offsetof (3)

Leading comments

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 ...

(The comments found at the beginning of the groff file "man3/offsetof.3".)

NAME

offsetof - offset of a structure member

SYNOPSIS

#include <stddef.h>

size_t offsetof(type, member);

DESCRIPTION

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).

RETURN VALUE

offsetof() returns the offset of the given member within the given type, in units of bytes.

CONFORMING TO

POSIX.1-2001, POSIX.1-2008, C89, C99.

EXAMPLE

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);
}

COLOPHON

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