offsetof - offset of a structure member

       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).

       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

   Programsource

       #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 6.9.1                              2024-06-15                                        offsetof(3)

       POSIX.1-2001, C89.

       Standard C library (libc, -lc)

       offsetof - offset of a structure member

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

       C11, POSIX.1-2008.

#include<stddef.h>size_toffsetof(type,member);