Predefined Identifier __func__ in C

Before we start discussing __func__, let us write some code snippets and anticipate the output:

// C program to demonstrate working of a
// Predefined Identifier __func__

#include <stdio.h>

int main()
{
    // %s indicates that the program will read strings
    printf("%s", __func__);
    return 0;
}

main

C language standard (i.e. C99 and C11) defines a predefined identifier as follows in clause 6.4.2.2:

"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."

It means that the C compiler implicitly adds __func__ in every function so that it can be used in that function to get the function name. 

Example:

// C program to demonstrate __func__
#include <stdio.h>

// %s is used to read strings
void foo(void) { printf("%s", __func__); }
void bar(void) { printf("%s", __func__); }

int main()
{
    foo();
    bar();
    return 0;
}

foobar

A use case of this predefined identifier could be logging the output of a big program where a programmer can use __func__ to get the current function instead of mentioning the complete function name explicitly.

Now, what happens if we define one more variable of name __func__?

Example:

// C program to demonstrate double
// predefined identifier or __func__
#include <stdio.h>

int __func__ = 10;
int main()
{
    printf("%s", __func__);
    return 0;
}

Since the C standard says the compiler implicitly defines __func__ for each function as the function name, we should not define __func__ in the first place. You might get an error but the C standard says “undefined behavior” if someone explicitly defines __func__.

Just to finish the discussion on Predefined Identifier __func__, let us mention Predefined Macros as well (such as __FILE__ and __LINE__, etc.) Basically, C standard clause 6.10.8 mentions several predefined macros out of which __FILE__ and __LINE__ are of relevance here. 

It’s worthwhile to see the output of the following code snippet: 

// C program to demonstrate __FILE__, func, line
#include <stdio.h>

int main()
{
    printf("In file:%s, function:%s() and line:%d",
           __FILE__, __func__, __LINE__);

    return 0;
}

Instead of explaining the output, we will leave this to you to guess and understand the role of __FILE__ and __LINE__!