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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 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  bound-
     ary (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

     This  page  is  part of release 4.16 of the Linux man-pages project.  A
     description of the project, information about reporting bugs,  and  the
     latest     version     of     this    page,    can    be    found    at
     https://www.kernel.org/doc/man-pages/.

GNU 2017-09-15 OFFSETOF(3)