C offsetof() Macro Function

This document analyzes the offsetof() macro function.

1. offsetof()

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#define offsetof(TYPE, MEMBER) ((sizet) &((TYPE *)0)->MEMBER)

The offsetof() macro function calculates the memory offset of member variables that make up a struct. [Code 1] shows the offsetof() macro function. The offsetof() macro function works as follows:

• (TYPE *)0 : The address is 0, and this address is a pointer to a TYPE struct.
• &((TYPE *)0)->MEMBER : Gets the offset of MEMBER.
• ((sizet) &((TYPE *)0)->MEMBER) : Casts the obtained offset to sizet.

2. Example

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#include <stdio.h>
#define offsetof(TYPE, MEMBER) ((size_t) &((TYPE *)0)->MEMBER)

struct offset{
    int a;
    int b;
    char c;
    double d;
    int e;
};

int main(void)
{
    printf("a : %d\n", offsetof(struct offset, a));
    printf("b : %d\n", offsetof(struct offset, b));
    printf("c : %d\n", offsetof(struct offset, c));
    printf("d : %d\n", offsetof(struct offset, d));
    printf("e : %d\n", offsetof(struct offset, e));

    return 0;
}

a : 0
b : 4
c : 8
d : 12
e : 20

You can check the memory offset of each member variable of the offset struct through the offsetof() macro function.