| NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | CONFORMING TO | EXAMPLE | COLOPHON | The Linux Programming Interface |
OFFSETOF(3) Linux Programmer's Manual OFFSETOF(3)
offsetof - offset of a structure member
#include <stddef.h>
size_t offsetof(type, member);
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 boundary
(i.e., it is a bit field).
offsetof() returns the offset of the given member within the given type, in
units of bytes.
C89, C99, POSIX.1-2001.
On a Linux/i386 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(void)
{
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);
}
This page is part of release 3.32 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can be found
at http://www.kernel.org/doc/man-pages/.
GNU 2008-07-12 OFFSETOF(3)
HTML rendering created 2010-12-03 by Michael Kerrisk, author of The Linux Programming Interface