OFFSETOF(3)               Linux Programmer's Manual              OFFSETOF(3)

NAME         top

       offsetof - offset of a structure member

SYNOPSIS         top

       #include <stddef.h>

       size_t offsetof(type, member);

DESCRIPTION         top

       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         top

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

CONFORMING TO         top

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

EXAMPLE         top

       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>

           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));


COLOPHON         top

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GNU                              2015-08-08                      OFFSETOF(3)