1OFFSETOF(3)                Linux Programmer's Manual               OFFSETOF(3)
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NAME

6       offsetof - offset of a structure member
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SYNOPSIS

9       #include <stddef.h>
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11       size_t offsetof(type, member);
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DESCRIPTION

14       The  macro  offsetof()  returns the offset of the field member from the
15       start of the structure type.
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17       This macro is useful because the sizes of the  fields  that  compose  a
18       structure  can  vary  across  implementations, and compilers may insert
19       different numbers of padding bytes between  fields.   Consequently,  an
20       element's  offset  is  not necessarily given by the sum of the sizes of
21       the previous elements.
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23       A compiler error will result if member is not aligned to a byte  bound‐
24       ary (i.e., it is a bit field).
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RETURN VALUE

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

31       POSIX.1-2001, POSIX.1-2008, C89, C99.
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EXAMPLE

34       On a Linux/i386 system, when compiled using the default gcc(1) options,
35       the program below produces the following output:
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37           $ ./a.out
38           offsets: i=0; c=4; d=8 a=16
39           sizeof(struct s)=16
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41   Program source
42
43       #include <stddef.h>
44       #include <stdio.h>
45       #include <stdlib.h>
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47       int
48       main(void)
49       {
50           struct s {
51               int i;
52               char c;
53               double d;
54               char a[];
55           };
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57           /* Output is compiler dependent */
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59           printf("offsets: i=%zd; c=%zd; d=%zd a=%zd\n",
60                   offsetof(struct s, i), offsetof(struct s, c),
61                   offsetof(struct s, d), offsetof(struct s, a));
62           printf("sizeof(struct s)=%zd\n", sizeof(struct s));
63
64           exit(EXIT_SUCCESS);
65       }
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COLOPHON

68       This  page  is  part of release 4.16 of the Linux man-pages project.  A
69       description of the project, information about reporting bugs,  and  the
70       latest     version     of     this    page,    can    be    found    at
71       https://www.kernel.org/doc/man-pages/.
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75GNU                               2017-09-15                       OFFSETOF(3)
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