offsetof(3) — Linux manual page


offsetof(3)             Library Functions Manual             offsetof(3)

NAME         top

       offsetof - offset of a structure member

LIBRARY         top

       Standard C library (libc, -lc)

SYNOPSIS         top

       #include <stddef.h>

       size_t offsetof(type, member);

DESCRIPTION         top

       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         top

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

STANDARDS         top

       C11, POSIX.1-2008.

HISTORY         top

       POSIX.1-2001, C89.

EXAMPLES         top

       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>

           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));


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