The keyword, __attribute__, allows you to specify special attributes when making a declaration. This keyword is followed by an attribute specification inside double parentheses. Nine attributes (noreturn, const, format, no_instrument_function, section, constructor, destructor, unused and weak) are currently defined for functions. Other attributes (including section) are supported for variables declarations (see Specifying attributes of variables) and for types (see 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
A few standard library functions, such as abort and exit, cannot return. GNU CC knows this automatically. Some programs define their own functions that never return. You can declare them noreturn to tell the compiler this fact as the following example shows.

void fatal () __attribute__ ((noreturn)); 

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

The noreturn keyword tells the compiler to assume that fatal cannot return. It can then optimize without regard to what would happen if fatal ever did return. This makes slightly better code. More importantly, it helps avoid spurious warnings of uninitialized variables.

Do not assume that registers saved by the calling function are restored before calling the noreturn function. It does not make sense for a noreturn function to have a return type other than void.

The attribute noreturn is not implemented in GNU C versions earlier than 2.5.

An alternative way to declare that a function does not return, which works in the current version and in some older versions, is as follows:

typedef void voidfn (); 
volatile voidfn fatal;

const
Many functions do not examine any values except their arguments, and have no effects except the return value. Such a function can be subject to common subexpression elimination and loop optimization just as an arithmetic operator would be. These functions should be declared with the attribute, const. For example, the following says that the hypothetical function, square, is safe to call fewer times than the program says.

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

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 const must be attached to the return value.
Note:

A function that has pointer arguments and examines the data pointed to must not be declared const. Likewise, a function that calls a non-const function usually must not be const. It does not make sense for a const function to return void.
format (archetype, string-index, first-to-check)
The format attribute specifies that a function takes printf or scanf style arguments which should be type-checked against a format string. For example, the following declaration causes the compiler to check the arguments in calls to my_ printf for consistency with the printf style format string argument, my_format.

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

The parameter, archetype, determines how the format string is interpreted, and should be either printf, scanf or strftime.
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 previous example, 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, strftime, vprintf, vfprintf and vsprintf whenever such warnings are requested (using ‘-Wformat’), so there is no need to modify the header file, ‘stdio.h’.
format_arg (string-index)
The format_arg attribute specifies that a function takes printf or scanf style arguments, modifies it (for example, to translate it into another language), and passes it to a printf or scanf style function. For example, the following declaration causes the compiler to check the arguments in calls to my_dgettext, whose result is passed to a printf, or scanf, or strftime type function for consistency with the printf style format string argument, my_format.
 

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

The parameter, string-index, specifies which argument is the format string argument (starting from 1).
The format-arg attribute allows you to identify your own functions which modify format strings, so that GNU CC can check the calls to printf, scanf, or strftime functions whose operands are a call to one of your own function. The compiler always treats gettext, dgettext, and dcgettext in this manner.
no-instrument-function
If -finstrument-functions is given, profiling function calls will be generated at entry and exit of most user-compiled functions. Functions with this attribute will not be so instrumented.
section ("section-name")
Normally, the compiler places the code it generates in the text section. Sometimes, however, you need additional sections, or you need certain particular functions to appear in special sections. The section attribute specifies that a function lives in a particular section. For example, the following declaration puts the function, foobar, in the bar section.
 
extern void foobar (void) __attribute__ \
                                    ((section ("bar")));
 
Some file formats do not support arbitrary sections so the section attribute is not available on all platforms. If you need to map the entire contents of a module to a particular section, consider using the facilities of the linker instead.
constructor
destructor
The constructor attribute causes the function to be called automatically before execution enters main(). Similarly, the destructor attribute causes the function to be called automatically after main() has completed or exit() has been called. Functions with these attributes are useful for initializing data that will be used implicitly during the execution of the program. These attributes are not currently implemented for Objective C.
unused
This attribute, attached to a function, means that the function is meant to be possibly unused. GNU CC will not produce a warning for this function. GNU C++ does not currently support this attribute as definitions without parameters are valid in C++.
weak
The weak attribute causes the declaration to be emitted as a weak symbol rather than a global. This is primarily useful in defining library functions which can be overridden in user code, though it can also be used with non-function declarations. Weak symbols are supported for ELF targets, and also for a.out targets when using the GNU assembler and linker.
alias ("target")
The alias attribute causes the declaration to be emitted as an alias for another symbol, which must be specified. For instance, the following declares ‘f’ to be a weak alias for ‘__f’. In C++, the mangled name for the target must be used.

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

Not all target machines support this attribute.
regparm (number)
On the Intel 386, the regparm attribute causes the compiler to pass up to number integer arguments in registers EAX, EDX, and ECX instead of on the stack. Functions that take a variable number of arguments will continue to be passed all of their arguments on the stack.
stdcall
On the Intel 386, the stdcall attribute causes the compiler to assume that the called function will pop off the stack space used to pass arguments, unless it takes a variable number of arguments. The PowerPC compiler for Windows NT currently ignores the stdcall attribute.
cdecl
On the Intel 386, the cdecl attribute causes the compiler to assume that the called function will pop off the stack space used to pass arguments. This is useful to override the effects of the switch,‘-mrtd’. The PowerPC compiler for Windows NT currently ignores the cdecl attribute.
longcall
On the RS/6000 and PowerPC, the longcall attribute causes the compiler to always call the function via a pointer, so that functions which reside further than 64 megabytes (67,108,864 bytes) from the current location can be called.
dllimport
On the PowerPC running Windows NT, the dllimport attribute causes the compiler to call the function via a global pointer to the function pointer that is set up by the Windows NT dll library. The pointer name is formed by combining __imp_ and the function name.
dllexport
On the PowerPC running Windows NT, the dllexport attribute causes the compiler to provide a global pointer to the function pointer, so that it can be called with the dllimport attribute. The pointer name is formed by combining __imp_ and the function name.
exception (except-func[, except-arg])
On the PowerPC running Windows NT, the exception attribute causes the compiler to modify the structured exception table entry it emits for the declared function. The string or identifier, except-func, is placed in the third entry of the structured exception table. It represents a function which is called by the exception handling mechanism if an exception occurs. If it was specified, the string or identifier, except-arg, is placed in the fourth entry of the structured exception table.
function_vector
Use this option on the H8/300 and H8/300H to indicate that the specified function should be called through the function vector. Calling a function through the function vector will reduce code size, however; the function vector has a limited size (maximum 128 entries on the H8/300 and 64 entries on the H8/300H) and shares space with the interrupt vector.
You must use GAS and LD from GNU binutils version 2.7 or later for this option to work correctly.
interrupt_handler
Use this option on the H8/300 and H8/300H to indicate that the specified function is an interrupt handler. The compiler will generate function entry and exit sequences suitable for use in an interrupt handler when this attribute is present.
eightbit_data
Use this option on the H8/300 and H8/300H to indicate that the specified variable should be placed into the eight bit data section. The compiler will generate more efficient code for certain operations on data in the eight bit data area. Note the eight bit data area is limited to 256 bytes of data.
You must use as and ld from GNU binutils version 2.7 or later for this option to work correctly.
tiny_data

Use this option on the H8/300H to indicate that the specified variable should be placed into the tiny data section. The compiler will generate more efficient code for loads and stores on data in the tiny data section. Note the tiny data area is limited to slightly under 32kbytes of data.

interrupt

Use this option on the M32R/D to indicate that the speci ed function is an interrupt handler. The compiler will generate function entry and exit sequences suitable for use in an interrupt handler when this attribute is present.

model (model-name)

Use this attribute on the M32R/D to set the addressability of an object, and the code generated for a function. The identifier model-name is one of small, medium, or large, representing each of the code models.
Small model objects live in the lower 16MB of memory (so that their addresses can be loaded with the ld24 instruction), and are callable with the bl instruction.
Medium model objects may live anywhere in the 32 bit address space (the compiler will generate seth/add3 instructions to load their addresses), and are callable with the bl instruction.
Large model objects may live anywhere in the 32 bit address space (the compiler will generate seth/add3 instructions to load their addresses), and may not be reachable with the bl instruction (the compiler will generate the much slower seth/add3/jl instruction sequence).
Some people object to the __attribute__ feature, suggesting the 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 most any application that might be proposed for #pragma. It is basically a mistake to use #pragma for anything.