sizeof Post: 86196

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offsetof

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 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
",
		   offsetof(struct s, i), offsetof(struct s, c),
		   offsetof(struct s, d), offsetof(struct s, a));
	   printf("sizeof(struct s)=%zd
", sizeof(struct s));

	   exit(EXIT_SUCCESS);
       }

COLOPHON

       This  page is part of release 4.15 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)