5.41 Function Names as Strings

GNU CC 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__.

GNU CC also supports the magic word __func__, 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__.