IBM extension

Function attributes

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.

A function attribute is specified with the keyword __attribute__ followed by the attribute name and any additional arguments the attribute name requires. A function __attribute__ specification is included in the declaration or definition of a function. The syntax takes the following forms:

Read syntax diagramSkip visual syntax diagramFunction attribute syntax: function declaration
 
>>-function declarator--__attribute__--------------------------->
 
       .-,--------------------------.
       V                            |
>--((----+-attribute_name---------+-+--))--;-------------------><
         '-__--attribute_name--__-'
 
C only

Read syntax diagramSkip visual syntax diagramFunction attribute syntax: function definition
 
                      .-,--------------------------.
                      V                            |
>>-__attribute__--((----+-attribute name---------+-+--))-------->
                        '-__--attribute_name--__-'
 
>--function_declarator--{--function body--}--------------------><
 
End of C only
C++ only

Read syntax diagramSkip visual syntax diagramFunction attribute syntax: function definition
 
>>-function declarator--__attribute__--------------------------->
 
       .-,--------------------------.
       V                            |
>--((----+-attribute_name---------+-+--))--;-------------------><
         '-__--attribute_name--__-'
 
End of C++ only

The function attribute in a function declaration is always placed after the declarator, including the parenthesized parameter declaration:

   /* Specify the attribute on a function prototype declaration */
   void f(int i, int j) __attribute__((individual_attribute_name));
   void f(int i, int j) { }
C++ only

In C++, the attribute specification must also follow any exception declaration that may be present for the function.

End of C++ only
C only

Due to ambiguities in parsing old-style parameter declarations, a function definition must have the attribute specification precede the declarator:

int __attribute__((individual_attribute_name)) foo(int i) { }
End of C only

A function attribute specification using the form __attribute_name__ (that is, the attribute name with double underscore characters leading and trailing) reduces the likelihood of a name conflict with a macro of the same name.

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