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 element within the given
28       type, in units of bytes.
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EXAMPLE

31       On a Linux/x86 system, when compiled using the default gcc(1)  options,
32       the program below produces the following output:
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34           $ ./a.out
35           offsets: i=0; c=4; d=8 a=16
36           sizeof(struct s)=16
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38       #include <stddef.h>
39       #include <stdio.h>
40       #include <stdlib.h>
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42       int main()
43       {
44           struct s {
45               int i;
46               char c;
47               double d;
48               char a[];
49           };
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51           /* Output is compiler dependent */
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53           printf("offsets: i=%ld; c=%ld; d=%ld a=%ld\n",
54                   (long) offsetof(struct s, i),
55                   (long) offsetof(struct s, c),
56                   (long) offsetof(struct s, d),
57                   (long) offsetof(struct s, a));
58           printf("sizeof(struct s)=%ld\n", (long) sizeof(struct s));
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60           exit(EXIT_SUCCESS);
61       }
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CONFORMING TO

64       C89, C99, POSIX.1-2001.
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68GNU                               2006-05-23                       OFFSETOF(3)
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