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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | CONFORMING TO | EXAMPLE | COLOPHON |
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.
POSIX.1-2001, POSIX.1-2008, C89, C99.
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
Program source
#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=%zd; c=%zd; d=%zd a=%zd\n",
offsetof(struct s, i), offsetof(struct s, c),
offsetof(struct s, d), offsetof(struct s, a));
printf("sizeof(struct s)=%zd\n", sizeof(struct s));
exit(EXIT_SUCCESS);
}
This page is part of release 4.12 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
GNU 2015-08-08 OFFSETOF(3)
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