In GNU C, you declare certain things about functions called in your program that 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 2.28.) and for types (see Section 2.29.).
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.
A few standard library functions, such as abort and exit, cannot return. The compiler 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.
Many functions do not examine any values except their arguments, and have no effects except the return value. Such a function can be subject to common subexpression elimination and loop optimization just as an arithmetic operator would be. These functions should be declared with the attribute const. For example,
int square (int) __attribute__ ((const));
says that the hypothetical function square is safe to call fewer times than the program says.
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.
The format attribute specifies that a function takes printf or scanf style arguments that 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 that take format strings as arguments, so that the compiler 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.
The format_arg attribute specifies that a function takes printf or scanf style arguments, modifies it (for example, to translate it into another language), and passes it to a printf or scanf style function. For example, the declaration:
extern char *
my_dgettext (char *my_domain, const char *my_format)
__attribute__ ((format_arg (2)));
causes the compiler to check the arguments in calls to my_dgettext whose result is passed to a printf or scanf type function for consistency with the printf style format string argument my_format.
The parameter string-index specifies which argument is the format string argument (starting from 1).
The format-arg attribute allows you to identify your own functions that modify format strings, so that the compiler can check the calls to printf and scanf function whose operands are a call to one of your own function. The compiler always treats gettext, dgettext, and dcgettext in this manner.
Normally, the compiler places the code it generates in the text section. Sometimes, however, you need additional sections, or you need certain particular functions to appear in special sections. The section attribute specifies that a function lives in a particular section. For example, the declaration:
extern void foobar (void) __attribute__ ((section ("bar")));
puts the function foobar in the bar section.
Some file formats do not support arbitrary sections so the section attribute is not available on all platforms. If you need to map the entire contents of a module to a particular section, consider using the facilities of the linker instead.
The constructor attribute causes the function to be called automatically before execution enters main (). Similarly, the destructor attribute causes the function to be called automatically after main () has completed or exit () has been called. Functions with these attributes are useful for initializing data that will be used implicitly during the execution of the program.
This attribute, attached to a function, means that the function is meant to be possibly unused. The compiler will not produce a warning for this function. GNU C++ does not currently support this attribute as definitions without parameters are valid in C++.
The weak attribute causes the declaration to be emitted as a weak symbol rather than a global. This is primarily useful in defining library functions that 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.
The alias attribute causes the declaration to be emitted as an alias for another symbol, which must be specified. For instance,
void __f () { /* do something */; }
void f () __attribute__ ((weak, alias ("__f")));
declares “f” to be a weak alias for “__f”. In C++, the mangled name for the target must be used.
Not all target machines support this attribute.
On the Intel 386, the regparm attribute causes the compiler to pass up to number integer arguments in registers EAX, EDX, and ECX instead of on the stack. Functions that take a variable number of arguments will continue to be passed all of their arguments on the stack.
On the Intel 386, the stdcall attribute causes the compiler to assume that the called function will pop off the stack space used to pass arguments, unless it takes a variable number of arguments.
On the Intel 386, the cdecl attribute causes the compiler to assume that the calling function will pop off the stack space used to pass arguments. This is useful to override the effects of the “-mrtd” switch.
The PowerPC compiler for Windows NT currently ignores the cdecl attribute.
On the RS/6000 and PowerPC, the longcall attribute causes the compiler to always call the function via a pointer, so that functions which reside further than 64 megabytes (67,108,864 bytes) from the current location can be called.
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.
It is impossible to generate #pragma commands from a macro.
There is no telling what the same #pragma might mean in another compiler.
These two reasons apply to almost any application that might be proposed for #pragma. It is basically a mistake to use #pragma for anything.