OFFSETOF(3) Linux Programmer's Manual OFFSETOF(3)NAMEoffsetof - offset of a structure member
SYNOPSIS
#include <stddef.h>
size_t offsetof(type, member);
DESCRIPTION
The macro offsetof() returns the offset of the field member from the
start of the structure type.
This macro is useful because the sizes of the fields that compose a
structure can vary across implementations, and compilers may insert
different numbers of padding bytes between fields. Consequently, an
element's offset is not necessarily given by the sum of the sizes of
the previous elements.
A compiler error will result if member is not aligned to a byte bound‐
ary (i.e., it is a bit field).
RETURN VALUEoffsetof() returns the offset of the given element within the given
type, in units of bytes.
EXAMPLE
On a Linux/x86 system, when compiled using the default gcc(1) options,
the program below produces the following output:
$ ./a.out
offsets: i=0; c=4; d=8 a=16
sizeof(struct s)=16
#include <stddef.h>
#include <stdio.h>
#include <stdlib.h>
int main()
{
struct s {
int i;
char c;
double d;
char a[];
};
/* Output is compiler dependent */
printf("offsets: i=%ld; c=%ld; d=%ld a=%ld\n",
(long) offsetof(struct s, i),
(long) offsetof(struct s, c),
(long) offsetof(struct s, d),
(long) offsetof(struct s, a));
printf("sizeof(struct s)=%ld\n", (long) sizeof(struct s));
exit(EXIT_SUCCESS);
}
CONFORMING TO
POSIX.1-2001.
GNU 2006-05-23 OFFSETOF(3)