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.

[edit] Example

Run this code

#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));
    printf("the double is at offset %zu\n", offsetof(struct S, d));
}

Possible output:

the first element is at offset 0
the double is at offset 8

[edit] See also

size_t	unsigned integer type returned by the `sizeof` operator (typedef)
C++ documentation for offsetof

Language
headers
Type support
Dynamic memory management
Error handling
Program utilities
Variadic function support
Date and time utilities
Strings library
Algorithms
Numerics
Input/output support
Localization support
Thread support (C11)
Atomic operations (C11)
Technical Specifications

size_t
ptrdiff_t
NULL
max_align_t (since C11)
offsetof
Numeric limits