itself demand any alignment.

   A related feature which lets you specify the alignment of an object
is `__attribute__ ((aligned (ALIGNMENT)))'; see the following section.


File: gcc.info,  Node: Variable Attributes,  Next: Type Attributes,  Prev: Character Escapes,  Up: C Extensions

Specifying Attributes of Variables
==================================

   The keyword `__attribute__' allows you to specify special attributes
of variables or structure fields.  This keyword is followed by an
attribute specification inside double parentheses.  Eight attributes
are currently defined for variables: `aligned', `mode', `nocommon',
`packed', `section', `transparent_union', `unused', and `weak'.  Other
attributes are available for functions (*note Function Attributes::.)
and for types (*note Type Attributes::.).

   You may also specify attributes with `__' 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 `__aligned__' instead of `aligned'.

`aligned (ALIGNMENT)'
     This attribute specifies a minimum alignment for the variable or
     structure field, measured in bytes.  For example, the declaration:

          int x __attribute__ ((aligned (16))) = 0;

     causes the compiler to allocate the global variable `x' on a
     16-byte boundary.  On a 68040, this could be used in conjunction
     with an `asm' expression to access the `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 `int' pair, you could
     write:

          struct foo { int x[2] __attribute__ ((aligned (8))); };

     This is an alternative to creating a union with a `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:

          short array[3] __attribute__ ((aligned));

     Whenever you leave out the alignment factor in an `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 `aligned' attribute can only increase the alignment; but you
     can decrease it by specifying `packed' as well.  See below.

     Note that the effectiveness of `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 `aligned(16)' in an `__attribute__' will still
     only provide you with 8 byte alignment.  See your linker
     documentation for further information.

`mode (MODE)'
     This attribute specifies the data type for the
     declaration--whichever type corresponds to the mode MODE.  This in
     effect lets you request an integer or floating point type
     according to its width.

     You may also specify a mode of `byte' or `__byte__' to indicate
     the mode corresponding to a one-byte integer, `word' or `__word__'
     for the mode of a one-word integer, and `pointer' or `__pointer__'
     for the mode used to represent pointers.

`nocommon'
     This attribute specifies requests GNU CC not to place a variable
     "common" but instead to allocate space for it directly.  If you
     specify the `-fno-common' flag, GNU CC will do this for all
     variables.

     Specifying the `nocommon' attribute for a variable provides an
     initialization of zeros.  A variable may only be initialized in one
     source file.

`packed'
     The `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 `aligned' attribute.

     Here is a structure in which the field `x' is packed, so that it
     immediately follows `a':

          struct foo
          {
            char a;
            int x[2] __attribute__ ((packed));
          };

`section ("section-name")'
     Normally, the compiler places the objects it generates in sections
     like `data' and `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
     `section' attribute specifies that a variable (or function) lives
     in a particular section.  For example, this small program uses
     several specific section names:

          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);
          }

     Use the `section' attribute with an *initialized* definition of a
     *global* variable, as shown in the example.  GNU CC issues a
     warning and otherwise ignores the `section' attribute in
     uninitialized variable declarations.

     You may only use the `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 `common' (or `bss')
     section and can be multiply "defined".  You can force a variable
     to be initialized with the `-fno-common' flag or the `nocommon'
     attribute.

     Some file formats do not support arbitrary sections so the
     `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.

`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 *Note Type
     Attributes::.  You can also use this attribute on a `typedef' for
     a union data type; then it applies to all function parameters with
     that type.

`unused'
     This attribute, attached to a variable, means that the variable is
     meant to be possibly unused.  GNU CC will not produce a warning
     for this variable.

`weak'
     The `weak' attribute is described in *Note Function Attributes::.

`model (MODEL-NAME)'
     Use this attribute on the M32R/D to set the addressability of an
     object.  The identifier MODEL-NAME is one of `small', `medium', or
     `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 `ld24' instruction).

     Medium and large model objects may live anywhere in the 32 bit
     address space (the compiler will generate `seth/add3' instructions
     to load their addresses).

   To specify multiple attributes, separate them by commas within the
double parentheses: for example, `__attribute__ ((aligned (16),
packed))'.


File: gcc.info,  Node: Type Attributes,  Next: Alignment,  Prev: Variable Attributes,  Up: C Extensions

Specifying Attributes of Types
==============================

   The keyword `__attribute__' allows you to specify special attributes
of `struct' and `union' types when you define such types.  This keyword
is followed by an attribute specification inside double parentheses.
Three attributes are currently defined for types: `aligned', `packed',
and `transparent_union'.  Other attributes are defined for functions
(*note Function Attributes::.) and for variables (*note Variable
Attributes::.).

   You may also specify any one of these attributes with `__' 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 `__aligned__' instead of `aligned'.

   You may specify the `aligned' and `transparent_union' attributes
either in a `typedef' declaration or just past the closing curly brace
of a complete enum, struct or union type *definition* and the `packed'
attribute only past the closing brace of a definition.

`aligned (ALIGNMENT)'
     This attribute specifies a minimum alignment (in bytes) for
     variables of the specified type.  For example, the declarations:

          struct S { short f[3]; } __attribute__ ((aligned (8));
          typedef int more_aligned_int __attribute__ ((aligned (8));

     force the compiler to insure (as far as it can) that each variable
     whose type is `struct S' or `more_aligned_int' will be allocated
     and aligned *at least* on a 8-byte boundary.  On a Sparc, having
     all variables of type `struct S' aligned to 8-byte boundaries
     allows the compiler to use the `ldd' and `std' (doubleword load and
     store) instructions when copying one variable of type `struct S' to
     another, thus improving run-time efficiency.

     Note that the alignment of any given `struct' or `union' type is
     required by the ANSI C standard to be at least a perfect multiple
     of the lowest common multiple of the alignments of all of the
     members of the `struct' or `union' in question.  This means that
     you *can* effectively adjust the alignment of a `struct' or `union'
     type by attaching an `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 `struct' or `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
     `struct' or `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:

          struct S { short f[3]; } __attribute__ ((aligned));

     Whenever you leave out the alignment factor in an `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 `short' is 2 bytes, then
     the size of the entire `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 `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 `aligned' attribute can only increase the alignment; but you
     can decrease it by specifying `packed' as well.  See below.

     Note that the effectiveness of `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 `aligned(16)' in an `__attribute__' will still
     only provide you with 8 byte alignment.  See your linker
     documentation for further information.

`packed'
     This attribute, attached to an `enum', `struct', or `union' type
     definition, specified that the minimum required memory be used to
     represent the type.

     Specifying this attribute for `struct' and `union' types is
     equivalent to specifying the `packed' attribute on each of the
     structure or union members.  Specifying the `-fshort-enums' flag
     on the line is equivalent to specifying the `packed' attribute on
     all `enum' definitions.

     You may only specify this attribute after a closing curly brace on
     an `enum' definition, not in a `typedef' declaration, unless that
     declaration also contains the definition of the `enum'.

`transparent_union'
     This attribute, attached to a `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 `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 `wait' function must accept either a value of type
     `int *' to comply with Posix, or a value of type `union wait *' to
     comply with the 4.1BSD interface.  If `wait''s parameter were
     `void *', `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, `<sys/wait.h>' might
     define the interface as follows:

          typedef union
            {
              int *__ip;
              union wait *__up;
            } wait_status_ptr_t __attribute__ ((__transparent_union__));
          
          pid_t wait (wait_status_ptr_t);

     This interface allows either `int *' or `union wait *' arguments
     to be passed, using the `int *' calling convention.  The program
     can call `wait' with arguments of either type:

          int w1 () { int w; return wait (&w); }
          int w2 () { union wait w; return wait (&w); }

     With this interface, `wait''s implementation might look like this:

          pid_t wait (wait_status_ptr_t p)
          {
            return waitpid (-1, p.__ip, 0);
          }

`unused'
     When attached to a type (including a `union' or a `struct'), this
     attribute means that variables of that type are meant to appear
     possibly unused.  GNU CC 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.

   To specify multiple attributes, separate them by commas within the
double parentheses: for example, `__attribute__ ((aligned (16),
packed))'.