Manpage of OFFSETOF

OFFSETOF

Section: Linux Programmer's Manual (3)
Updated: 2015-08-08
Index
 

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 memberfrom 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 memberis not aligned to a byte boundary (i.e., it is a bit field).  

RETURN VALUE

offsetof() returns the offset of the given memberwithin 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.outoffsets: 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);
}


 

Index

NAME
SYNOPSIS
DESCRIPTION
RETURN VALUE
CONFORMING TO
EXAMPLE
Program source

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Time: 22:27:49 GMT, June 20, 2016