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1 | .\" Copyright (C) 2006 Justin Pryzby <pryzbyj@justinpryzby.com> | |
2 | .\" and Copyright (C) 2006 Michael Kerrisk <mtk.manpages@gmail.com> | |
3 | .\" | |
4 | .\" Permission is hereby granted, free of charge, to any person obtaining | |
5 | .\" a copy of this software and associated documentation files (the | |
6 | .\" "Software"), to deal in the Software without restriction, including | |
7 | .\" without limitation the rights to use, copy, modify, merge, publish, | |
8 | .\" distribute, sublicense, and/or sell copies of the Software, and to | |
9 | .\" permit persons to whom the Software is furnished to do so, subject to | |
10 | .\" the following conditions: | |
11 | .\" | |
12 | .\" The above copyright notice and this permission notice shall be | |
13 | .\" included in all copies or substantial portions of the Software. | |
14 | .\" | |
15 | .\" THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, | |
16 | .\" EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF | |
17 | .\" MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. | |
18 | .\" IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY | |
19 | .\" CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, | |
20 | .\" TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE | |
21 | .\" SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE. | |
22 | .\" | |
23 | .\" References: | |
24 | .\" /usr/lib/gcc/i486-linux-gnu/4.1.1/include/stddef.h | |
25 | .\" glibc-doc | |
26 | .TH OFFSETOF 3 2008-07-12 "GNU" "Linux Programmer's Manual" | |
27 | .SH NAME | |
28 | offsetof \- offset of a structure member | |
29 | .SH SYNOPSIS | |
30 | .nf | |
31 | .B #include <stddef.h> | |
32 | .sp | |
33 | .BI "size_t offsetof(" type ", " member ); | |
34 | .fi | |
35 | .SH DESCRIPTION | |
36 | The macro | |
37 | .BR offsetof () | |
38 | returns the offset of the field | |
39 | \fImember\fP from the start of the structure \fItype\fP. | |
40 | ||
41 | This macro is useful because the sizes of the fields that compose | |
42 | a structure can vary across implementations, | |
43 | and compilers may insert different numbers of padding | |
44 | bytes between fields. | |
45 | Consequently, an element's offset is not necessarily | |
46 | given by the sum of the sizes of the previous elements. | |
47 | ||
48 | A compiler error will result if | |
49 | \fImember\fP is not aligned to a byte boundary | |
50 | (i.e., it is a bit field). | |
51 | .SH "RETURN VALUE" | |
52 | .BR offsetof () | |
53 | returns the offset of the given | |
54 | .I member | |
55 | within the given | |
56 | .IR type , | |
57 | in units of bytes. | |
58 | .SH "CONFORMING TO" | |
59 | C89, C99, POSIX.1-2001. | |
60 | .SH EXAMPLE | |
61 | On a Linux/i386 system, when compiled using the default | |
62 | .BR gcc (1) | |
63 | options, the program below produces the following output: | |
64 | .in +4n | |
65 | .nf | |
66 | ||
67 | .RB "$" " ./a.out" | |
68 | offsets: i=0; c=4; d=8 a=16 | |
69 | sizeof(struct s)=16 | |
70 | .fi | |
71 | .nf | |
72 | .SS Program source | |
73 | .R " " | |
74 | .nf | |
75 | #include <stddef.h> | |
76 | #include <stdio.h> | |
77 | #include <stdlib.h> | |
78 | ||
79 | int | |
80 | main(void) | |
81 | { | |
82 | struct s { | |
83 | int i; | |
84 | char c; | |
85 | double d; | |
86 | char a[]; | |
87 | }; | |
88 | ||
89 | /* Output is compiler dependent */ | |
90 | ||
91 | printf("offsets: i=%ld; c=%ld; d=%ld a=%ld\\n", | |
92 | (long) offsetof(struct s, i), | |
93 | (long) offsetof(struct s, c), | |
94 | (long) offsetof(struct s, d), | |
95 | (long) offsetof(struct s, a)); | |
96 | printf("sizeof(struct s)=%ld\\n", (long) sizeof(struct s)); | |
97 | ||
98 | exit(EXIT_SUCCESS); | |
99 | } | |
100 | .fi | |
101 | .\" .SH SEE ALSO | |
102 | .\" FIXME . When one day readdir_r(3) is documented, it should have | |
103 | .\" a SEE ALSO that refers to this page. |