In GNU C, you declare certain things about functions called in your program which help the compiler optimize function calls and check your code more carefully.
The keyword __attribute__ allows you to specify special
attributes when making a declaration. This keyword is followed by an
attribute specification inside double parentheses. Eight attributes,
noreturn, const, format, section,
constructor, destructor, unused and weak are
currently defined for functions. Other attributes, including
section are supported for variables declarations (see section Specifying Attributes of Variables) and for types (see section Specifying Attributes of Types).
You may also specify attributes with `__' preceding and following
each keyword. This allows you to use them in header files without
being concerned about a possible macro of the same name. For example,
you may use __noreturn__ instead of noreturn.
noreturn
abort and exit,
cannot return. GNU CC knows this automatically. Some programs define
their own functions that never return. You can declare them
noreturn to tell the compiler this fact. For example,
void fatal () __attribute__ ((noreturn));
void
fatal (...)
{
... /* Print error message. */ ...
exit (1);
}
The noreturn keyword tells the compiler to assume that
fatal cannot return. It can then optimize without regard to what
would happen if fatal ever did return. This makes slightly
better code. More importantly, it helps avoid spurious warnings of
uninitialized variables.
Do not assume that registers saved by the calling function are
restored before calling the noreturn function.
It does not make sense for a noreturn function to have a return
type other than void.
The attribute noreturn is not implemented in GNU C versions
earlier than 2.5. An alternative way to declare that a function does
not return, which works in the current version and in some older
versions, is as follows:
typedef void voidfn (); volatile voidfn fatal;
const
const. For example,
int square (int) __attribute__ ((const));says that the hypothetical function
square is safe to call
fewer times than the program says.
The attribute const is not implemented in GNU C versions earlier
than 2.5. An alternative way to declare that a function has no side
effects, which works in the current version and in some older versions,
is as follows:
typedef int intfn (); extern const intfn square;This approach does not work in GNU C++ from 2.6.0 on, since the language specifies that the `const' must be attached to the return value. Note that a function that has pointer arguments and examines the data pointed to must not be declared
const. Likewise, a
function that calls a non-const function usually must not be
const. It does not make sense for a const function to
return void.
format (archetype, string-index, first-to-check)
format attribute specifies that a function takes printf
or scanf style arguments which should be type-checked against a
format string. For example, the declaration:
extern int
my_printf (void *my_object, const char *my_format, ...)
__attribute__ ((format (printf, 2, 3)));
causes the compiler to check the arguments in calls to my_printf
for consistency with the printf style format string argument
my_format.
The parameter archetype determines how the format string is
interpreted, and should be either printf or scanf. The
parameter string-index specifies which argument is the format
string argument (starting from 1), while first-to-check is the
number of the first argument to check against the format string. For
functions where the arguments are not available to be checked (such as
vprintf), specify the third parameter as zero. In this case the
compiler only checks the format string for consistency.
In the example above, the format string (my_format) is the second
argument of the function my_print, and the arguments to check
start with the third argument, so the correct parameters for the format
attribute are 2 and 3.
The format attribute allows you to identify your own functions
which take format strings as arguments, so that GNU CC can check the
calls to these functions for errors. The compiler always checks formats
for the ANSI library functions printf, fprintf,
sprintf, scanf, fscanf, sscanf,
vprintf, vfprintf and vsprintf whenever such
warnings are requested (using `-Wformat'), so there is no need to
modify the header file `stdio.h'.
section ("section-name")
text section.
Sometimes, however, you need additional sections, or you need certain
particular functions to appear in special sections. The section
attribute specifies that a function lives in a particular section.
For example, the declaration:
extern void foobar (void) __attribute__ ((section ("bar")));
puts the function foobar in the bar section.
Some file formats do not support arbitrary sections so the section
attribute is not available on all platforms.
If you need to map the entire contents of a module to a particular
section, consider using the facilities of the linker instead.
constructor
destructor
constructor attribute causes the function to be called
automatically before execution enters main (). Similarly, the
destructor attribute causes the function to be called
automatically after main () has completed or exit () has
been called. Functions with these attributes are useful for
initializing data that will be used implicitly during the execution of
the program.
These attributes are not currently implemented for Objective C.
unused
weak
weak attribute causes the declaration to be emitted as a weak
symbol rather than a global. This is primarily useful in defining
library functions which can be overridden in user code, though it can
also be used with non-function declarations. Weak symbols are supported
for ELF targets, and also for a.out targets when using the GNU assembler
and linker.
alias ("target")
alias attribute causes the declaration to be emitted as an
alias for another symbol, which must be specified. For instance,
void __f () { /* do something */; }
void f () __attribute__ ((weak, alias ("__f")));
declares `f' to be a weak alias for `__f'. In C++, the
mangled name for the target must be used.
regparm (number)
regparm attribute causes the compiler to
pass up to number integer arguments in registers EAX,
EDX, and ECX instead of on the stack. Functions that take a
variable number of arguments will continue to be passed all of their
arguments on the stack.
stdcall
stdcall attribute causes the compiler to
assume that the called function will pop off the stack space used to
pass arguments, unless it takes a variable number of arguments.
cdecl
cdecl attribute causes the compiler to
assume that the called function will pop off the stack space used to
pass arguments, unless it takes a variable number of arguments. This is
useful to override the effects of the `-mrtd' switch.
You can specify multiple attributes in a declaration by separating them by commas within the double parentheses or by immediately following an attribute declaration with another attribute declaration.
Some people object to the __attribute__ feature, suggesting that ANSI C's
#pragma should be used instead. There are two reasons for not
doing this.
#pragma commands from a macro.
#pragma might mean in another
compiler.
These two reasons apply to almost any application that might be proposed
for #pragma. It is basically a mistake to use #pragma for
anything.