offsetof

 

C

Language

Headers

Type support

Program utilities

Variadic function support

Error handling

Dynamic memory management

Date and time utilities

Strings library

Algorithms

Numerics

Input/output support

Localization support

Atomic operations (C11)

Thread support (C11)

Technical Specifications

 

Type support

size_t

ptrdiff_t

NULL

max_align_t (since C11)

offsetof

Numeric limits

 

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

The macro offsetof expands to an integer constant expression 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

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

See also

size_t

unsigned integer type returned by the sizeof operator (typedef)