Node:Function Names, Next:Return Address, Previous:Incomplete Enums, Up:C Extensions

### Function Names as Strings

GCC predefines two magic identifiers to hold the name of the current function. The identifier `__FUNCTION__` holds the name of the function as it appears in the source. The identifier `__PRETTY_FUNCTION__` holds the name of the function pretty printed in a language specific fashion.

These names are always the same in a C function, but in a C++ function they may be different. For example, this program:

         extern "C" {
         extern int printf (char *, ...);
         }

         class a {
          public:
           sub (int i)
             {
               printf ("__FUNCTION__ = %s\n", __FUNCTION__);
               printf ("__PRETTY_FUNCTION__ = %s\n", __PRETTY_FUNCTION__);
             }
         };

         int
         main (void)
         {
           a ax;
           ax.sub (0);
           return 0;
         }

gives this output:

         __FUNCTION__ = sub
         __PRETTY_FUNCTION__ = int  a::sub (int)

The compiler automagically replaces the identifiers with a string literal containing the appropriate name. Thus, they are neither preprocessor macros, like `__FILE__` and `__LINE__`, nor variables. This means that they catenate with other string literals, and that they can be used to initialize char arrays. For example

         char here[] = "Function " __FUNCTION__ " in " __FILE__;

On the other hand, `#ifdef __FUNCTION__` does not have any special meaning inside a function, since the preprocessor does not do anything special with the identifier `__FUNCTION__`.

Note that these semantics are deprecated, and that GCC 3.2 will handle `__FUNCTION__` and `__PRETTY_FUNCTION__` the same way as `__func__`. `__func__` is defined by the ISO standard C99:

         The identifier __func__ is implicitly declared by the translator
         as if, immediately following the opening brace of each function
         definition, the declaration
                   static const char __func__[] = "function-name";

         appeared, where function-name is the name of the lexically-enclosing
         function.  This name is the unadorned name of the function.

By this definition, `__func__` is a variable, not a string literal. In particular, `__func__` does not catenate with other string literals.

In `C++`, `__FUNCTION__` and `__PRETTY_FUNCTION__` are variables, declared in the same way as `__func__`.