offsetof

offsetof

Defined in header <stddef.h>
#define offsetof(type, member) /*implementation-defined*/

The macro offsetof expands to a constant of type size_t, the value of which is the offset, in bytes, from the beginning of an object of specified type to its specified member, including padding if any.

Example

#include <stdio.h>
#include <stddef.h>
 
struct S {
    char c;
    double d;
};
 
int main(void)
{
    printf("the first element is at offset %zu\n", offsetof(struct S, c));
    pr