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### 5.41 Function Names as Strings

GCC provides three magic variables which hold the name of the current function, as a string. The first of these is `__func__`, which is part of the C99 standard:

         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.

`__FUNCTION__` is another name for `__func__`. Older versions of GCC recognize only this name. However, it is not standardized. For maximum portability, we recommend you use `__func__`, but provide a fallback definition with the preprocessor:

         #if __STDC_VERSION__ < 199901L
         # if __GNUC__ >= 2
         #  define __func__ __FUNCTION__
         # else
         #  define __func__ "<unknown>"
         # endif
         #endif

In C, `__PRETTY_FUNCTION__` is yet another name for `__func__`. However, in C++, `__PRETTY_FUNCTION__` contains the type signature of the function as well as its bare name. For example, this program:

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

         class a {
          public:
           void 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__ = void a::sub(int)

These identifiers are not preprocessor macros. In GCC 3.3 and earlier, in C only, `__FUNCTION__` and `__PRETTY_FUNCTION__` were treated as string literals; they could be used to initialize `char` arrays, and they could be concatenated with other string literals. GCC 3.4 and later treat them as variables, like `__func__`. In C++, `__FUNCTION__` and `__PRETTY_FUNCTION__` have always been variables.