offsetof

OFFSETOF(3)                Linux Programmer's Manual               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  bound-
       ary (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
       This  page  is  part of release 5.05 of the Linux man-pages project.  A
       description of the project, information about reporting bugs,  and  the
       latest     version     of     this    page,    can    be    found    at
       https://www.kernel.org/doc/man-pages/.

GNU                               2019-03-06                       OFFSETOF(3)