__func__: The 'magic' constant

__func__ is an implicitly declared ‘magic constant' which will ‘hold the name of the current function as a string'. ¹

It is mentioned in C99 under 6.4.2.2 Predefined identifiers (draft version):

The identifier __func__ shall be 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.

__func__ will have the unadorned name of the function. No return type, argument types or arguments themselves.

#include<stdio.h>
int main()
{
    printf("\nName of function: %s", __func__);
}

The above program will print:

Name of function: main

__func__ is not a macro as the function names will be known only after parsing. Hence __func__ does not catenate with strings. Meaning things like

char here[] = "Function " __func__ " is used";

will give an error.

As mentioned in this stackoverflow post:

Expanding __func__ at preprocessing time requires the preprocessor to know which function it's processing. The preprocessor generally doesn't know that, because parsing happens after the preprocessing

GCC has another constant __FUNCTION__ which is basically like an alias of __func__.

Provided for backward compatibility with old version of GCC. ¹

Fun fact: GCC provides yet another 'magic constant' __PRETTY_FUNCTION__ as an extension which is similar to __func__ for C programs but gives the function signature as well in C++.

`func`: The 'magic' constant

References