How does sizeof work? How can I write my own?

Question

I know C++ and know the function sizeof itself but I need to write my own sizeof function so please explain how it works exactly? What does it do with the parameter

Answer 1

sizeof isn't a function. It's an operator in C. We can implement its functionality something like follows.

#include 

#define mysizeof(X)  \

           ({                    \
                  __typeof(X) x;  \
                  (char *) (&x+1) - (char*) (&x); \
           })
int main()
{

     struct sample
     {
         int a;float b; char c;
     };

     printf("%d", mysizeof(struct sample));
     return 0;
}

Answer : 12