C: why the sizeof main() is 2 byte?

Question

Used the sizeof of function, which gives 1; why?
I want to know the size of the entire function. How to achive it?

#include <stdio.h>
void (*p)(int); 
void test_func(int data)
{
  printf("%d\n",data);
}

main(void)
{
    p = test_func;
    (*p)(4);
    printf("%d",sizeof(test_func));
}

Answer 1

sizeof is a compile-time constant and main may not been called by that time.

However manul of C clearly says "The size operator shall not be applied to an expression that has function or incomplete type" however sizeof() can take any valid identifier, but it won't return a valid (an honestly true and valid) result for function names.

So there is no explanation why sizeof(main) is returning 2, however i wrote a small program as following which is giving me 1.

main()
{
  printf("%d\n", sizeof(main));
}