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
   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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   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/.

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