Google Git
Sign in
gnu / gcc / f49e3d28be44179f07b8a06159139ce77096dda7 / . / gcc / doc / extend.texi
blob: 133b82eef381cdd763647066f56239e9fb32ce8d [file] [log] [blame]
c Copyright (C) 1988-2021 Free Software Foundation, Inc.
@c This is part of the GCC manual.
@c For copying conditions, see the file gcc.texi.
@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 C90 or C++ are also, as
extensions, accepted by GCC in C90 mode and in C++.
@menu
* Statement Exprs:: Putting statements and declarations inside expressions.
* Local Labels:: Labels local to a block.
* Labels as Values:: Getting pointers to labels, and computed gotos.
* Nested Functions:: Nested function in GNU C.
* Nonlocal Gotos:: Nonlocal gotos.
* Constructing Calls:: Dispatching a call to another function.
* Typeof:: @code{typeof}: referring to the type of an expression.
* Conditionals:: Omitting the middle operand of a @samp{?:} expression.
* __int128:: 128-bit integers---@code{__int128}.
* Long Long:: Double-word integers---@code{long long int}.
* Complex:: Data types for complex numbers.
* Floating Types:: Additional Floating Types.
* Half-Precision:: Half-Precision Floating Point.
* Decimal Float:: Decimal Floating Types.
* Hex Floats:: Hexadecimal floating-point constants.
* Fixed-Point:: Fixed-Point Types.
* Named Address Spaces::Named address spaces.
* Zero Length:: Zero-length arrays.
* Empty Structures:: Structures with no members.
* 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.
* Subscripting:: Any array can be subscripted, even if not an lvalue.
* Pointer Arith:: Arithmetic on @code{void}-pointers and function pointers.
* Variadic Pointer Args:: Pointer arguments to variadic functions.
* Pointers to Arrays:: Pointers to arrays with qualifiers work as expected.
* Initializers:: Non-constant initializers.
* Compound Literals:: Compound literals give structures, unions
or arrays as values.
* Designated Inits:: Labeling elements of initializers.
* Case Ranges:: `case 1 ... 9' and such.
* Cast to Union:: Casting to union type from any member of the union.
* Mixed Labels and Declarations:: Mixing declarations, labels and code.
* Function Attributes:: Declaring that functions have no side effects,
or that they can never return.
* Variable Attributes:: Specifying attributes of variables.
* Type Attributes:: Specifying attributes of types.
* Label Attributes:: Specifying attributes on labels.
* Enumerator Attributes:: Specifying attributes on enumerators.
* Statement Attributes:: Specifying attributes on statements.
* 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}.
* Alignment:: Determining the alignment of a function, type or variable.
* Inline:: Defining inline functions (as fast as macros).
* Volatiles:: What constitutes an access to a volatile object.
* Using Assembly Language with C:: Instructions and extensions for interfacing C with assembler.
* 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.
* Offsetof:: Special syntax for implementing @code{offsetof}.
* __sync Builtins:: Legacy built-in functions for atomic memory access.
* __atomic Builtins:: Atomic built-in functions with memory model.
* Integer Overflow Builtins:: Built-in functions to perform arithmetics and
arithmetic overflow checking.
* x86 specific memory model extensions for transactional memory:: x86 memory models.
* Object Size Checking:: Built-in functions for limited buffer overflow
checking.
* Other Builtins:: Other built-in functions.
* Target Builtins:: Built-in functions specific to particular targets.
* Target Format Checks:: Format checks specific to particular targets.
* Pragmas:: Pragmas accepted by GCC.
* Unnamed Fields:: Unnamed struct/union fields within structs/unions.
* Thread-Local:: Per-thread variables.
* Binary constants:: Binary constants using the @samp{0b} prefix.
@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:
@smallexample
(@{ int y = foo (); int z;
if (y > 0) z = y;
else z = - y;
z; @})
@end smallexample
@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:
@smallexample
#define max(a,b) ((a) > (b) ? (a) : (b))
@end smallexample
@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 taken as @code{int}), you can avoid this
problem by defining the macro as follows:
@smallexample
#define maxint(a,b) \
(@{int _a = (a), _b = (b); _a > _b ? _a : _b; @})
@end smallexample
Note that introducing variable declarations (as we do in @code{maxint}) can
cause variable shadowing, so while this example using the @code{max} macro
produces correct results:
@smallexample
int _a = 1, _b = 2, c;
c = max (_a, _b);
@end smallexample
@noindent
this example using maxint will not:
@smallexample
int _a = 1, _b = 2, c;
c = maxint (_a, _b);
@end smallexample
This problem may for instance occur when we use this pattern recursively, like
so:
@smallexample
#define maxint3(a, b, c) \
(@{int _a = (a), _b = (b), _c = (c); maxint (maxint (_a, _b), _c); @})
@end smallexample
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} or @code{__auto_type} (@pxref{Typeof}).
In G++, the result value of a statement expression undergoes array and
function pointer decay, and is returned by value to the enclosing
expression. For instance, if @code{A} is a class, then
@smallexample
A a;
(@{a;@}).Foo ()
@end smallexample
@noindent
constructs a temporary @code{A} object to hold the result of the
statement expression, and that is used to invoke @code{Foo}.
Therefore the @code{this} pointer observed by @code{Foo} is not the
address of @code{a}.
In a statement expression, any temporaries created within a statement
are destroyed at that statement's end. This makes statement
expressions inside macros slightly different from function calls. In
the latter case temporaries introduced during argument evaluation are
destroyed at the end of the statement that includes the function
call. In the statement expression case they are destroyed during
the statement expression. For instance,
@smallexample
#define macro(a) (@{__typeof__(a) b = (a); b + 3; @})
template<typename T> T function(T a) @{ T b = a; return b + 3; @}
void foo ()
@{
macro (X ());
function (X ());
@}
@end smallexample
@noindent
has different places where temporaries are destroyed. For the
@code{macro} case, the temporary @code{X} is destroyed just after
the initialization of @code{b}. In the @code{function} case that
temporary is destroyed when the function returns.
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 expressions that lead to precisely this
bug.)
Jumping into a statement expression with @code{goto} or using a
@code{switch} statement outside the statement expression with a
@code{case} or @code{default} label inside the statement expression is
not permitted. Jumping into a statement expression with a computed
@code{goto} (@pxref{Labels as Values}) has undefined behavior.
Jumping out of a statement expression is permitted, but if the
statement expression is part of a larger expression then it is
unspecified which other subexpressions of that expression have been
evaluated except where the language definition requires certain
subexpressions to be evaluated before or after the statement
expression. A @code{break} or @code{continue} statement inside of
a statement expression used in @code{while}, @code{do} or @code{for}
loop or @code{switch} statement condition
or @code{for} statement init or increment expressions jumps to an
outer loop or @code{switch} statement if any (otherwise it is an error),
rather than to the loop or @code{switch} statement in whose condition
or init or increment expression it appears.
In any case, as with a function call, the evaluation of a
statement expression is not interleaved with the evaluation of other
parts of the containing expression. For example,
@smallexample
foo (), ((@{ bar1 (); goto a; 0; @}) + bar2 ()), baz();
@end smallexample
@noindent
calls @code{foo} and @code{bar1} and does not call @code{baz} but
may or may not call @code{bar2}. If @code{bar2} is called, it is
called after @code{foo} and before @code{bar1}.
@node Local Labels
@section Locally Declared Labels
@cindex local labels
@cindex macros, local labels
GCC allows you to declare @dfn{local labels} in any nested block
scope. A local label is just like an ordinary label, but you can
only reference it (with a @code{goto} statement, or by taking its
address) within the block in which it is declared.
A local label declaration looks like this:
@smallexample
__label__ @var{label};
@end smallexample
@noindent
or
@smallexample
__label__ @var{label1}, @var{label2}, /* @r{@dots{}} */;
@end smallexample
Local label declarations must come at the beginning of the block,
before any ordinary declarations or statements.
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 for complex macros. If a 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 is multiply defined in that function. A
local label avoids this problem. For example:
@smallexample
#define SEARCH(value, array, target) \
do @{ \
__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:; \
@} while (0)
@end smallexample
This could also be written using a statement expression:
@smallexample
#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 smallexample
Local label declarations also make the labels they declare visible to
nested functions, if there are any. @xref{Nested Functions}, for details.
@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:
@smallexample
void *ptr;
/* @r{@dots{}} */
ptr = &&foo;
@end smallexample
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,
@smallexample
goto *ptr;
@end smallexample
@noindent
Any expression of type @code{void *} is allowed.
One way of using these constants is in initializing a static array that
serves as a jump table:
@smallexample
static void *array[] = @{ &&foo, &&bar, &&hack @};
@end smallexample
@noindent
Then you can select a label with indexing, like this:
@smallexample
goto *array[i];
@end smallexample
@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 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
@smallexample
static const int array[] = @{ &&foo - &&foo, &&bar - &&foo,
&&hack - &&foo @};
goto *(&&foo + array[i]);
@end smallexample
@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.
This alternative with label differences is not supported for the AVR target,
please use the first approach for AVR programs.
The @code{&&foo} expressions for the same label might have different
values if the containing function is inlined or cloned. If a program
relies on them being always the same,
@code{__attribute__((__noinline__,__noclone__))} should be used to
prevent inlining and cloning. If @code{&&foo} is used in a static
variable initializer, inlining and cloning is forbidden.
@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 supported as an extension in GNU C, but are not
supported by 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:
@smallexample
@group
foo (double a, double b)
@{
double square (double z) @{ return z * z; @}
return square (a) + square (b);
@}
@end group
@end smallexample
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}:
@smallexample
@group
bar (int *array, int offset, int size)
@{
int access (int *array, int index)
@{ return array[index + offset]; @}
int i;
/* @r{@dots{}} */
for (i = 0; i < size; i++)
/* @r{@dots{}} */ access (array, i) /* @r{@dots{}} */
@}
@end group
@end smallexample
Nested function definitions are permitted within functions in the places
where variable definitions are allowed; that is, in any block, mixed
with the other declarations and statements 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:
@smallexample
hack (int *array, int size)
@{
void store (int index, int value)
@{ array[index] = value; @}
intermediate (store, size);
@}
@end smallexample
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 exits, all hell breaks loose. If you try
to call it after a containing scope level exits, 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}. This technique was described in
@cite{Lexical Closures for C++} (Thomas M. Breuel, USENIX
C++ Conference Proceedings, October 17-21, 1988).
A nested function can jump to a label inherited from a containing
function, provided the label is explicitly declared in the containing
function (@pxref{Local Labels}). Such a jump returns instantly to the
containing function, exiting the nested function that did the
@code{goto} and any intermediate functions as well. Here is an example:
@smallexample
@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;
/* @r{@dots{}} */
for (i = 0; i < size; i++)
/* @r{@dots{}} */ access (array, i) /* @r{@dots{}} */
/* @r{@dots{}} */
return 0;
/* @r{Control comes here from @code{access}
if it detects an error.} */
failure:
return -1;
@}
@end group
@end smallexample
A nested function always has no linkage. Declaring one with
@code{extern} or @code{static} is erroneous. If you need to declare the nested function
before its definition, use @code{auto} (which is otherwise meaningless
for function declarations).
@smallexample
bar (int *array, int offset, int size)
@{
__label__ failure;
auto int access (int *, int);
/* @r{@dots{}} */
int access (int *array, int index)
@{
if (index > size)
goto failure;
return array[index + offset];
@}
/* @r{@dots{}} */
@}
@end smallexample
@node Nonlocal Gotos
@section Nonlocal Gotos
@cindex nonlocal gotos
GCC provides the built-in functions @code{__builtin_setjmp} and
@code{__builtin_longjmp} which are similar to, but not interchangeable
with, the C library functions @code{setjmp} and @code{longjmp}.
The built-in versions are used internally by GCC's libraries
to implement exception handling on some targets. You should use the
standard C library functions declared in @code{<setjmp.h>} in user code
instead of the builtins.
The built-in versions of these functions use GCC's normal
mechanisms to save and restore registers using the stack on function
entry and exit. The jump buffer argument @var{buf} holds only the
information needed to restore the stack frame, rather than the entire
set of saved register values.
An important caveat is that GCC arranges to save and restore only
those registers known to the specific architecture variant being
compiled for. This can make @code{__builtin_setjmp} and
@code{__builtin_longjmp} more efficient than their library
counterparts in some cases, but it can also cause incorrect and
mysterious behavior when mixing with code that uses the full register
set.
You should declare the jump buffer argument @var{buf} to the
built-in functions as:
@smallexample
#include <stdint.h>
intptr_t @var{buf}[5];
@end smallexample
@deftypefn {Built-in Function} {int} __builtin_setjmp (intptr_t *@var{buf})
This function saves the current stack context in @var{buf}.
@code{__builtin_setjmp} returns 0 when returning directly,
and 1 when returning from @code{__builtin_longjmp} using the same
@var{buf}.
@end deftypefn
@deftypefn {Built-in Function} {void} __builtin_longjmp (intptr_t *@var{buf}, int @var{val})
This function restores the stack context in @var{buf},
saved by a previous call to @code{__builtin_setjmp}. After
@code{__builtin_longjmp} is finished, the program resumes execution as
if the matching @code{__builtin_setjmp} returns the value @var{val},
which must be 1.
Because @code{__builtin_longjmp} depends on the function return
mechanism to restore the stack context, it cannot be called
from the same function calling @code{__builtin_setjmp} to
initialize @var{buf}. It can only be called from a function called
(directly or indirectly) from the function calling @code{__builtin_setjmp}.
@end deftypefn
@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).
However, these built-in functions may interact badly with some
sophisticated features or other extensions of the language. It
is, therefore, not recommended to use them outside very simple
functions acting as mere forwarders for their arguments.
@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 are 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 is 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
@deftypefn {Built-in Function} {} __builtin_va_arg_pack ()
This built-in function represents all anonymous arguments of an inline
function. It can be used only in inline functions that are always
inlined, never compiled as a separate function, such as those using
@code{__attribute__ ((__always_inline__))} or
@code{__attribute__ ((__gnu_inline__))} extern inline functions.
It must be only passed as last argument to some other function
with variable arguments. This is useful for writing small wrapper
inlines for variable argument functions, when using preprocessor
macros is undesirable. For example:
@smallexample
extern int myprintf (FILE *f, const char *format, ...);
extern inline __attribute__ ((__gnu_inline__)) int
myprintf (FILE *f, const char *format, ...)
@{
int r = fprintf (f, "myprintf: ");
if (r < 0)
return r;
int s = fprintf (f, format, __builtin_va_arg_pack ());
if (s < 0)
return s;
return r + s;
@}
@end smallexample
@end deftypefn
@deftypefn {Built-in Function} {size_t} __builtin_va_arg_pack_len ()
This built-in function returns the number of anonymous arguments of
an inline function. It can be used only in inline functions that
are always inlined, never compiled as a separate function, such
as those using @code{__attribute__ ((__always_inline__))} or
@code{__attribute__ ((__gnu_inline__))} extern inline functions.
For example following does link- or run-time checking of open
arguments for optimized code:
@smallexample
#ifdef __OPTIMIZE__
extern inline __attribute__((__gnu_inline__)) int
myopen (const char *path, int oflag, ...)
@{
if (__builtin_va_arg_pack_len () > 1)
warn_open_too_many_arguments ();
if (__builtin_constant_p (oflag))
@{
if ((oflag & O_CREAT) != 0 && __builtin_va_arg_pack_len () < 1)
@{
warn_open_missing_mode ();
return __open_2 (path, oflag);
@}
return open (path, oflag, __builtin_va_arg_pack ());
@}
if (__builtin_va_arg_pack_len () < 1)
return __open_2 (path, oflag);
return open (path, oflag, __builtin_va_arg_pack ());
@}
#endif
@end smallexample
@end deftypefn
@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:
@smallexample
typeof (x[0](1))
@end smallexample
@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:
@smallexample
typeof (int *)
@end smallexample
@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 can be
used. For example, you can use it in a declaration, in a cast, or inside
of @code{sizeof} or @code{typeof}.
The operand of @code{typeof} is evaluated for its side effects if and
only if it is an expression of variably modified type or the name of
such a type.
@code{typeof} is often useful in conjunction with
statement expressions (@pxref{Statement Exprs}).
Here is how the two together can
be used to define a safe ``maximum'' macro which operates on any
arithmetic type and evaluates each of its arguments exactly once:
@smallexample
#define max(a,b) \
(@{ typeof (a) _a = (a); \
typeof (b) _b = (b); \
_a > _b ? _a : _b; @})
@end smallexample
@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.
@noindent
Some more examples of the use of @code{typeof}:
@itemize @bullet
@item
This declares @code{y} with the type of what @code{x} points to.
@smallexample
typeof (*x) y;
@end smallexample
@item
This declares @code{y} as an array of such values.
@smallexample
typeof (*x) y[4];
@end smallexample
@item
This declares @code{y} as an array of pointers to characters:
@smallexample
typeof (typeof (char *)[4]) y;
@end smallexample
@noindent
It is equivalent to the following traditional C declaration:
@smallexample
char *y[4];
@end smallexample
To see the meaning of the declaration using @code{typeof}, and why it
might be a useful way to write, rewrite it with these macros:
@smallexample
#define pointer(T) typeof(T *)
#define array(T, N) typeof(T [N])
@end smallexample
@noindent
Now the declaration can be rewritten this way:
@smallexample
array (pointer (char), 4) y;
@end smallexample
@noindent
Thus, @code{array (pointer (char), 4)} is the type of arrays of 4
pointers to @code{char}.
@end itemize
In GNU C, but not GNU C++, you may also declare the type of a variable
as @code{__auto_type}. In that case, the declaration must declare
only one variable, whose declarator must just be an identifier, the
declaration must be initialized, and the type of the variable is
determined by the initializer; the name of the variable is not in
scope until after the initializer. (In C++, you should use C++11
@code{auto} for this purpose.) Using @code{__auto_type}, the
``maximum'' macro above could be written as:
@smallexample
#define max(a,b) \
(@{ __auto_type _a = (a); \
__auto_type _b = (b); \
_a > _b ? _a : _b; @})
@end smallexample
Using @code{__auto_type} instead of @code{typeof} has two advantages:
@itemize @bullet
@item Each argument to the macro appears only once in the expansion of
the macro. This prevents the size of the macro expansion growing
exponentially when calls to such macros are nested inside arguments of
such macros.
@item If the argument to the macro has variably modified type, it is
evaluated only once when using @code{__auto_type}, but twice if
@code{typeof} is used.
@end itemize
@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
@smallexample
x ? : y
@end smallexample
@noindent
has the value of @code{x} if that is nonzero; otherwise, the value of
@code{y}.
This example is perfectly equivalent to
@smallexample
x ? x : y
@end smallexample
@cindex side effect in @code{?:}
@cindex @code{?:} 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 __int128
@section 128-bit Integers
@cindex @code{__int128} data types
As an extension the integer scalar type @code{__int128} is supported for
targets which have an integer mode wide enough to hold 128 bits.
Simply write @code{__int128} for a signed 128-bit integer, or
@code{unsigned __int128} for an unsigned 128-bit integer. There is no
support in GCC for expressing an integer constant of type @code{__int128}
for targets with @code{long long} integer less than 128 bits wide.
@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 and ISO C++11 support data types for integers that are at least
64 bits wide, and as an extension GCC supports them in C90 and C++98 modes.
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 a fullword-to-doubleword 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 without function prototypes. If a function
expects type @code{int} for its argument, and you pass a value of type
@code{long long int}, confusion results because the caller and the
subroutine 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 C90 mode and in C++. GCC also 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 the GNU C Library), 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.
The ISO C++14 library also defines the @samp{i} suffix, so C++14 code
that includes the @samp{<complex>} header cannot use @samp{i} for the
GNU extension. The @samp{j} suffix still has the GNU meaning.
@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). Only the DWARF
debug info format can represent this, so use of DWARF is recommended.
If you are using the stabs debug info format, 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.
@node Floating Types
@section Additional Floating Types
@cindex additional floating types
@cindex @code{_Float@var{n}} data types
@cindex @code{_Float@var{n}x} data types
@cindex @code{__float80} data type
@cindex @code{__float128} data type
@cindex @code{__ibm128} data type
@cindex @code{w} floating point suffix
@cindex @code{q} floating point suffix
@cindex @code{W} floating point suffix
@cindex @code{Q} floating point suffix
ISO/IEC TS 18661-3:2015 defines C support for additional floating
types @code{_Float@var{n}} and @code{_Float@var{n}x}, and GCC supports
these type names; the set of types supported depends on the target
architecture. These types are not supported when compiling C++.
Constants with these types use suffixes @code{f@var{n}} or
@code{F@var{n}} and @code{f@var{n}x} or @code{F@var{n}x}. These type
names can be used together with @code{_Complex} to declare complex
types.
As an extension, GNU C and GNU C++ support additional floating
types, which are not supported by all targets.
@itemize @bullet
@item @code{__float128} is available on i386, x86_64, IA-64, and
hppa HP-UX, as well as on PowerPC GNU/Linux targets that enable
the vector scalar (VSX) instruction set. @code{__float128} supports
the 128-bit floating type. On i386, x86_64, PowerPC, and IA-64
other than HP-UX, @code{__float128} is an alias for @code{_Float128}.
On hppa and IA-64 HP-UX, @code{__float128} is an alias for @code{long
double}.
@item @code{__float80} is available on the i386, x86_64, and IA-64
targets, and supports the 80-bit (@code{XFmode}) floating type. It is
an alias for the type name @code{_Float64x} on these targets.
@item @code{__ibm128} is available on PowerPC targets, and provides
access to the IBM extended double format which is the current format
used for @code{long double}. When @code{long double} transitions to
@code{__float128} on PowerPC in the future, @code{__ibm128} will remain
for use in conversions between the two types.
@end itemize
Support for these additional types includes the arithmetic operators:
add, subtract, multiply, divide; unary arithmetic operators;
relational operators; equality operators; and conversions to and from
integer and other floating types. Use a suffix @samp{w} or @samp{W}
in a literal constant of type @code{__float80} or type
@code{__ibm128}. Use a suffix @samp{q} or @samp{Q} for @code{_float128}.
In order to use @code{_Float128}, @code{__float128}, and @code{__ibm128}
on PowerPC Linux systems, you must use the @option{-mfloat128} option. It is
expected in future versions of GCC that @code{_Float128} and @code{__float128}
will be enabled automatically.
The @code{_Float128} type is supported on all systems where
@code{__float128} is supported or where @code{long double} has the
IEEE binary128 format. The @code{_Float64x} type is supported on all
systems where @code{__float128} is supported. The @code{_Float32}
type is supported on all systems supporting IEEE binary32; the
@code{_Float64} and @code{_Float32x} types are supported on all systems
supporting IEEE binary64. The @code{_Float16} type is supported on AArch64
systems by default, on ARM systems when the IEEE format for 16-bit
floating-point types is selected with @option{-mfp16-format=ieee} and,
for both C and C++, on x86 systems with SSE2 enabled. GCC does not currently
support @code{_Float128x} on any systems.
On the i386, x86_64, IA-64, and HP-UX targets, you can declare complex
types using the corresponding internal complex type, @code{XCmode} for
@code{__float80} type and @code{TCmode} for @code{__float128} type:
@smallexample
typedef _Complex float __attribute__((mode(TC))) _Complex128;
typedef _Complex float __attribute__((mode(XC))) _Complex80;
@end smallexample
On the PowerPC Linux VSX targets, you can declare complex types using
the corresponding internal complex type, @code{KCmode} for
@code{__float128} type and @code{ICmode} for @code{__ibm128} type:
@smallexample
typedef _Complex float __attribute__((mode(KC))) _Complex_float128;
typedef _Complex float __attribute__((mode(IC))) _Complex_ibm128;
@end smallexample
@node Half-Precision
@section Half-Precision Floating Point
@cindex half-precision floating point
@cindex @code{__fp16} data type
@cindex @code{__Float16} data type
On ARM and AArch64 targets, GCC supports half-precision (16-bit) floating
point via the @code{__fp16} type defined in the ARM C Language Extensions.
On ARM systems, you must enable this type explicitly with the
@option{-mfp16-format} command-line option in order to use it.
On x86 targets with SSE2 enabled, GCC supports half-precision (16-bit)
floating point via the @code{_Float16} type. For C++, x86 provides a builtin
type named @code{_Float16} which contains same data format as C.
ARM targets support two incompatible representations for half-precision
floating-point values. You must choose one of the representations and
use it consistently in your program.
Specifying @option{-mfp16-format=ieee} selects the IEEE 754-2008 format.
This format can represent normalized values in the range of @math{2^{-14}} to 65504.
There are 11 bits of significand precision, approximately 3
decimal digits.
Specifying @option{-mfp16-format=alternative} selects the ARM
alternative format. This representation is similar to the IEEE
format, but does not support infinities or NaNs. Instead, the range
of exponents is extended, so that this format can represent normalized
values in the range of @math{2^{-14}} to 131008.
The GCC port for AArch64 only supports the IEEE 754-2008 format, and does
not require use of the @option{-mfp16-format} command-line option.
The @code{__fp16} type may only be used as an argument to intrinsics defined
in @code{<arm_fp16.h>}, or as a storage format. For purposes of
arithmetic and other operations, @code{__fp16} values in C or C++
expressions are automatically promoted to @code{float}.
The ARM target provides hardware support for conversions between
@code{__fp16} and @code{float} values
as an extension to VFP and NEON (Advanced SIMD), and from ARMv8-A provides
hardware support for conversions between @code{__fp16} and @code{double}
values. GCC generates code using these hardware instructions if you
compile with options to select an FPU that provides them;
for example, @option{-mfpu=neon-fp16 -mfloat-abi=softfp},
in addition to the @option{-mfp16-format} option to select
a half-precision format.
Language-level support for the @code{__fp16} data type is
independent of whether GCC generates code using hardware floating-point
instructions. In cases where hardware support is not specified, GCC
implements conversions between @code{__fp16} and other types as library
calls.
It is recommended that portable code use the @code{_Float16} type defined
by ISO/IEC TS 18661-3:2015. @xref{Floating Types}.
On x86 targets with SSE2 enabled, without @option{-mavx512fp16},
all operations will be emulated by software emulation and the @code{float}
instructions. The default behavior for @code{FLT_EVAL_METHOD} is to keep the
intermediate result of the operation as 32-bit precision. This may lead to
inconsistent behavior between software emulation and AVX512-FP16 instructions.
Using @option{-fexcess-precision=16} will force round back after each operation.
Using @option{-mavx512fp16} will generate AVX512-FP16 instructions instead of
software emulation. The default behavior of @code{FLT_EVAL_METHOD} is to round
after each operation. The same is true with @option{-fexcess-precision=standard}
and @option{-mfpmath=sse}. If there is no @option{-mfpmath=sse},
@option{-fexcess-precision=standard} alone does the same thing as before,
It is useful for code that does not have @code{_Float16} and runs on the x87
FPU.
@node Decimal Float
@section Decimal Floating Types
@cindex decimal floating types
@cindex @code{_Decimal32} data type
@cindex @code{_Decimal64} data type
@cindex @code{_Decimal128} data type
@cindex @code{df} integer suffix
@cindex @code{dd} integer suffix
@cindex @code{dl} integer suffix
@cindex @code{DF} integer suffix
@cindex @code{DD} integer suffix
@cindex @code{DL} integer suffix
As an extension, GNU C supports decimal floating types as
defined in the N1312 draft of ISO/IEC WDTR24732. Support for decimal
floating types in GCC will evolve as the draft technical report changes.
Calling conventions for any target might also change. Not all targets
support decimal floating types.
The decimal floating types are @code{_Decimal32}, @code{_Decimal64}, and
@code{_Decimal128}. They use a radix of ten, unlike the floating types
@code{float}, @code{double}, and @code{long double} whose radix is not
specified by the C standard but is usually two.
Support for decimal floating types includes the arithmetic operators
add, subtract, multiply, divide; unary arithmetic operators;
relational operators; equality operators; and conversions to and from
integer and other floating types. Use a suffix @samp{df} or
@samp{DF} in a literal constant of type @code{_Decimal32}, @samp{dd}
or @samp{DD} for @code{_Decimal64}, and @samp{dl} or @samp{DL} for
@code{_Decimal128}.
GCC support of decimal float as specified by the draft technical report
is incomplete:
@itemize @bullet
@item
When the value of a decimal floating type cannot be represented in the
integer type to which it is being converted, the result is undefined
rather than the result value specified by the draft technical report.
@item
GCC does not provide the C library functionality associated with
@file{math.h}, @file{fenv.h}, @file{stdio.h}, @file{stdlib.h}, and
@file{wchar.h}, which must come from a separate C library implementation.
Because of this the GNU C compiler does not define macro
@code{__STDC_DEC_FP__} to indicate that the implementation conforms to
the technical report.
@end itemize
Types @code{_Decimal32}, @code{_Decimal64}, and @code{_Decimal128}
are supported by the DWARF debug information format.
@node Hex Floats
@section Hex Floats
@cindex hex floats
ISO C99 and ISO C++17 support 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 C90 mode (except in some cases when strictly
conforming) and in C++98, C++11 and C++14 modes. 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 is 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 Fixed-Point
@section Fixed-Point Types
@cindex fixed-point types
@cindex @code{_Fract} data type
@cindex @code{_Accum} data type
@cindex @code{_Sat} data type
@cindex @code{hr} fixed-suffix
@cindex @code{r} fixed-suffix
@cindex @code{lr} fixed-suffix
@cindex @code{llr} fixed-suffix
@cindex @code{uhr} fixed-suffix
@cindex @code{ur} fixed-suffix
@cindex @code{ulr} fixed-suffix
@cindex @code{ullr} fixed-suffix
@cindex @code{hk} fixed-suffix
@cindex @code{k} fixed-suffix
@cindex @code{lk} fixed-suffix
@cindex @code{llk} fixed-suffix
@cindex @code{uhk} fixed-suffix
@cindex @code{uk} fixed-suffix
@cindex @code{ulk} fixed-suffix
@cindex @code{ullk} fixed-suffix
@cindex @code{HR} fixed-suffix
@cindex @code{R} fixed-suffix
@cindex @code{LR} fixed-suffix
@cindex @code{LLR} fixed-suffix
@cindex @code{UHR} fixed-suffix
@cindex @code{UR} fixed-suffix
@cindex @code{ULR} fixed-suffix
@cindex @code{ULLR} fixed-suffix
@cindex @code{HK} fixed-suffix
@cindex @code{K} fixed-suffix
@cindex @code{LK} fixed-suffix
@cindex @code{LLK} fixed-suffix
@cindex @code{UHK} fixed-suffix
@cindex @code{UK} fixed-suffix
@cindex @code{ULK} fixed-suffix
@cindex @code{ULLK} fixed-suffix
As an extension, GNU C supports fixed-point types as
defined in the N1169 draft of ISO/IEC DTR 18037. Support for fixed-point
types in GCC will evolve as the draft technical report changes.
Calling conventions for any target might also change. Not all targets
support fixed-point types.
The fixed-point types are
@code{short _Fract},
@code{_Fract},
@code{long _Fract},
@code{long long _Fract},
@code{unsigned short _Fract},
@code{unsigned _Fract},
@code{unsigned long _Fract},
@code{unsigned long long _Fract},
@code{_Sat short _Fract},
@code{_Sat _Fract},
@code{_Sat long _Fract},
@code{_Sat long long _Fract},
@code{_Sat unsigned short _Fract},
@code{_Sat unsigned _Fract},
@code{_Sat unsigned long _Fract},
@code{_Sat unsigned long long _Fract},
@code{short _Accum},
@code{_Accum},
@code{long _Accum},
@code{long long _Accum},
@code{unsigned short _Accum},
@code{unsigned _Accum},
@code{unsigned long _Accum},
@code{unsigned long long _Accum},
@code{_Sat short _Accum},
@code{_Sat _Accum},
@code{_Sat long _Accum},
@code{_Sat long long _Accum},
@code{_Sat unsigned short _Accum},
@code{_Sat unsigned _Accum},
@code{_Sat unsigned long _Accum},
@code{_Sat unsigned long long _Accum}.
Fixed-point data values contain fractional and optional integral parts.
The format of fixed-point data varies and depends on the target machine.
Support for fixed-point types includes:
@itemize @bullet
@item
prefix and postfix increment and decrement operators (@code{++}, @code{--})
@item
unary arithmetic operators (@code{+}, @code{-}, @code{!})
@item
binary arithmetic operators (@code{+}, @code{-}, @code{*}, @code{/})
@item
binary shift operators (@code{<<}, @code{>>})
@item
relational operators (@code{<}, @code{<=}, @code{>=}, @code{>})
@item
equality operators (@code{==}, @code{!=})
@item
assignment operators (@code{+=}, @code{-=}, @code{*=}, @code{/=},
@code{<<=}, @code{>>=})
@item
conversions to and from integer, floating-point, or fixed-point types
@end itemize
Use a suffix in a fixed-point literal constant:
@itemize
@item @samp{hr} or @samp{HR} for @code{short _Fract} and
@code{_Sat short _Fract}
@item @samp{r} or @samp{R} for @code{_Fract} and @code{_Sat _Fract}
@item @samp{lr} or @samp{LR} for @code{long _Fract} and
@code{_Sat long _Fract}
@item @samp{llr} or @samp{LLR} for @code{long long _Fract} and
@code{_Sat long long _Fract}
@item @samp{uhr} or @samp{UHR} for @code{unsigned short _Fract} and
@code{_Sat unsigned short _Fract}
@item @samp{ur} or @samp{UR} for @code{unsigned _Fract} and
@code{_Sat unsigned _Fract}
@item @samp{ulr} or @samp{ULR} for @code{unsigned long _Fract} and
@code{_Sat unsigned long _Fract}
@item @samp{ullr} or @samp{ULLR} for @code{unsigned long long _Fract}
and @code{_Sat unsigned long long _Fract}
@item @samp{hk} or @samp{HK} for @code{short _Accum} and
@code{_Sat short _Accum}
@item @samp{k} or @samp{K} for @code{_Accum} and @code{_Sat _Accum}
@item @samp{lk} or @samp{LK} for @code{long _Accum} and
@code{_Sat long _Accum}
@item @samp{llk} or @samp{LLK} for @code{long long _Accum} and
@code{_Sat long long _Accum}
@item @samp{uhk} or @samp{UHK} for @code{unsigned short _Accum} and
@code{_Sat unsigned short _Accum}
@item @samp{uk} or @samp{UK} for @code{unsigned _Accum} and
@code{_Sat unsigned _Accum}
@item @samp{ulk} or @samp{ULK} for @code{unsigned long _Accum} and
@code{_Sat unsigned long _Accum}
@item @samp{ullk} or @samp{ULLK} for @code{unsigned long long _Accum}
and @code{_Sat unsigned long long _Accum}
@end itemize
GCC support of fixed-point types as specified by the draft technical report
is incomplete:
@itemize @bullet
@item
Pragmas to control overflow and rounding behaviors are not implemented.
@end itemize
Fixed-point types are supported by the DWARF debug information format.
@node Named Address Spaces
@section Named Address Spaces
@cindex Named Address Spaces
As an extension, GNU C supports named address spaces as
defined in the N1275 draft of ISO/IEC DTR 18037. Support for named
address spaces in GCC will evolve as the draft technical report
changes. Calling conventions for any target might also change. At
present, only the AVR, M32C, PRU, RL78, and x86 targets support
address spaces other than the generic address space.
Address space identifiers may be used exactly like any other C type
qualifier (e.g., @code{const} or @code{volatile}). See the N1275
document for more details.
@anchor{AVR Named Address Spaces}
@subsection AVR Named Address Spaces
On the AVR target, there are several address spaces that can be used
in order to put read-only data into the flash memory and access that
data by means of the special instructions @code{LPM} or @code{ELPM}
needed to read from flash.
Devices belonging to @code{avrtiny} and @code{avrxmega3} can access
flash memory by means of @code{LD*} instructions because the flash
memory is mapped into the RAM address space. There is @emph{no need}
for language extensions like @code{__flash} or attribute
@ref{AVR Variable Attributes,,@code{progmem}}.
The default linker description files for these devices cater for that
feature and @code{.rodata} stays in flash: The compiler just generates
@code{LD*} instructions, and the linker script adds core specific
offsets to all @code{.rodata} symbols: @code{0x4000} in the case of
@code{avrtiny} and @code{0x8000} in the case of @code{avrxmega3}.
See @ref{AVR Options} for a list of respective devices.
For devices not in @code{avrtiny} or @code{avrxmega3},
any data including read-only data is located in RAM (the generic
address space) because flash memory is not visible in the RAM address
space. In order to locate read-only data in flash memory @emph{and}
to generate the right instructions to access this data without
using (inline) assembler code, special address spaces are needed.
@table @code
@item __flash
@cindex @code{__flash} AVR Named Address Spaces
The @code{__flash} qualifier locates data in the
@code{.progmem.data} section. Data is read using the @code{LPM}
instruction. Pointers to this address space are 16 bits wide.
@item __flash1
@itemx __flash2
@itemx __flash3
@itemx __flash4
@itemx __flash5
@cindex @code{__flash1} AVR Named Address Spaces
@cindex @code{__flash2} AVR Named Address Spaces
@cindex @code{__flash3} AVR Named Address Spaces
@cindex @code{__flash4} AVR Named Address Spaces
@cindex @code{__flash5} AVR Named Address Spaces
These are 16-bit address spaces locating data in section
@code{.progmem@var{N}.data} where @var{N} refers to
address space @code{__flash@var{N}}.
The compiler sets the @code{RAMPZ} segment register appropriately
before reading data by means of the @code{ELPM} instruction.
@item __memx
@cindex @code{__memx} AVR Named Address Spaces
This is a 24-bit address space that linearizes flash and RAM:
If the high bit of the address is set, data is read from
RAM using the lower two bytes as RAM address.
If the high bit of the address is clear, data is read from flash
with @code{RAMPZ} set according to the high byte of the address.
@xref{AVR Built-in Functions,,@code{__builtin_avr_flash_segment}}.
Objects in this address space are located in @code{.progmemx.data}.
@end table
@b{Example}
@smallexample
char my_read (const __flash char ** p)
@{
/* p is a pointer to RAM that points to a pointer to flash.
The first indirection of p reads that flash pointer
from RAM and the second indirection reads a char from this
flash address. */
return **p;
@}
/* Locate array[] in flash memory */
const __flash int array[] = @{ 3, 5, 7, 11, 13, 17, 19 @};
int i = 1;
int main (void)
@{
/* Return 17 by reading from flash memory */
return array[array[i]];
@}
@end smallexample
@noindent
For each named address space supported by avr-gcc there is an equally
named but uppercase built-in macro defined.
The purpose is to facilitate testing if respective address space
support is available or not:
@smallexample
#ifdef __FLASH
const __flash int var = 1;
int read_var (void)
@{
return var;
@}
#else
#include <avr/pgmspace.h> /* From AVR-LibC */
const int var PROGMEM = 1;
int read_var (void)
@{
return (int) pgm_read_word (&var);
@}
#endif /* __FLASH */
@end smallexample
@noindent
Notice that attribute @ref{AVR Variable Attributes,,@code{progmem}}
locates data in flash but
accesses to these data read from generic address space, i.e.@:
from RAM,
so that you need special accessors like @code{pgm_read_byte}
from @w{@uref{http://nongnu.org/avr-libc/user-manual/,AVR-LibC}}
together with attribute @code{progmem}.
@noindent
@b{Limitations and caveats}
@itemize
@item
Reading across the 64@tie{}KiB section boundary of
the @code{__flash} or @code{__flash@var{N}} address spaces
shows undefined behavior. The only address space that
supports reading across the 64@tie{}KiB flash segment boundaries is
@code{__memx}.
@item
If you use one of the @code{__flash@var{N}} address spaces
you must arrange your linker script to locate the
@code{.progmem@var{N}.data} sections according to your needs.
@item
Any data or pointers to the non-generic address spaces must
be qualified as @code{const}, i.e.@: as read-only data.
This still applies if the data in one of these address
spaces like software version number or calibration lookup table are intended to
be changed after load time by, say, a boot loader. In this case
the right qualification is @code{const} @code{volatile} so that the compiler
must not optimize away known values or insert them
as immediates into operands of instructions.
@item
The following code initializes a variable @code{pfoo}
located in static storage with a 24-bit address:
@smallexample
extern const __memx char foo;
const __memx void *pfoo = &foo;
@end smallexample
@item
On the reduced Tiny devices like ATtiny40, no address spaces are supported.
Just use vanilla C / C++ code without overhead as outlined above.
Attribute @code{progmem} is supported but works differently,
see @ref{AVR Variable Attributes}.
@end itemize
@subsection M32C Named Address Spaces
@cindex @code{__far} M32C Named Address Spaces
On the M32C target, with the R8C and M16C CPU variants, variables
qualified with @code{__far} are accessed using 32-bit addresses in
order to access memory beyond the first 64@tie{}Ki bytes. If
@code{__far} is used with the M32CM or M32C CPU variants, it has no
effect.
@subsection PRU Named Address Spaces
@cindex @code{__regio_symbol} PRU Named Address Spaces
On the PRU target, variables qualified with @code{__regio_symbol} are
aliases used to access the special I/O CPU registers. They must be
declared as @code{extern} because such variables will not be allocated in
any data memory. They must also be marked as @code{volatile}, and can
only be 32-bit integer types. The only names those variables can have
are @code{__R30} and @code{__R31}, representing respectively the
@code{R30} and @code{R31} special I/O CPU registers. Hence the following
example is the only valid usage of @code{__regio_symbol}:
@smallexample
extern volatile __regio_symbol uint32_t __R30;
extern volatile __regio_symbol uint32_t __R31;
@end smallexample
@subsection RL78 Named Address Spaces
@cindex @code{__far} RL78 Named Address Spaces
On the RL78 target, variables qualified with @code{__far} are accessed
with 32-bit pointers (20-bit addresses) rather than the default 16-bit
addresses. Non-far variables are assumed to appear in the topmost
64@tie{}KiB of the address space.
@subsection x86 Named Address Spaces
@cindex x86 named address spaces
On the x86 target, variables may be declared as being relative
to the @code{%fs} or @code{%gs} segments.
@table @code
@item __seg_fs
@itemx __seg_gs
@cindex @code{__seg_fs} x86 named address space
@cindex @code{__seg_gs} x86 named address space
The object is accessed with the respective segment override prefix.
The respective segment base must be set via some method specific to
the operating system. Rather than require an expensive system call
to retrieve the segment base, these address spaces are not considered
to be subspaces of the generic (flat) address space. This means that
explicit casts are required to convert pointers between these address
spaces and the generic address space. In practice the application
should cast to @code{uintptr_t} and apply the segment base offset
that it installed previously.
The preprocessor symbols @code{__SEG_FS} and @code{__SEG_GS} are
defined when these address spaces are supported.
@end table
@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
Declaring zero-length arrays is allowed in GNU C as an extension.
A zero-length array can be useful as the last element of a structure
that is really a header for a variable-length object:
@smallexample
struct line @{
int length;
char contents[0];
@};
struct line *thisline = (struct line *)
malloc (sizeof (struct line) + this_length);
thisline->length = this_length;
@end smallexample
Although the size of a zero-length array is zero, an array member of
this kind may increase the size of the enclosing type as a result of tail
padding. The offset of a zero-length array member from the beginning
of the enclosing structure is the same as the offset of an array with
one or more elements of the same type. The alignment of a zero-length
array is the same as the alignment of its elements.
Declaring zero-length arrays in other contexts, including as interior
members of structure objects or as non-member objects, is discouraged.
Accessing elements of zero-length arrays declared in such contexts is
undefined and may be diagnosed.
In the absence of the zero-length array extension, in ISO C90
the @code{contents} array in the example above would typically be declared
to have a single element. Unlike a zero-length array which only contributes
to the size of the enclosing structure for the purposes of alignment,
a one-element array always occupies at least as much space as a single
object of the type. Although using one-element arrays this way is
discouraged, GCC handles accesses to trailing one-element array members
analogously to zero-length arrays.
The preferred mechanism to declare variable-length types like
@code{struct line} above is the ISO C99 @dfn{flexible array member},
with slightly different 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.
@item
A structure containing a flexible array member, or a union containing
such a structure (possibly recursively), may not be a member of a
structure or an element of an array. (However, these uses are
permitted by GCC as extensions.)
@end itemize
Non-empty initialization of zero-length
arrays is 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.
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.
E.g.@: in the following, @code{f1} is constructed as if it were declared
like @code{f2}.
@smallexample
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 smallexample
@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:
@smallexample
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 smallexample
@node Empty Structures
@section Structures with No Members
@cindex empty structures
@cindex zero-size structures
GCC permits a C structure to have no members:
@smallexample
struct empty @{
@};
@end smallexample
The structure has size zero. In C++, empty structures are part
of the language. G++ treats empty structures as if they had a single
member of type @code{char}.
@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 C90 mode and in 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 block scope containing the declaration
exits. For
example:
@smallexample
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 smallexample
@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 variable-length array in a structure
As an extension, GCC accepts variable-length arrays as a member of
a structure or a union. For example:
@smallexample
void
foo (int n)
@{
struct S @{ int x[n]; @};
@}
@end smallexample
@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, unless you also use @code{alloca} in this scope.
You can also use variable-length arrays as arguments to functions:
@smallexample
struct entry
tester (int len, char data[len][len])
@{
/* @r{@dots{}} */
@}
@end smallexample
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.
@smallexample
struct entry
tester (int len; char data[len][len], int len)
@{
/* @r{@dots{}} */
@}
@end smallexample
@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:
@smallexample
#define debug(format, ...) fprintf (stderr, format, __VA_ARGS__)
@end smallexample
@noindent
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:
@smallexample
#define debug(format, args...) fprintf (stderr, format, args)
@end smallexample
@noindent
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:
@smallexample
debug ("A message")
@end smallexample
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
@smallexample
#define debug(format, ...) fprintf (stderr, format, ## __VA_ARGS__)
@end smallexample
@noindent
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)
The preprocessor treatment of escaped newlines is more relaxed
than that specified by the C90 standard, which requires the newline
to immediately follow a backslash.
GCC's 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, 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 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, GNU C allows such arrays to be
subscripted in C90 mode, though otherwise they do not decay to
pointers outside C99 mode. For example,
this is valid in GNU C though not valid in C90:
@smallexample
@group
struct foo @{int a[4];@};
struct foo f();
bar (int index)
@{
return f().a[index];
@}
@end group
@end smallexample
@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 Variadic Pointer Args
@section Pointer Arguments in Variadic Functions
@cindex pointer arguments in variadic functions
@cindex variadic functions, pointer arguments
Standard C requires that pointer types used with @code{va_arg} in
functions with variable argument lists either must be compatible with
that of the actual argument, or that one type must be a pointer to
@code{void} and the other a pointer to a character type. GNU C
implements the POSIX XSI extension that additionally permits the use
of @code{va_arg} with a pointer type to receive arguments of any other
pointer type.
In particular, in GNU C @samp{va_arg (ap, void *)} can safely be used
to consume an argument of any pointer type.
@node Pointers to Arrays
@section Pointers to Arrays with Qualifiers Work as Expected
@cindex pointers to arrays
@cindex const qualifier
In GNU C, pointers to arrays with qualifiers work similar to pointers
to other qualified types. For example, a value of type @code{int (*)[5]}
can be used to initialize a variable of type @code{const int (*)[5]}.
These types are incompatible in ISO C because the @code{const} qualifier
is formally attached to the element type of the array and not the
array itself.
@smallexample
extern void
transpose (int N, int M, double out[M][N], const double in[N][M]);
double x[3][2];
double y[2][3];
@r{@dots{}}
transpose(3, 2, y, x);
@end smallexample
@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:
@smallexample
foo (float f, float g)
@{
float beat_freqs[2] = @{ f-g, f+g @};
/* @r{@dots{}} */
@}
@end smallexample
@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".
A compound literal looks like a cast of a brace-enclosed aggregate
initializer list. Its value is an object of the type specified in
the cast, containing the elements specified in the initializer.
Unlike the result of a cast, a compound literal is an lvalue. ISO
C99 and later support compound literals. As an extension, GCC
supports compound literals also in C90 mode and in C++, although
as explained below, the C++ semantics are somewhat different.
Usually, the specified type of a compound literal is a structure. Assume
that @code{struct foo} and @code{structure} are declared as shown:
@smallexample
struct foo @{int a; char b[2];@} structure;
@end smallexample
@noindent
Here is an example of constructing a @code{struct foo} with a compound literal:
@smallexample
structure = ((struct foo) @{x + y, 'a', 0@});
@end smallexample
@noindent
This is equivalent to writing the following:
@smallexample
@{
struct foo temp = @{x + y, 'a', 0@};
structure = temp;
@}
@end smallexample
You can also construct an array, though this is dangerous in C++, as
explained below. 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:
@smallexample
char **foo = (char *[]) @{ "x", "y", "z" @};
@end smallexample
Compound literals for scalar types and union types are also allowed. In
the following example the variable @code{i} is initialized to the value
@code{2}, the result of incrementing the unnamed object created by
the compound literal.
@smallexample
int i = ++(int) @{ 1 @};
@end smallexample
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 were initialized only with the brace-enclosed
list if the types of the compound literal and the object match.
The elements of the compound literal must be constant.
If the object being initialized has array type of unknown size, the size is
determined by the size of the compound literal.
@smallexample
static struct foo x = (struct foo) @{1, 'a', 'b'@};
static int y[] = (int []) @{1, 2, 3@};
static int z[] = (int [3]) @{1@};
@end smallexample
@noindent
The above lines are equivalent to the following:
@smallexample
static struct foo x = @{1, 'a', 'b'@};
static int y[] = @{1, 2, 3@};
static int z[] = @{1, 0, 0@};
@end smallexample
In C, a compound literal designates an unnamed object with static or
automatic storage duration. In C++, a compound literal designates a
temporary object that only lives until the end of its full-expression.
As a result, well-defined C code that takes the address of a subobject
of a compound literal can be undefined in C++, so G++ rejects
the conversion of a temporary array to a pointer. For instance, if
the array compound literal example above appeared inside a function,
any subsequent use of @code{foo} in C++ would have undefined behavior
because the lifetime of the array ends after the declaration of @code{foo}.
As an optimization, G++ sometimes gives array compound literals longer
lifetimes: when the array either appears outside a function or has
a @code{const}-qualified type. If @code{foo} and its initializer had
elements of type @code{char *const} rather than @code{char *}, or if
@code{foo} were a global variable, the array would have static storage
duration. But it is probably safest just to avoid the use of array
compound literals in C++ code.
@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 C90 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 C90 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,
@smallexample
int a[6] = @{ [4] = 29, [2] = 15 @};
@end smallexample
@noindent
is equivalent to
@smallexample
int a[6] = @{ 0, 0, 15, 0, 29, 0 @};
@end smallexample
@noindent
The index values must be constant expressions, even if the array being
initialized is automatic.
An alternative syntax for this that 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,
@smallexample
int widths[] = @{ [0 ... 9] = 1, [10 ... 99] = 2, [100] = 3 @};
@end smallexample
@noindent
If the value in it has side effects, the side effects 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,
@smallexample
struct point @{ int x, y; @};
@end smallexample
@noindent
the following initialization
@smallexample
struct point p = @{ .y = yvalue, .x = xvalue @};
@end smallexample
@noindent
is equivalent to
@smallexample
struct point p = @{ xvalue, yvalue @};
@end smallexample
Another syntax that has the same meaning, obsolete since GCC 2.5, is
@samp{@var{fieldname}:}, as shown here:
@smallexample
struct point p = @{ y: yvalue, x: xvalue @};
@end smallexample
Omitted fields are implicitly initialized the same as for objects
that have static storage duration.
@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,
@smallexample
union foo @{ int i; double d; @};
union foo f = @{ .d = 4 @};
@end smallexample
@noindent
converts 4 to a @code{double} to store it in the union using
the second element. By contrast, casting 4 to type @code{union foo}
stores 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,
@smallexample
int a[6] = @{ [1] = v1, v2, [4] = v4 @};
@end smallexample
@noindent
is equivalent to
@smallexample
int a[6] = @{ 0, v1, v2, 0, v4, 0 @};
@end smallexample
Labeling the elements of an array initializer is especially useful
when the indices are characters or belong to an @code{enum} type.
For example:
@smallexample
int whitespace[256]
= @{ [' '] = 1, ['\t'] = 1, ['\h'] = 1,
['\f'] = 1, ['\n'] = 1, ['\r'] = 1 @};
@end smallexample
@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:
@smallexample
struct point ptarray[10] = @{ [2].y = yv2, [2].x = xv2, [0].x = xv0 @};
@end smallexample
If the same field is initialized multiple times, or overlapping
fields of a union are initialized, the value from the last
initialization is used. When a field of a union is itself a structure,
the entire structure from the last field initialized is used. If any previous
initializer has side effect, it is unspecified whether the side effect
happens or not. Currently, GCC discards the side-effecting
initializer expressions and issues 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:
@smallexample
case @var{low} ... @var{high}:
@end smallexample
@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:
@smallexample
case 'A' ... 'Z':
@end smallexample
@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:
@smallexample
case 1 ... 5:
@end smallexample
@noindent
rather than this:
@smallexample
case 1...5:
@end smallexample
@node Cast to Union
@section Cast to a Union Type
@cindex cast to a union
@cindex union, casting to a
A cast to a union type is a C extension not available in C++. It looks
just like ordinary casts with the constraint that the type specified is
a union type. You can specify the type either with the @code{union}
keyword or with a @code{typedef} name that refers to a union. The result
of a cast to a union is a temporary rvalue of the union type with a member
whose type matches that of the operand initialized to the value of
the operand. The effect of a cast to a union is similar to a compound
literal except that it yields an rvalue like standard casts do.
@xref{Compound Literals}.
Expressions that may be cast to the union type are those whose type matches
at least one of the members of the union. Thus, given the following union
and variables:
@smallexample
union foo @{ int i; double d; @};
int x;
double y;
union foo z;
@end smallexample
@noindent
both @code{x} and @code{y} can be cast to type @code{union foo} and
the following assignments
@smallexample
z = (union foo) x;
z = (union foo) y;
@end smallexample
are shorthand equivalents of these
@smallexample
z = (union foo) @{ .i = x @};
z = (union foo) @{ .d = y @};
@end smallexample
However, @code{(union foo) FLT_MAX;} is not a valid cast because the union
has no member of type @code{float}.
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 with
the same type
@smallexample
union foo u;
/* @r{@dots{}} */
u = (union foo) x @equiv{} u.i = x
u = (union foo) y @equiv{} u.d = y
@end smallexample
You can also use the union cast as a function argument:
@smallexample
void hack (union foo);
/* @r{@dots{}} */
hack ((union foo) x);
@end smallexample
@node Mixed Labels and Declarations
@section Mixed Declarations, Labels 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. ISO C2X allows labels to be
placed before declarations and at the end of a compound statement.
As an extension, GNU C also allows all this in C90 mode. For example,
you could do:
@smallexample
int i;
/* @r{@dots{}} */
i++;
int j = i + 2;
@end smallexample
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 @code{volatile} applied to function
@cindex @code{const} applied to function
In GNU C and C++, you can use function attributes to specify certain
function properties that may help the compiler optimize calls or
check code more carefully for correctness. For example, you
can use attributes to specify that a function never returns
(@code{noreturn}), returns a value depending only on the values of
its arguments (@code{const}), or has @code{printf}-style arguments
(@code{format}).
You can also use attributes to control memory placement, code
generation options or call/return conventions within the function
being annotated. Many of these attributes are target-specific. For
example, many targets support attributes for defining interrupt
handler functions, which typically must follow special register usage
and return conventions. Such attributes are described in the subsection
for each target. However, a considerable number of attributes are
supported by most, if not all targets. Those are described in
the @ref{Common Function Attributes} section.
Function attributes are introduced by the @code{__attribute__} keyword
in the declaration of a function, followed by an attribute specification
enclosed in double parentheses. You can specify multiple attributes in
a declaration by separating them by commas within the double parentheses
or by immediately following one attribute specification with another.
@xref{Attribute Syntax}, for the exact rules on attribute syntax and
placement. Compatible attribute specifications on distinct declarations
of the same function are merged. An attribute specification that is not
compatible with attributes already applied to a declaration of the same
function is ignored with a warning.
Some function attributes take one or more arguments that refer to
the function's parameters by their positions within the function parameter
list. Such attribute arguments are referred to as @dfn{positional arguments}.
Unless specified otherwise, positional arguments that specify properties
of parameters with pointer types can also specify the same properties of
the implicit C++ @code{this} argument in non-static member functions, and
of parameters of reference to a pointer type. For ordinary functions,
position one refers to the first parameter on the list. In C++ non-static
member functions, position one refers to the implicit @code{this} pointer.
The same restrictions and effects apply to function attributes used with
ordinary functions or C++ member functions.
GCC also supports attributes on
variable declarations (@pxref{Variable Attributes}),
labels (@pxref{Label Attributes}),
enumerators (@pxref{Enumerator Attributes}),
statements (@pxref{Statement Attributes}),
and types (@pxref{Type Attributes}).
There is some overlap between the purposes of attributes and pragmas
(@pxref{Pragmas,,Pragmas Accepted by GCC}). It has been
found convenient to use @code{__attribute__} to achieve a natural
attachment of attributes to their corresponding declarations, whereas
@code{#pragma} is of use for compatibility with other compilers
or constructs that do not naturally form part of the grammar.
In addition to the attributes documented here,
GCC plugins may provide their own attributes.
@menu
* Common Function Attributes::
* AArch64 Function Attributes::
* AMD GCN Function Attributes::
* ARC Function Attributes::
* ARM Function Attributes::
* AVR Function Attributes::
* Blackfin Function Attributes::
* BPF Function Attributes::
* CR16 Function Attributes::
* C-SKY Function Attributes::
* Epiphany Function Attributes::
* H8/300 Function Attributes::
* IA-64 Function Attributes::
* M32C Function Attributes::
* M32R/D Function Attributes::
* m68k Function Attributes::
* MCORE Function Attributes::
* MeP Function Attributes::
* MicroBlaze Function Attributes::
* Microsoft Windows Function Attributes::
* MIPS Function Attributes::
* MSP430 Function Attributes::
* NDS32 Function Attributes::
* Nios II Function Attributes::
* Nvidia PTX Function Attributes::
* PowerPC Function Attributes::
* RISC-V Function Attributes::
* RL78 Function Attributes::
* RX Function Attributes::
* S/390 Function Attributes::
* SH Function Attributes::
* Symbian OS Function Attributes::
* V850 Function Attributes::
* Visium Function Attributes::
* x86 Function Attributes::
* Xstormy16 Function Attributes::
@end menu
@node Common Function Attributes
@subsection Common Function Attributes
The following attributes are supported on most targets.
@table @code
@c Keep this table alphabetized by attribute name. Treat _ as space.
@item access
@itemx access (@var{access-mode}, @var{ref-index})
@itemx access (@var{access-mode}, @var{ref-index}, @var{size-index})
The @code{access} attribute enables the detection of invalid or unsafe
accesses by functions to which they apply or their callers, as well as
write-only accesses to objects that are never read from. Such accesses
may be diagnosed by warnings such as @option{-Wstringop-overflow},
@option{-Wuninitialized}, @option{-Wunused}, and others.
The @code{access} attribute specifies that a function to whose by-reference
arguments the attribute applies accesses the referenced object according to
@var{access-mode}. The @var{access-mode} argument is required and must be
one of four names: @code{read_only}, @code{read_write}, @code{write_only},
or @code{none}. The remaining two are positional arguments.
The required @var{ref-index} positional argument denotes a function
argument of pointer (or in C++, reference) type that is subject to
the access. The same pointer argument can be referenced by at most one
distinct @code{access} attribute.
The optional @var{size-index} positional argument denotes a function
argument of integer type that specifies the maximum size of the access.
The size is the number of elements of the type referenced by @var{ref-index},
or the number of bytes when the pointer type is @code{void*}. When no
@var{size-index} argument is specified, the pointer argument must be either
null or point to a space that is suitably aligned and large for at least one
object of the referenced type (this implies that a past-the-end pointer is
not a valid argument). The actual size of the access may be less but it
must not be more.
The @code{read_only} access mode specifies that the pointer to which it
applies is used to read the referenced object but not write to it. Unless
the argument specifying the size of the access denoted by @var{size-index}
is zero, the referenced object must be initialized. The mode implies
a stronger guarantee than the @code{const} qualifier which, when cast away
from a pointer, does not prevent the pointed-to object from being modified.
Examples of the use of the @code{read_only} access mode is the argument to
the @code{puts} function, or the second and third arguments to
the @code{memcpy} function.
@smallexample
__attribute__ ((access (read_only, 1))) int puts (const char*);
__attribute__ ((access (read_only, 2, 3))) void* memcpy (void*, const void*, size_t);
@end smallexample
The @code{read_write} access mode applies to arguments of pointer types
without the @code{const} qualifier. It specifies that the pointer to which
it applies is used to both read and write the referenced object. Unless
the argument specifying the size of the access denoted by @var{size-index}
is zero, the object referenced by the pointer must be initialized. An example
of the use of the @code{read_write} access mode is the first argument to
the @code{strcat} function.
@smallexample
__attribute__ ((access (read_write, 1), access (read_only, 2))) char* strcat (char*, const char*);
@end smallexample
The @code{write_only} access mode applies to arguments of pointer types
without the @code{const} qualifier. It specifies that the pointer to which
it applies is used to write to the referenced object but not read from it.
The object referenced by the pointer need not be initialized. An example
of the use of the @code{write_only} access mode is the first argument to
the @code{strcpy} function, or the first two arguments to the @code{fgets}
function.
@smallexample
__attribute__ ((access (write_only, 1), access (read_only, 2))) char* strcpy (char*, const char*);
__attribute__ ((access (write_only, 1, 2), access (read_write, 3))) int fgets (char*, int, FILE*);
@end smallexample
The access mode @code{none} specifies that the pointer to which it applies
is not used to access the referenced object at all. Unless the pointer is
null the pointed-to object must exist and have at least the size as denoted
by the @var{size-index} argument. The object need not be initialized.
The mode is intended to be used as a means to help validate the expected
object size, for example in functions that call @code{__builtin_object_size}.
@xref{Object Size Checking}.
@item alias ("@var{target}")
@cindex @code{alias} function attribute
The @code{alias} attribute causes the declaration to be emitted as an alias
for another symbol, which must have been previously declared with the same
type, and for variables, also the same size and alignment. Declaring an alias
with a different type than the target is undefined and may be diagnosed. As
an example, the following declarations:
@smallexample
void __f () @{ /* @r{Do something.} */; @}
void f () __attribute__ ((weak, alias ("__f")));
@end smallexample
@noindent
define @samp{f} to be a weak alias for @samp{__f}. In C++, the mangled name
for the target must be used. It is an error if @samp{__f} is not defined in
the same translation unit.
This attribute requires assembler and object file support,
and may not be available on all targets.
@item aligned
@itemx aligned (@var{alignment})
@cindex @code{aligned} function attribute
The @code{aligned} attribute specifies a minimum alignment for
the first instruction of the function, measured in bytes. When specified,
@var{alignment} must be an integer constant power of 2. Specifying no
@var{alignment} argument implies the ideal alignment for the target.
The @code{__alignof__} operator can be used to determine what that is
(@pxref{Alignment}). The attribute has no effect when a definition for
the function is not provided in the same translation unit.
The attribute cannot be used to decrease the alignment of a function
previously declared with a more restrictive alignment; only to increase
it. Attempts to do otherwise are diagnosed. Some targets specify
a minimum default alignment for functions that is greater than 1. On
such targets, specifying a less restrictive alignment is silently ignored.
Using the attribute overrides the effect of the @option{-falign-functions}
(@pxref{Optimize Options}) option for this function.
Note that the effectiveness of @code{aligned} attributes may be
limited by inherent limitations in the system linker
and/or object file format. On some systems, the
linker is only able to arrange for functions to be aligned up to a
certain maximum alignment. (For some linkers, the maximum supported
alignment may be very very small.) See your linker documentation for
further information.
The @code{aligned} attribute can also be used for variables and fields
(@pxref{Variable Attributes}.)
@item alloc_align (@var{position})
@cindex @code{alloc_align} function attribute
The @code{alloc_align} attribute may be applied to a function that
returns a pointer and takes at least one argument of an integer or
enumerated type.
It indicates that the returned pointer is aligned on a boundary given
by the function argument at @var{position}. Meaningful alignments are
powers of 2 greater than one. GCC uses this information to improve
pointer alignment analysis.
The function parameter denoting the allocated alignment is specified by
one constant integer argument whose number is the argument of the attribute.
Argument numbering starts at one.
For instance,
@smallexample
void* my_memalign (size_t, size_t) __attribute__ ((alloc_align (1)));
@end smallexample
@noindent
declares that @code{my_memalign} returns memory with minimum alignment
given by parameter 1.
@item alloc_size (@var{position})
@itemx alloc_size (@var{position-1}, @var{position-2})
@cindex @code{alloc_size} function attribute
The @code{alloc_size} attribute may be applied to a function that
returns a pointer and takes at least one argument of an integer or
enumerated type.
It indicates that the returned pointer points to memory whose size is
given by the function argument at @var{position-1}, or by the product
of the arguments at @var{position-1} and @var{position-2}. Meaningful
sizes are positive values less than @code{PTRDIFF_MAX}. GCC uses this
information to improve the results of @code{__builtin_object_size}.
The function parameter(s) denoting the allocated size are specified by
one or two integer arguments supplied to the attribute. The allocated size
is either the value of the single function argument specified or the product
of the two function arguments specified. Argument numbering starts at
one for ordinary functions, and at two for C++ non-static member functions.
For instance,
@smallexample
void* my_calloc (size_t, size_t) __attribute__ ((alloc_size (1, 2)));
void* my_realloc (void*, size_t) __attribute__ ((alloc_size (2)));
@end smallexample
@noindent
declares that @code{my_calloc} returns memory of the size given by
the product of parameter 1 and 2 and that @code{my_realloc} returns memory
of the size given by parameter 2.
@item always_inline
@cindex @code{always_inline} function attribute
Generally, functions are not inlined unless optimization is specified.
For functions declared inline, this attribute inlines the function
independent of any restrictions that otherwise apply to inlining.
Failure to inline such a function is diagnosed as an error.
Note that if such a function is called indirectly the compiler may
or may not inline it depending on optimization level and a failure
to inline an indirect call may or may not be diagnosed.
@item artificial
@cindex @code{artificial} function attribute
This attribute is useful for small inline wrappers that if possible
should appear during debugging as a unit. Depending on the debug
info format it either means marking the function as artificial
or using the caller location for all instructions within the inlined
body.
@item assume_aligned (@var{alignment})
@itemx assume_aligned (@var{alignment}, @var{offset})
@cindex @code{assume_aligned} function attribute
The @code{assume_aligned} attribute may be applied to a function that
returns a pointer. It indicates that the returned pointer is aligned
on a boundary given by @var{alignment}. If the attribute has two
arguments, the second argument is misalignment @var{offset}. Meaningful
values of @var{alignment} are powers of 2 greater than one. Meaningful
values of @var{offset} are greater than zero and less than @var{alignment}.
For instance
@smallexample
void* my_alloc1 (size_t) __attribute__((assume_aligned (16)));
void* my_alloc2 (size_t) __attribute__((assume_aligned (32, 8)));
@end smallexample
@noindent
declares that @code{my_alloc1} returns 16-byte aligned pointers and
that @code{my_alloc2} returns a pointer whose value modulo 32 is equal
to 8.
@item cold
@cindex @code{cold} function attribute
The @code{cold} attribute on functions is used to inform the compiler that
the function is unlikely to be executed. The function is optimized for
size rather than speed and on many targets it is placed into a special
subsection of the text section so all cold functions appear close together,
improving code locality of non-cold parts of program. The paths leading
to calls of cold functions within code are marked as unlikely by the branch
prediction mechanism. It is thus useful to mark functions used to handle
unlikely conditions, such as @code{perror}, as cold to improve optimization
of hot functions that do call marked functions in rare occasions.
When profile feedback is available, via @option{-fprofile-use}, cold functions
are automatically detected and this attribute is ignored.
@item const
@cindex @code{const} function attribute
@cindex functions that have no side effects
Calls to functions whose return value is not affected by changes to
the observable state of the program and that have no observable effects
on such state other than to return a value may lend themselves to
optimizations such as common subexpression elimination. Declaring such
functions with the @code{const} attribute allows GCC to avoid emitting
some calls in repeated invocations of the function with the same argument
values.
For example,
@smallexample
int square (int) __attribute__ ((const));
@end smallexample
@noindent
tells GCC that subsequent calls to function @code{square} with the same
argument value can be replaced by the result of the first call regardless
of the statements in between.
The @code{const} attribute prohibits a function from reading objects
that affect its return value between successive invocations. However,
functions declared with the attribute can safely read objects that do
not change their return value, such as non-volatile constants.
The @code{const} attribute imposes greater restrictions on a function's
definition than the similar @code{pure} attribute. Declaring the same
function with both the @code{const} and the @code{pure} attribute is
diagnosed. Because a const function cannot have any observable side
effects it does not make sense for it to return @code{void}. Declaring
such a function is diagnosed.
@cindex pointer arguments
Note that a function that has pointer arguments and examines the data
pointed to must @emph{not} be declared @code{const} if the pointed-to
data might change between successive invocations of the function. In
general, since a function cannot distinguish data that might change
from data that cannot, const functions should never take pointer or,
in C++, reference arguments. Likewise, a function that calls a non-const
function usually must not be const itself.
@item constructor
@itemx destructor
@itemx constructor (@var{priority})
@itemx destructor (@var{priority})
@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 ()} completes or @code{exit ()} is
called. Functions with these attributes are useful for
initializing data that is used implicitly during the execution of
the program.
On some targets the attributes also accept an integer argument to
specify a priority to control the order in which constructor and
destructor functions are run. A constructor
with a smaller priority number runs before a constructor with a larger
priority number; the opposite relationship holds for destructors. Note
that priorities 0-100 are reserved. So, if you have a constructor that
allocates a resource and a destructor that deallocates the same
resource, both functions typically have the same priority. The
priorities for constructor and destructor functions are the same as
those specified for namespace-scope C++ objects (@pxref{C++ Attributes}).
However, at present, the order in which constructors for C++ objects
with static storage duration and functions decorated with attribute
@code{constructor} are invoked is unspecified. In mixed declarations,
attribute @code{init_priority} can be used to impose a specific ordering.
Using the argument forms of the @code{constructor} and @code{destructor}
attributes on targets where the feature is not supported is rejected with
an error.
@item copy
@itemx copy (@var{function})
@cindex @code{copy} function attribute
The @code{copy} attribute applies the set of attributes with which
@var{function} has been declared to the declaration of the function
to which the attribute is applied. The attribute is designed for
libraries that define aliases or function resolvers that are expected
to specify the same set of attributes as their targets. The @code{copy}
attribute can be used with functions, variables, or types. However,
the kind of symbol to which the attribute is applied (either function
or variable) must match the kind of symbol to which the argument refers.
The @code{copy} attribute copies only syntactic and semantic attributes
but not attributes that affect a symbol's linkage or visibility such as
@code{alias}, @code{visibility}, or @code{weak}. The @code{deprecated}
and @code{target_clones} attribute are also not copied.
@xref{Common Type Attributes}.
@xref{Common Variable Attributes}.
For example, the @var{StrongAlias} macro below makes use of the @code{alias}
and @code{copy} attributes to define an alias named @var{alloc} for function
@var{allocate} declared with attributes @var{alloc_size}, @var{malloc}, and
@var{nothrow}. Thanks to the @code{__typeof__} operator the alias has
the same type as the target function. As a result of the @code{copy}
attribute the alias also shares the same attributes as the target.
@smallexample
#define StrongAlias(TargetFunc, AliasDecl) \
extern __typeof__ (TargetFunc) AliasDecl \
__attribute__ ((alias (#TargetFunc), copy (TargetFunc)));
extern __attribute__ ((alloc_size (1), malloc, nothrow))
void* allocate (size_t);
StrongAlias (allocate, alloc);
@end smallexample
@item deprecated
@itemx deprecated (@var{msg})
@cindex @code{deprecated} function attribute
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
@noindent
results in a warning on line 3 but not line 2. The optional @var{msg}
argument, which must be a string, is printed in the warning if
present.
The @code{deprecated} attribute can also be used for variables and
types (@pxref{Variable Attributes}, @pxref{Type Attributes}.)
The message attached to the attribute is affected by the setting of
the @option{-fmessage-length} option.
@item unavailable
@itemx unavailable (@var{msg})
@cindex @code{unavailable} function attribute
The @code{unavailable} attribute results in an error if the function
is used anywhere in the source file. This is useful when identifying
functions that have been removed from a particular variation of an
interface. Other than emitting an error rather than a warning, the
@code{unavailable} attribute behaves in the same manner as
@code{deprecated}.
The @code{unavailable} attribute can also be used for variables and
types (@pxref{Variable Attributes}, @pxref{Type Attributes}.)
@item error ("@var{message}")
@itemx warning ("@var{message}")
@cindex @code{error} function attribute
@cindex @code{warning} function attribute
If the @code{error} or @code{warning} attribute
is used on a function declaration and a call to such a function
is not eliminated through dead code elimination or other optimizations,
an error or warning (respectively) that includes @var{message} is diagnosed.
This is useful
for compile-time checking, especially together with @code{__builtin_constant_p}
and inline functions where checking the inline function arguments is not
possible through @code{extern char [(condition) ? 1 : -1];} tricks.
While it is possible to leave the function undefined and thus invoke
a link failure (to define the function with
a message in @code{.gnu.warning*} section),
when using these attributes the problem is diagnosed
earlier and with exact location of the call even in presence of inline
functions or when not emitting debugging information.
@item externally_visible
@cindex @code{externally_visible} function attribute
This attribute, attached to a global variable or function, nullifies
the effect of the @option{-fwhole-program} command-line option, so the
object remains visible outside the current compilation unit.
If @option{-fwhole-program} is used together with @option{-flto} and
@command{gold} is used as the linker plugin,
@code{externally_visible} attributes are automatically added to functions
(not variable yet due to a current @command{gold} issue)
that are accessed outside of LTO objects according to resolution file
produced by @command{gold}.
For other linkers that cannot generate resolution file,
explicit @code{externally_visible} attributes are still necessary.
@item flatten
@cindex @code{flatten} function attribute
Generally, inlining into a function is limited. For a function marked with
this attribute, every call inside this function is inlined, if possible.
Functions declared with attribute @code{noinline} and similar are not
inlined. Whether the function itself is considered for inlining depends
on its size and the current inlining parameters.
@item format (@var{archetype}, @var{string-index}, @var{first-to-check})
@cindex @code{format} function attribute
@cindex functions with @code{printf}, @code{scanf}, @code{strftime} or @code{strfmon} style arguments
@opindex Wformat
The @code{format} attribute specifies that a function takes @code{printf},
@code{scanf}, @code{strftime} or @code{strfmon} style arguments that
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},
@code{gnu_printf}, @code{gnu_scanf}, @code{gnu_strftime} or
@code{strfmon}. (You can also use @code{__printf__},
@code{__scanf__}, @code{__strftime__} or @code{__strfmon__}.) On
MinGW targets, @code{ms_printf}, @code{ms_scanf}, and
@code{ms_strftime} are also present.
@var{archetype} values such as @code{printf} refer to the formats accepted
by the system's C runtime library,
while values prefixed with @samp{gnu_} always refer
to the formats accepted by the GNU C Library. On Microsoft Windows
targets, values prefixed with @samp{ms_} refer to the formats accepted by the
@file{msvcrt.dll} library.
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.
Since non-static C++ methods have an implicit @code{this} argument, the
arguments of such methods should be counted from two, not one, when
giving values for @var{string-index} and @var{first-to-check}.
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
@opindex fno-builtin
The @code{format} attribute allows you to identify your own functions
that take format strings as arguments, so that GCC can check the
calls to these functions for errors. The compiler always (unless
@option{-ffreestanding} or @option{-fno-builtin} 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}.
For Objective-C dialects, @code{NSString} (or @code{__NSString__}) is
recognized in the same context. Declarations including these format attributes
are parsed for correct syntax, however the result of checking of such format
strings is not yet defined, and is not carried out by this version of the
compiler.
The target may also provide additional types of format checks.
@xref{Target Format Checks,,Format Checks Specific to Particular
Target Machines}.
@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 one or
more format strings 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). Multiple
@code{format_arg} attributes may be applied to the same function, each
designating a distinct parameter as a format 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.
In calls to a function declared with more than one @code{format_arg}
attribute, each with a distinct argument value, the corresponding
actual function arguments are checked against all format strings
designated by the attributes. This capability is designed to support
the GNU @code{ngettext} family of functions.
The parameter @var{string-index} specifies which argument is the format
string argument (starting from one). Since non-static C++ methods have
an implicit @code{this} argument, the arguments of such methods should
be counted from two.
The @code{format_arg} attribute allows you to identify your own
functions that 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} or @option{-fno-builtin}
is used. @xref{C Dialect Options,,Options
Controlling C Dialect}.
For Objective-C dialects, the @code{format-arg} attribute may refer to an
@code{NSString} reference for compatibility with the @code{format} attribute
above.
The target may also allow additional types in @code{format-arg} attributes.
@xref{Target Format Checks,,Format Checks Specific to Particular
Target Machines}.
@item gnu_inline
@cindex @code{gnu_inline} function attribute
This attribute should be used with a function that is also declared
with the @code{inline} keyword. It directs GCC to treat the function
as if it were defined in gnu90 mode even when compiling in C99 or
gnu99 mode.
If the function is declared @code{extern}, then this definition of the
function is used only for inlining. In no case is the function
compiled as a standalone function, not even if you take its address
explicitly. Such an address becomes an external reference, as if you
had only declared the function, and had not defined it. This has
almost the effect of a macro. The way to use this is to put a
function definition in a header file with this attribute, and put
another copy of the function, without @code{extern}, in a library
file. The definition in the header file causes most calls to the
function to be inlined. If any uses of the function remain, they
refer to the single copy in the library. Note that the two
definitions of the functions need not be precisely the same, although
if they do not have the same effect your program may behave oddly.
In C, if the function is neither @code{extern} nor @code{static}, then
the function is compiled as a standalone function, as well as being
inlined where possible.
This is how GCC traditionally handled functions declared
@code{inline}. Since ISO C99 specifies a different semantics for
@code{inline}, this function attribute is provided as a transition
measure and as a useful feature in its own right. This attribute is
available in GCC 4.1.3 and later. It is available if either of the
preprocessor macros @code{__GNUC_GNU_INLINE__} or
@code{__GNUC_STDC_INLINE__} are defined. @xref{Inline,,An Inline
Function is As Fast As a Macro}.
In C++, this attribute does not depend on @code{extern} in any way,
but it still requires the @code{inline} keyword to enable its special
behavior.
@item hot
@cindex @code{hot} function attribute
The @code{hot} attribute on a function is used to inform the compiler that
the function is a hot spot of the compiled program. The function is
optimized more aggressively and on many targets it is placed into a special
subsection of the text section so all hot functions appear close together,
improving locality.
When profile feedback is available, via @option{-fprofile-use}, hot functions
are automatically detected and this attribute is ignored.
@item ifunc ("@var{resolver}")
@cindex @code{ifunc} function attribute
@cindex indirect functions
@cindex functions that are dynamically resolved
The @code{ifunc} attribute is used to mark a function as an indirect
function using the STT_GNU_IFUNC symbol type extension to the ELF
standard. This allows the resolution of the symbol value to be
determined dynamically at load time, and an optimized version of the
routine to be selected for the particular processor or other system
characteristics determined then. To use this attribute, first define
the implementation functions available, and a resolver function that
returns a pointer to the selected implementation function. The
implementation functions' declarations must match the API of the
function being implemented. The resolver should be declared to
be a function taking no arguments and returning a pointer to
a function of the same type as the implementation. For example:
@smallexample
void *my_memcpy (void *dst, const void *src, size_t len)
@{
@dots{}
return dst;
@}
static void * (*resolve_memcpy (void))(void *, const void *, size_t)
@{
return my_memcpy; // we will just always select this routine
@}
@end smallexample
@noindent
The exported header file declaring the function the user calls would
contain:
@smallexample
extern void *memcpy (void *, const void *, size_t);
@end smallexample
@noindent
allowing the user to call @code{memcpy} as a regular function, unaware of
the actual implementation. Finally, the indirect function needs to be
defined in the same translation unit as the resolver function:
@smallexample
void *memcpy (void *, const void *, size_t)
__attribute__ ((ifunc ("resolve_memcpy")));
@end smallexample
In C++, the @code{ifunc} attribute takes a string that is the mangled name
of the resolver function. A C++ resolver for a non-static member function
of class @code{C} should be declared to return a pointer to a non-member
function taking pointer to @code{C} as the first argument, followed by
the same arguments as of the implementation function. G++ checks
the signatures of the two functions and issues
a @option{-Wattribute-alias} warning for mismatches. To suppress a warning
for the necessary cast from a pointer to the implementation member function
to the type of the corresponding non-member function use
the @option{-Wno-pmf-conversions} option. For example:
@smallexample
class S
@{
private:
int debug_impl (int);
int optimized_impl (int);
typedef int Func (S*, int);
static Func* resolver ();
public:
int interface (int);
@};
int S::debug_impl (int) @{ /* @r{@dots{}} */ @}
int S::optimized_impl (int) @{ /* @r{@dots{}} */ @}
S::Func* S::resolver ()
@{
int (S::*pimpl) (int)
= getenv ("DEBUG") ? &S::debug_impl : &S::optimized_impl;
// Cast triggers -Wno-pmf-conversions.
return reinterpret_cast<Func*>(pimpl);
@}
int S::interface (int) __attribute__ ((ifunc ("_ZN1S8resolverEv")));
@end smallexample
Indirect functions cannot be weak. Binutils version 2.20.1 or higher
and GNU C Library version 2.11.1 are required to use this feature.
@item interrupt
@itemx interrupt_handler
Many GCC back ends support attributes to indicate that a function is
an interrupt handler, which tells the compiler to generate function
entry and exit sequences that differ from those from regular
functions. The exact syntax and behavior are target-specific;
refer to the following subsections for details.
@item leaf
@cindex @code{leaf} function attribute
Calls to external functions with this attribute must return to the
current compilation unit only by return or by exception handling. In
particular, a leaf function is not allowed to invoke callback functions
passed to it from the current compilation unit, directly call functions
exported by the unit, or @code{longjmp} into the unit. Leaf functions
might still call functions from other compilation units and thus they
are not necessarily leaf in the sense that they contain no function
calls at all.
The attribute is intended for library functions to improve dataflow
analysis. The compiler takes the hint that any data not escaping the
current compilation unit cannot be used or modified by the leaf
function. For example, the @code{sin} function is a leaf function, but
@code{qsort} is not.
Note that leaf functions might indirectly run a signal handler defined
in the current compilation unit that uses static variables. Similarly,
when lazy symbol resolution is in effect, leaf functions might invoke
indirect functions whose resolver function or implementation function is
defined in the current compilation unit and uses static variables. There
is no standard-compliant way to write such a signal handler, resolver
function, or implementation function, and the best that you can do is to
remove the @code{leaf} attribute or mark all such static variables
@code{volatile}. Lastly, for ELF-based systems that support symbol
interposition, care should be taken that functions defined in the
current compilation unit do not unexpectedly interpose other symbols
based on the defined standards mode and defined feature test macros;
otherwise an inadvertent callback would be added.
The attribute has no effect on functions defined within the current
compilation unit. This is to allow easy merging of multiple compilation
units into one, for example, by using the link-time optimization. For
this reason the attribute is not allowed on types to annotate indirect
calls.
@item malloc
@item malloc (@var{deallocator})
@item malloc (@var{deallocator}, @var{ptr-index})
@cindex @code{malloc} function attribute
@cindex functions that behave like malloc
Attribute @code{malloc} indicates that a function is @code{malloc}-like,
i.e., that the pointer @var{P} returned by the function cannot alias any
other pointer valid when the function returns, and moreover no
pointers to valid objects occur in any storage addressed by @var{P}. In
addition, the GCC predicts that a function with the attribute returns
non-null in most cases.
Independently, the form of the attribute with one or two arguments
associates @code{deallocator} as a suitable deallocation function for
pointers returned from the @code{malloc}-like function. @var{ptr-index}
denotes the positional argument to which when the pointer is passed in
calls to @code{deallocator} has the effect of deallocating it.
Using the attribute with no arguments is designed to improve optimization
by relying on the aliasing property it implies. Functions like @code{malloc}
and @code{calloc} have this property because they return a pointer to
uninitialized or zeroed-out, newly obtained storage. However, functions
like @code{realloc} do not have this property, as they may return pointers
to storage containing pointers to existing objects. Additionally, since
all such functions are assumed to return null only infrequently, callers
can be optimized based on that assumption.
Associating a function with a @var{deallocator} helps detect calls to
mismatched allocation and deallocation functions and diagnose them under
the control of options such as @option{-Wmismatched-dealloc}. It also
makes it possible to diagnose attempts to deallocate objects that were not
allocated dynamically, by @option{-Wfree-nonheap-object}. To indicate
that an allocation function both satisifies the nonaliasing property and
has a deallocator associated with it, both the plain form of the attribute
and the one with the @var{deallocator} argument must be used. The same
function can be both an allocator and a deallocator. Since inlining one
of the associated functions but not the other could result in apparent
mismatches, this form of attribute @code{malloc} is not accepted on inline
functions. For the same reason, using the attribute prevents both
the allocation and deallocation functions from being expanded inline.
For example, besides stating that the functions return pointers that do
not alias any others, the following declarations make @code{fclose}
a suitable deallocator for pointers returned from all functions except
@code{popen}, and @code{pclose} as the only suitable deallocator for
pointers returned from @code{popen}. The deallocator functions must
be declared before they can be referenced in the attribute.
@smallexample
int fclose (FILE*);
int pclose (FILE*);
__attribute__ ((malloc, malloc (fclose, 1)))
FILE* fdopen (int, const char*);
__attribute__ ((malloc, malloc (fclose, 1)))
FILE* fopen (const char*, const char*);
__attribute__ ((malloc, malloc (fclose, 1)))
FILE* fmemopen(void *, size_t, const char *);
__attribute__ ((malloc, malloc (pclose, 1)))
FILE* popen (const char*, const char*);
__attribute__ ((malloc, malloc (fclose, 1)))
FILE* tmpfile (void);
@end smallexample
The warnings guarded by @option{-fanalyzer} respect allocation and
deallocation pairs marked with the @code{malloc}. In particular:
@itemize @bullet
@item
The analyzer will emit a @option{-Wanalyzer-mismatching-deallocation}
diagnostic if there is an execution path in which the result of an
allocation call is passed to a different deallocator.
@item
The analyzer will emit a @option{-Wanalyzer-double-free}
diagnostic if there is an execution path in which a value is passed
more than once to a deallocation call.
@item
The analyzer will consider the possibility that an allocation function
could fail and return NULL. It will emit
@option{-Wanalyzer-possible-null-dereference} and
@option{-Wanalyzer-possible-null-argument} diagnostics if there are
execution paths in which an unchecked result of an allocation call is
dereferenced or passed to a function requiring a non-null argument.
If the allocator always returns non-null, use
@code{__attribute__ ((returns_nonnull))} to suppress these warnings.
For example:
@smallexample
char *xstrdup (const char *)
__attribute__((malloc (free), returns_nonnull));
@end smallexample
@item
The analyzer will emit a @option{-Wanalyzer-use-after-free}
diagnostic if there is an execution path in which the memory passed
by pointer to a deallocation call is used after the deallocation.
@item
The analyzer will emit a @option{-Wanalyzer-malloc-leak} diagnostic if
there is an execution path in which the result of an allocation call
is leaked (without being passed to the deallocation function).
@item
The analyzer will emit a @option{-Wanalyzer-free-of-non-heap} diagnostic
if a deallocation function is used on a global or on-stack variable.
@end itemize
The analyzer assumes that deallocators can gracefully handle the @code{NULL}
pointer. If this is not the case, the deallocator can be marked with
@code{__attribute__((nonnull))} so that @option{-fanalyzer} can emit
a @option{-Wanalyzer-possible-null-argument} diagnostic for code paths
in which the deallocator is called with NULL.
@item no_icf
@cindex @code{no_icf} function attribute
This function attribute prevents a functions from being merged with another
semantically equivalent function.
@item no_instrument_function
@cindex @code{no_instrument_function} function attribute
@opindex finstrument-functions
@opindex p
@opindex pg
If any of @option{-finstrument-functions}, @option{-p}, or @option{-pg} are
given, profiling function calls are
generated at entry and exit of most user-compiled functions.
Functions with this attribute are not so instrumented.
@item no_profile_instrument_function
@cindex @code{no_profile_instrument_function} function attribute
The @code{no_profile_instrument_function} attribute on functions is used
to inform the compiler that it should not process any profile feedback based
optimization code instrumentation.
@item no_reorder
@cindex @code{no_reorder} function attribute
Do not reorder functions or variables marked @code{no_reorder}
against each other or top level assembler statements the executable.
The actual order in the program will depend on the linker command
line. Static variables marked like this are also not removed.
This has a similar effect
as the @option{-fno-toplevel-reorder} option, but only applies to the
marked symbols.
@item no_sanitize ("@var{sanitize_option}")
@cindex @code{no_sanitize} function attribute
The @code{no_sanitize} attribute on functions is used
to inform the compiler that it should not do sanitization of any option
mentioned in @var{sanitize_option}. A list of values acceptable by
the @option{-fsanitize} option can be provided.
@smallexample
void __attribute__ ((no_sanitize ("alignment", "object-size")))
f () @{ /* @r{Do something.} */; @}
void __attribute__ ((no_sanitize ("alignment,object-size")))
g () @{ /* @r{Do something.} */; @}
@end smallexample
@item no_sanitize_address
@itemx no_address_safety_analysis
@cindex @code{no_sanitize_address} function attribute
The @code{no_sanitize_address} attribute on functions is used
to inform the compiler that it should not instrument memory accesses
in the function when compiling with the @option{-fsanitize=address} option.
The @code{no_address_safety_analysis} is a deprecated alias of the
@code{no_sanitize_address} attribute, new code should use
@code{no_sanitize_address}.
@item no_sanitize_thread
@cindex @code{no_sanitize_thread} function attribute
The @code{no_sanitize_thread} attribute on functions is used
to inform the compiler that it should not instrument memory accesses
in the function when compiling with the @option{-fsanitize=thread} option.
@item no_sanitize_undefined
@cindex @code{no_sanitize_undefined} function attribute
The @code{no_sanitize_undefined} attribute on functions is used
to inform the compiler that it should not check for undefined behavior
in the function when compiling with the @option{-fsanitize=undefined} option.
@item no_sanitize_coverage
@cindex @code{no_sanitize_coverage} function attribute
The @code{no_sanitize_coverage} attribute on functions is used
to inform the compiler that it should not do coverage-guided
fuzzing code instrumentation (@option{-fsanitize-coverage}).
@item no_split_stack
@cindex @code{no_split_stack} function attribute
@opindex fsplit-stack
If @option{-fsplit-stack} is given, functions have a small
prologue which decides whether to split the stack. Functions with the
@code{no_split_stack} attribute do not have that prologue, and thus
may run with only a small amount of stack space available.
@item no_stack_limit
@cindex @code{no_stack_limit} function attribute
This attribute locally overrides the @option{-fstack-limit-register}
and @option{-fstack-limit-symbol} command-line options; it has the effect
of disabling stack limit checking in the function it applies to.
@item noclone
@cindex @code{noclone} function attribute
This function attribute prevents a function from being considered for
cloning---a mechanism that produces specialized copies of functions
and which is (currently) performed by interprocedural constant
propagation.
@item noinline
@cindex @code{noinline} function attribute
This function attribute prevents a function from being considered for
inlining.
@c Don't enumerate the optimizations by name here; we try to be
@c future-compatible with this mechanism.
If the function does not have side effects, there are optimizations
other than inlining that cause function calls to be optimized away,
although the function call is live. To keep such calls from being
optimized away, put
@smallexample
asm ("");
@end smallexample
@noindent
(@pxref{Extended Asm}) in the called function, to serve as a special
side effect.
@item noipa
@cindex @code{noipa} function attribute
Disable interprocedural optimizations between the function with this
attribute and its callers, as if the body of the function is not available
when optimizing callers and the callers are unavailable when optimizing
the body. This attribute implies @code{noinline}, @code{noclone} and
@code{no_icf} attributes. However, this attribute is not equivalent
to a combination of other attributes, because its purpose is to suppress
existing and future optimizations employing interprocedural analysis,
including those that do not have an attribute suitable for disabling
them individually. This attribute is supported mainly for the purpose
of testing the compiler.
@item nonnull
@itemx nonnull (@var{arg-index}, @dots{})
@cindex @code{nonnull} function attribute
@cindex functions with non-null pointer arguments
The @code{nonnull} attribute may be applied to a function that takes at
least one argument of a pointer type. It indicates that the referenced
arguments must be non-null pointers. For instance, the declaration:
@smallexample
extern void *
my_memcpy (void *dest, const void *src, size_t len)
__attribute__((nonnull (1, 2)));
@end smallexample
@noindent
informs the compiler that, in calls to @code{my_memcpy}, arguments
@var{dest} and @var{src} must be non-null.
The attribute has an effect both on functions calls and function definitions.
For function calls:
@itemize @bullet
@item If the compiler determines that a null pointer is
passed in an argument slot marked as non-null, and the
@option{-Wnonnull} option is enabled, a warning is issued.
@xref{Warning Options}.
@item The @option{-fisolate-erroneous-paths-attribute} option can be
specified to have GCC transform calls with null arguments to non-null
functions into traps. @xref{Optimize Options}.
@item The compiler may also perform optimizations based on the
knowledge that certain function arguments cannot be null. These
optimizations can be disabled by the
@option{-fno-delete-null-pointer-checks} option. @xref{Optimize Options}.
@end itemize
For function definitions:
@itemize @bullet
@item If the compiler determines that a function parameter that is
marked with nonnull is compared with null, and
@option{-Wnonnull-compare} option is enabled, a warning is issued.
@xref{Warning Options}.
@item The compiler may also perform optimizations based on the
knowledge that @code{nonnul} parameters cannot be null. This can
currently not be disabled other than by removing the nonnull
attribute.
@end itemize
If no @var{arg-index} is given to the @code{nonnull} attribute,
all pointer arguments are marked as non-null. To illustrate, the
following declaration is equivalent to the previous example:
@smallexample
extern void *
my_memcpy (void *dest, const void *src, size_t len)
__attribute__((nonnull));
@end smallexample
@item noplt
@cindex @code{noplt} function attribute
The @code{noplt} attribute is the counterpart to option @option{-fno-plt}.
Calls to functions marked with this attribute in position-independent code
do not use the PLT.
@smallexample
@group
/* Externally defined function foo. */
int foo () __attribute__ ((noplt));
int
main (/* @r{@dots{}} */)
@{
/* @r{@dots{}} */
foo ();
/* @r{@dots{}} */
@}
@end group
@end smallexample
The @code{noplt} attribute on function @code{foo}
tells the compiler to assume that
the function @code{foo} is externally defined and that the call to
@code{foo} must avoid the PLT
in position-independent code.
In position-dependent code, a few targets also convert calls to
functions that are marked to not use the PLT to use the GOT instead.
@item noreturn
@cindex @code{noreturn} function attribute
@cindex functions that never return
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 (/* @r{@dots{}} */)
@{
/* @r{@dots{}} */ /* @r{Print error message.} */ /* @r{@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.
The @code{noreturn} keyword does not affect the exceptional path when that
applies: a @code{noreturn}-marked function may still return to the caller
by throwing an exception or calling @code{longjmp}.
In order to preserve backtraces, GCC will never turn calls to
@code{noreturn} functions into tail calls.
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}.
@item nothrow
@cindex @code{nothrow} function attribute
The @code{nothrow} attribute is used to inform the compiler that a
function cannot throw an exception. For example, most functions in
the standard C library can be guaranteed not to throw an exception
with the notable exceptions of @code{qsort} and @code{bsearch} that
take function pointer arguments.
@item optimize (@var{level}, @dots{})
@item optimize (@var{string}, @dots{})
@cindex @code{optimize} function attribute
The @code{optimize} attribute is used to specify that a function is to
be compiled with different optimization options than specified on the
command line. The optimize attribute arguments of a function behave
behave as if appended to the command-line.
Valid arguments are constant non-negative integers and
strings. Each numeric argument specifies an optimization @var{level}.
Each @var{string} argument consists of one or more comma-separated
substrings. Each substring that begins with the letter @code{O} refers
to an optimization option such as @option{-O0} or @option{-Os}. Other
substrings are taken as suffixes to the @code{-f} prefix jointly
forming the name of an optimization option. @xref{Optimize Options}.
@samp{#pragma GCC optimize} can be used to set optimization options
for more than one function. @xref{Function Specific Option Pragmas},
for details about the pragma.
Providing multiple strings as arguments separated by commas to specify
multiple options is equivalent to separating the option suffixes with
a comma (@samp{,}) within a single string. Spaces are not permitted
within the strings.
Not every optimization option that starts with the @var{-f} prefix
specified by the attribute necessarily has an effect on the function.
The @code{optimize} attribute should be used for debugging purposes only.
It is not suitable in production code.
@item patchable_function_entry
@cindex @code{patchable_function_entry} function attribute
@cindex extra NOP instructions at the function entry point
In case the target's text segment can be made writable at run time by
any means, padding the function entry with a number of NOPs can be
used to provide a universal tool for instrumentation.
The @code{patchable_function_entry} function attribute can be used to
change the number of NOPs to any desired value. The two-value syntax
is the same as for the command-line switch
@option{-fpatchable-function-entry=N,M}, generating @var{N} NOPs, with
the function entry point before the @var{M}th NOP instruction.
@var{M} defaults to 0 if omitted e.g.@: function entry point is before
the first NOP.
If patchable function entries are enabled globally using the command-line
option @option{-fpatchable-function-entry=N,M}, then you must disable
instrumentation on all functions that are part of the instrumentation
framework with the attribute @code{patchable_function_entry (0)}
to prevent recursion.
@item pure
@cindex @code{pure} function attribute
@cindex functions that have no side effects
Calls to functions that have no observable effects on the state of
the program other than to return a value may lend themselves to optimizations
such as common subexpression elimination. Declaring such functions with
the @code{pure} attribute allows GCC to avoid emitting some calls in repeated
invocations of the function with the same argument values.
The @code{pure} attribute prohibits a function from modifying the state
of the program that is observable by means other than inspecting
the function's return value. However, functions declared with the @code{pure}
attribute can safely read any non-volatile objects, and modify the value of
objects in a way that does not affect their return value or the observable
state of the program.
For example,
@smallexample
int hash (char *) __attribute__ ((pure));
@end smallexample
@noindent
tells GCC that subsequent calls to the function @code{hash} with the same
string can be replaced by the result of the first call provided the state
of the program observable by @code{hash}, including the contents of the array
itself, does not change in between. Even though @code{hash} takes a non-const
pointer argument it must not modify the array it points to, or any other object
whose value the rest of the program may depend on. However, the caller may
safely change the contents of the array between successive calls to
the function (doing so disables the optimization). The restriction also
applies to member objects referenced by the @code{this} pointer in C++
non-static member functions.
Some 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
consecutive calls (such as the standard C @code{feof} function in
a multithreading environment).
The @code{pure} attribute imposes similar but looser restrictions on
a function's definition than the @code{const} attribute: @code{pure}
allows the function to read any non-volatile memory, even if it changes
in between successive invocations of the function. Declaring the same
function with both the @code{pure} and the @code{const} attribute is
diagnosed. Because a pure function cannot have any observable side
effects it does not make sense for such a function to return @code{void}.
Declaring such a function is diagnosed.
@item returns_nonnull
@cindex @code{returns_nonnull} function attribute
The @code{returns_nonnull} attribute specifies that the function
return value should be a non-null pointer. For instance, the declaration:
@smallexample
extern void *
mymalloc (size_t len) __attribute__((returns_nonnull));
@end smallexample
@noindent
lets the compiler optimize callers based on the knowledge
that the return value will never be null.
@item returns_twice
@cindex @code{returns_twice} function attribute
@cindex functions that return more than once
The @code{returns_twice} attribute tells the compiler that a function may
return more than one time. The compiler ensures that all registers
are dead before calling such a function and emits a warning about
the variables that may be clobbered after the second return from the
function. Examples of such functions are @code{setjmp} and @code{vfork}.
The @code{longjmp}-like counterpart of such function, if any, might need
to be marked with the @code{noreturn} attribute.
@item section ("@var{section-name}")
@cindex @code{section} function attribute
@cindex functions in arbitrary sections
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 sentinel
@itemx sentinel (@var{position})
@cindex @code{sentinel} function attribute
This function attribute indicates that an argument in a call to the function
is expected to be an explicit @code{NULL}. The attribute is only valid on
variadic functions. By default, the sentinel is expected to be the last
argument of the function call. If the optional @var{position} argument
is specified to the attribute, the sentinel must be located at
@var{position} counting backwards from the end of the argument list.
@smallexample
__attribute__ ((sentinel))
is equivalent to
__attribute__ ((sentinel(0)))
@end smallexample
The attribute is automatically set with a position of 0 for the built-in
functions @code{execl} and @code{execlp}. The built-in function
@code{execle} has the attribute set with a position of 1.
A valid @code{NULL} in this context is defined as zero with any object
pointer type. If your system defines the @code{NULL} macro with
an integer type then you need to add an explicit cast. During
installation GCC replaces the system @code{<stddef.h>} header with
a copy that redefines NULL appropriately.
The warnings for missing or incorrect sentinels are enabled with
@option{-Wformat}.
@item simd
@itemx simd("@var{mask}")
@cindex @code{simd} function attribute
This attribute enables creation of one or more function versions that
can process multiple arguments using SIMD instructions from a
single invocation. Specifying this attribute allows compiler to
assume that such versions are available at link time (provided
in the same or another translation unit). Generated versions are
target-dependent and described in the corresponding Vector ABI document. For
x86_64 target this document can be found
@w{@uref{https://sourceware.org/glibc/wiki/libmvec?action=AttachFile&do=view&target=VectorABI.txt,here}}.
The optional argument @var{mask} may have the value
@code{notinbranch} or @code{inbranch},
and instructs the compiler to generate non-masked or masked
clones correspondingly. By default, all clones are generated.
If the attribute is specified and @code{#pragma omp declare simd} is
present on a declaration and the @option{-fopenmp} or @option{-fopenmp-simd}
switch is specified, then the attribute is ignored.
@item stack_protect
@cindex @code{stack_protect} function attribute
This attribute adds stack protection code to the function if
flags @option{-fstack-protector}, @option{-fstack-protector-strong}
or @option{-fstack-protector-explicit} are set.
@item no_stack_protector
@cindex @code{no_stack_protector} function attribute
This attribute prevents stack protection code for the function.
@item target (@var{string}, @dots{})
@cindex @code{target} function attribute
Multiple target back ends implement the @code{target} attribute
to specify that a function is to
be compiled with different target options than specified on the
command line. The original target command-line options are ignored.
One or more strings can be provided as arguments.
Each string consists of one or more comma-separated suffixes to
the @code{-m} prefix jointly forming the name of a machine-dependent
option. @xref{Submodel Options,,Machine-Dependent Options}.
The @code{target} attribute can be used for instance to have a function
compiled with a different ISA (instruction set architecture) than the
default. @samp{#pragma GCC target} can be used to specify target-specific
options for more than one function. @xref{Function Specific Option Pragmas},
for details about the pragma.
For instance, on an x86, you could declare one function with the
@code{target("sse4.1,arch=core2")} attribute and another with
@code{target("sse4a,arch=amdfam10")}. This is equivalent to
compiling the first function with @option{-msse4.1} and
@option{-march=core2} options, and the second function with
@option{-msse4a} and @option{-march=amdfam10} options. It is up to you
to make sure that a function is only invoked on a machine that
supports the particular ISA it is compiled for (for example by using
@code{cpuid} on x86 to determine what feature bits and architecture
family are used).
@smallexample
int core2_func (void) __attribute__ ((__target__ ("arch=core2")));
int sse3_func (void) __attribute__ ((__target__ ("sse3")));
@end smallexample
Providing multiple strings as arguments separated by commas to specify
multiple options is equivalent to separating the option suffixes with
a comma (@samp{,}) within a single string. Spaces are not permitted
within the strings.
The options supported are specific to each target; refer to @ref{x86
Function Attributes}, @ref{PowerPC Function Attributes},
@ref{ARM Function Attributes}, @ref{AArch64 Function Attributes},
@ref{Nios II Function Attributes}, and @ref{S/390 Function Attributes}
for details.
@item symver ("@var{name2}@@@var{nodename}")
@cindex @code{symver} function attribute
On ELF targets this attribute creates a symbol version. The @var{name2} part
of the parameter is the actual name of the symbol by which it will be
externally referenced. The @code{nodename} portion should be the name of a
node specified in the version script supplied to the linker when building a
shared library. Versioned symbol must be defined and must be exported with
default visibility.
@smallexample
__attribute__ ((__symver__ ("foo@@VERS_1"))) int
foo_v1 (void)
@{
@}
@end smallexample
Will produce a @code{.symver foo_v1, foo@@VERS_1} directive in the assembler
output.
One can also define multiple version for a given symbol
(starting from binutils 2.35).
@smallexample
__attribute__ ((__symver__ ("foo@@VERS_2"), __symver__ ("foo@@VERS_3")))
int symver_foo_v1 (void)
@{
@}
@end smallexample
This example creates a symbol name @code{symver_foo_v1}
which will be version @code{VERS_2} and @code{VERS_3} of @code{foo}.
If you have an older release of binutils, then symbol alias needs to
be used:
@smallexample
__attribute__ ((__symver__ ("foo@@VERS_2")))
int foo_v1 (void)
@{
return 0;
@}
__attribute__ ((__symver__ ("foo@@VERS_3")))
__attribute__ ((alias ("foo_v1")))
int symver_foo_v1 (void);
@end smallexample
Finally if the parameter is @code{"@var{name2}@@@@@var{nodename}"} then in
addition to creating a symbol version (as if
@code{"@var{name2}@@@var{nodename}"} was used) the version will be also used
to resolve @var{name2} by the linker.
@item target_clones (@var{options})
@cindex @code{target_clones} function attribute
The @code{target_clones} attribute is used to specify that a function
be cloned into multiple versions compiled with different target options
than specified on the command line. The supported options and restrictions
are the same as for @code{target} attribute.
For instance, on an x86, you could compile a function with
@code{target_clones("sse4.1,avx")}. GCC creates two function clones,
one compiled with @option{-msse4.1} and another with @option{-mavx}.
On a PowerPC, you can compile a function with
@code{target_clones("cpu=power9,default")}. GCC will create two
function clones, one compiled with @option{-mcpu=power9} and another
with the default options. GCC must be configured to use GLIBC 2.23 or
newer in order to use the @code{target_clones} attribute.
It also creates a resolver function (see
the @code{ifunc} attribute above) that dynamically selects a clone
suitable for current architecture. The resolver is created only if there
is a usage of a function with @code{target_clones} attribute.
Note that any subsequent call of a function without @code{target_clone}
from a @code{target_clone} caller will not lead to copying
(target clone) of the called function.
If you want to enforce such behaviour,
we recommend declaring the calling function with the @code{flatten} attribute?
@item unused
@cindex @code{unused} function attribute
This attribute, attached to a function, means that the function is meant
to be possibly unused. GCC does not produce a warning for this
function.
@item used
@cindex @code{used} function attribute
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.
When applied to a member function of a C++ class template, the
attribute also means that the function is instantiated if the
class itself is instantiated.
@item retain
@cindex @code{retain} function attribute
For ELF targets that support the GNU or FreeBSD OSABIs, this attribute
will save the function from linker garbage collection. To support
this behavior, functions that have not been placed in specific sections
(e.g. by the @code{section} attribute, or the @code{-ffunction-sections}
option), will be placed in new, unique sections.
This additional functionality requires Binutils version 2.36 or later.
@item visibility ("@var{visibility_type}")
@cindex @code{visibility} function attribute
This attribute affects the linkage of the declaration to which it is attached.
It can be applied to variables (@pxref{Common Variable Attributes}) and types
(@pxref{Common Type Attributes}) as well as functions.
There are four supported @var{visibility_type} values: default,
hidden, protected or internal visibility.
@smallexample
void __attribute__ ((visibility ("protected")))
f () @{ /* @r{Do something.} */; @}
int i __attribute__ ((visibility ("hidden")));
@end smallexample
The possible values of @var{visibility_type} correspond to the
visibility settings in the ELF gABI.
@table @code
@c keep this list of visibilities in alphabetical order.
@item default
Default visibility is the normal case for the object file format.
This value is available for the visibility attribute to override other
options that may change the assumed visibility of entities.
On ELF, default visibility means that the declaration is visible to other
modules and, in shared libraries, means that the declared entity may be
overridden.
On Darwin, default visibility means that the declaration is visible to
other modules.
Default visibility corresponds to ``external linkage'' in the language.
@item hidden
Hidden visibility indicates that the entity declared has a new
form of linkage, which we call ``hidden linkage''. Two
declarations of an object with hidden linkage refer to the same object
if they are in the same shared object.
@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 a function is
@emph{never} called from another module. Compare this with hidden
functions which, while they cannot be referenced directly by other
modules, can be referenced indirectly via function pointers. By
indicating that a function 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.
@item protected
Protected visibility is like default visibility except that it
indicates that references within the defining module bind to the
definition in that module. That is, the declared entity cannot be
overridden by another module.
@end table
All visibilities are supported on many, but not all, ELF targets
(supported when the assembler supports the @samp{.visibility}
pseudo-op). Default visibility is supported everywhere. Hidden
visibility is supported on Darwin targets.
The visibility attribute should be applied only to declarations that
would otherwise have external linkage. The attribute should be applied
consistently, so that the same entity should not be declared with
different settings of the attribute.
In C++, the visibility attribute applies to types as well as functions
and objects, because in C++ types have linkage. A class must not have
greater visibility than its non-static data member types and bases,
and class members default to the visibility of their class. Also, a
declaration without explicit visibility is limited to the visibility
of its type.
In C++, you can mark member functions and static member variables of a
class with the visibility attribute. This is useful if you know a
particular method or static member variable should only be used from
one shared object; then you can mark it hidden while the rest of the
class has default visibility. Care must be taken to avoid breaking
the One Definition Rule; for example, it is usually not useful to mark
an inline method as hidden without marking the whole class as hidden.
A C++ namespace declaration can also have the visibility attribute.
@smallexample
namespace nspace1 __attribute__ ((visibility ("protected")))
@{ /* @r{Do something.} */; @}
@end smallexample
This attribute applies only to the particular namespace body, not to
other definitions of the same namespace; it is equivalent to using
@samp{#pragma GCC visibility} before and after the namespace
definition (@pxref{Visibility Pragmas}).
In C++, if a template argument has limited visibility, this
restriction is implicitly propagated to the template instantiation.
Otherwise, template instantiations and specializations default to the
visibility of their template.
If both the template and enclosing class have explicit visibility, the
visibility from the template is used.
@item warn_unused_result
@cindex @code{warn_unused_result} function attribute
The @code{warn_unused_result} attribute causes a warning to be emitted
if a caller of the function with this attribute does not use its
return value. This is useful for functions where not checking
the result is either a security problem or always a bug, such as
@code{realloc}.
@smallexample
int fn () __attribute__ ((warn_unused_result));
int foo ()
@{
if (fn () < 0) return -1;
fn ();
return 0;
@}
@end smallexample
@noindent
results in warning on line 5.
@item weak
@cindex @code{weak} function attribute
The @code{weak} attribute causes a declaration of an external symbol
to be emitted as a weak symbol rather than a global. This is primarily
useful in defining library functions that can be overridden in user code,
though it can also be used with non-function declarations. The overriding
symbol must have the same type as the weak symbol. In addition, if it
designates a variable it must also have the same size and alignment as
the weak symbol. Weak symbols are supported for ELF targets, and also
for a.out targets when using the GNU assembler and linker.
@item weakref
@itemx weakref ("@var{target}")
@cindex @code{weakref} function attribute
The @code{weakref} attribute marks a declaration as a weak reference.
Without arguments, it should be accompanied by an @code{alias} attribute
naming the target symbol. Alternatively, @var{target} may be given as
an argument to @code{weakref} itself, naming the target definition of
the alias. The @var{target} must have the same type as the declaration.
In addition, if it designates a variable it must also have the same size
and alignment as the declaration. In either form of the declaration
@code{weakref} implicitly marks the declared symbol as @code{weak}. Without
a @var{target} given as an argument to @code{weakref} or to @code{alias},
@code{weakref} is equivalent to @code{weak} (in that case the declaration
may be @code{extern}).
@smallexample
/* Given the declaration: */
extern int y (void);
/* the following... */
static int x (void) __attribute__ ((weakref ("y")));
/* is equivalent to... */
static int x (void) __attribute__ ((weakref, alias ("y")));
/* or, alternatively, to... */
static int x (void) __attribute__ ((weakref));
static int x (void) __attribute__ ((alias ("y")));
@end smallexample
A weak reference is an alias that does not by itself require a
definition to be given for the target symbol. If the target symbol is
only referenced through weak references, then it becomes a @code{weak}
undefined symbol. If it is directly referenced, however, then such
strong references prevail, and a definition is required for the
symbol, not necessarily in the same translation unit.
The effect is equivalent to moving all references to the alias to a
separate translation unit, renaming the alias to the aliased symbol,
declaring it as weak, compiling the two separate translation units and
performing a link with relocatable output (i.e.@: @code{ld -r}) on them.
A declaration to which @code{weakref} is attached and that is associated
with a named @code{target} must be @code{static}.
@item zero_call_used_regs ("@var{choice}")
@cindex @code{zero_call_used_regs} function attribute
The @code{zero_call_used_regs} attribute causes the compiler to zero
a subset of all call-used registers@footnote{A ``call-used'' register