offsetof(3)                Library Functions Manual                offsetof(3)

NAME
       offsetof - offset of a structure member

LIBRARY
       Standard C library (libc, -lc)

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.

STANDARDS
       C11, POSIX.1-2008.

HISTORY
       POSIX.1-2001, C89.

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);
       }

Linux man-pages 6.7               2023-10-31                       offsetof(3)