Function attributes are extensions implemented to enhance the portability of programs developed with GNU C. Specifiable attributes for functions provide explicit ways to help the compiler optimize function calls and to instruct it to check more aspects of the code. Others provide additional functionality.
IBM C and C++ implement a subset of the GNU C function attributes. If a particular function attribute is not implemented, its specification is accepted and the semantics are ignored. These language features are collectively available when compiling in any of the extended language levels.
Function attribute syntax: function declaration >>-function declarator--__attribute__---------------------------> .-,--------------------------. V | >--((----+-attribute_name---------+-+--))--;------------------->< '-__--attribute_name--__-'
/* Specify the attribute on a function prototype declaration */
void f(int i, int j) __attribute__((individual_attribute_name));
void f(int i, int j) { }
In C++, the attribute specification must also follow any exception declaration that may be present for the function.
int __attribute__((individual_attribute_name)) foo(int i) { }
You can specify attribute_name with or without leading and trailing double underscore characters; however, using the double underscore characters reduces the likelihood of name conflicts with macros of the same name.