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.\" Copyright (C) 2006 Justin Pryzby <pryzbyj@justinpryzby.com>
.\" and Copyright (C) 2006 Michael Kerrisk <mtk.manpages@gmail.com>
.\"
.\" %%%LICENSE_START(PERMISSIVE_MISC)
.\" Permission is hereby granted, free of charge, to any person obtaining
.\" a copy of this software and associated documentation files (the
.\" "Software"), to deal in the Software without restriction, including
.\" without limitation the rights to use, copy, modify, merge, publish,
.\" distribute, sublicense, and/or sell copies of the Software, and to
.\" permit persons to whom the Software is furnished to do so, subject to
.\" the following conditions:
.\"
.\" The above copyright notice and this permission notice shall be
.\" included in all copies or substantial portions of the Software.
.\"
.\" THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
.\" EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
.\" MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
.\" IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY
.\" CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT,
.\" TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE
.\" SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
.\" %%%LICENSE_END
.\"
.\" 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
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