offsetof

< c‎ | types
 
C
 
Type support
 
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

 unsigned integer type returned by the sizeof operator
(typedef)