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 pad‐
       ding 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.

EXAMPLES
       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=%zu; c=%zu; d=%zu a=%zu\n",
                   offsetof(struct s, i), offsetof(struct s, c),
                   offsetof(struct s, d), offsetof(struct s, a));
           printf("sizeof(struct s)=%zu\n", sizeof(struct s));

           exit(EXIT_SUCCESS);
       }

GNU                                                                            2020-11-01                                                                    OFFSETOF(3)