6.47 Function Names as Strings

GCC provides three magic variables that 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__, provided for backward compatibility with old versions of GCC.

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 variables, not preprocessor macros, and may not be used to initialize char arrays or be concatenated with other string literals.