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[thirdparty/man-pages.git] / man3 / offsetof.3

.\" Copyright (C) 2006 Justin Pryzby <pryzbyj@justinpryzby.com>
.\"     and Copyright (C) 2006 Michael Kerrisk <mtk-manpages@gmx.net>
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.\" References:
.\"   /usr/lib/gcc/i486-linux-gnu/4.1.1/include/stddef.h
.\"   glibc-doc
.TH OFFSETOF 3 2006-05-23 "GNU" "Linux Programmer's Manual"
.SH NAME
offsetof \- offset of a structure member
.SH SYNOPSIS
\fB#include <stddef.h>

\fBsize_t offsetof(\fItype\fP, \fPmember\fP);
.SH DESCRIPTION
The macro
.BR offsetof ()
returns the offset of the field
\fImember\fP from the start of the structure \fItype\fP.

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
\fImember\fP is not aligned to a byte boundary
(i.e., it is a bit field).
.SH "RETURN VALUE"
.BR offsetof ()
returns the offset of the given element within the
given type, in units of bytes.
.SH "CONFORMING TO"
C89, C99, POSIX.1-2001.
.SH EXAMPLE
On a Linux/x86 system, when compiled using the default
.BR gcc (1)
options, the program below produces the following output:
.fi

    $ ./a.out
    offsets: i=0; c=4; d=8 a=16
    sizeof(struct s)=16

.nf
.nf
#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);
}
.fi
.\" .SH SEE ALSO
.\" FIXME . When one day readdir_r(3) is documented, it should have
.\" a SEE ALSO that refers to this page.