why is sizeof() an operator and not a function ?

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 operator. To calculated size of an element (either for inbuilt data type or user defined data type) type information is necessary at the compile time along with operating system information(32 bit or 64 bit). After compilation phase, compiler generates object code. object code doesn't has type information. while executing sizeof() just return calculated size of object then we use malloc, calloc etc function to allocate memory dynamically.