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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