.\" Copyright (C) 2006 Justin Pryzby <pryzbyj@justinpryzby.com>
.\"     and Copyright (C) 2006 Michael Kerrisk <mtk.manpages@gmail.com>
.\"
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.\" References:
.\"   /usr/lib/gcc/i486-linux-gnu/4.1.1/include/stddef.h
.\"   glibc-doc
.TH offsetof 3 2023-03-30 "Linux man-pages 6.04"
.SH NAME
offsetof \- offset of a structure member
.SH LIBRARY
Standard C library
.RI ( libc ", " \-lc )
.SH SYNOPSIS
.nf
.B #include <stddef.h>
.PP
.BI "size_t offsetof(" type ", " member );
.fi
.SH DESCRIPTION
The macro
.BR offsetof ()
returns the offset of the field
.I member
from the start of the structure
.IR type .
.PP
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.
.PP
A compiler error will result if
.I member
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
.I member
within the given
.IR type ,
in units of bytes.
.SH STANDARDS
C11, POSIX.1-2008.
.SH HISTORY
POSIX.1-2001, C89.
.SH EXAMPLES
On a Linux/i386 system, when compiled using the default
.BR gcc (1)
options, the program below produces the following output:
.PP
.in +4n
.EX
.RB "$" " ./a.out"
offsets: i=0; c=4; d=8 a=16
sizeof(struct s)=16
.EE
.in
.SS Program source
\&
.\" SRC BEGIN (offsetof.c)
.EX
#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=%zu; c=%zu; d=%zu a=%zu\en",
           offsetof(struct s, i), offsetof(struct s, c),
           offsetof(struct s, d), offsetof(struct s, a));
    printf("sizeof(struct s)=%zu\en", sizeof(struct s));

    exit(EXIT_SUCCESS);
}
.EE
.\" SRC END