Why sizeof to function returns 1 in C?

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 operator does not evaluate its operand, it just yield the size of test_func(..) type, i.e. sizeof(void).

Referece C99 standard is 6.5.3.4.

The sizeof operator yields the size (in bytes) of its operand, which may be an expression or the 
parenthesized name of a type. The size is determined from the type of the operand. The result 
is an integer.  If the type of the operand is a variable length array type, the operand is evaluated; 
otherwise, the operand is not evaluated and the result is an integer constant.