implement "sizeof"

sizeof.c

#include <stdio.h>

typedef struct {
    int i;
    char a;
    char b[3];
    float * c;
    char s[100];
} T;

#define _sizeof(T) ({   \
    typeof(T *) p = 0;  \
    ((size_t) (p + 1)); \
})

int main() {
    printf("sizeof int = %zu\n", _sizeof(int));     // 4
    printf("sizeof struct T = %zu\n", _sizeof(T));  // 120
    return 0;
}