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12e41662 | 1 | .\" Copyright (C) 2006 Justin Pryzby <pryzbyj@justinpryzby.com> |
c11b1abf | 2 | .\" and Copyright (C) 2006 Michael Kerrisk <mtk.manpages@gmail.com> |
12e41662 | 3 | .\" |
7c576f45 | 4 | .\" %%%LICENSE_START(PERMISSIVE_MISC) |
12e41662 MK |
5 | .\" Permission is hereby granted, free of charge, to any person obtaining |
6 | .\" a copy of this software and associated documentation files (the | |
7 | .\" "Software"), to deal in the Software without restriction, including | |
8 | .\" without limitation the rights to use, copy, modify, merge, publish, | |
9 | .\" distribute, sublicense, and/or sell copies of the Software, and to | |
10 | .\" permit persons to whom the Software is furnished to do so, subject to | |
11 | .\" the following conditions: | |
12 | .\" | |
13 | .\" The above copyright notice and this permission notice shall be | |
14 | .\" included in all copies or substantial portions of the Software. | |
15 | .\" | |
16 | .\" THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, | |
17 | .\" EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF | |
18 | .\" MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. | |
19 | .\" IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY | |
20 | .\" CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, | |
21 | .\" TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE | |
22 | .\" SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE. | |
8ff7380d | 23 | .\" %%%LICENSE_END |
12e41662 MK |
24 | .\" |
25 | .\" References: | |
26 | .\" /usr/lib/gcc/i486-linux-gnu/4.1.1/include/stddef.h | |
27 | .\" glibc-doc | |
d7e02b73 | 28 | .TH OFFSETOF 3 2008-07-12 "GNU" "Linux Programmer's Manual" |
12e41662 MK |
29 | .SH NAME |
30 | offsetof \- offset of a structure member | |
31 | .SH SYNOPSIS | |
495846d9 MK |
32 | .nf |
33 | .B #include <stddef.h> | |
34 | .sp | |
da8cb51e | 35 | .BI "size_t offsetof(" type ", " member ); |
495846d9 | 36 | .fi |
12e41662 | 37 | .SH DESCRIPTION |
60a90ecd MK |
38 | The macro |
39 | .BR offsetof () | |
40 | returns the offset of the field | |
12e41662 MK |
41 | \fImember\fP from the start of the structure \fItype\fP. |
42 | ||
c13182ef MK |
43 | This macro is useful because the sizes of the fields that compose |
44 | a structure can vary across implementations, | |
45 | and compilers may insert different numbers of padding | |
12e41662 MK |
46 | bytes between fields. |
47 | Consequently, an element's offset is not necessarily | |
48 | given by the sum of the sizes of the previous elements. | |
49 | ||
c13182ef MK |
50 | A compiler error will result if |
51 | \fImember\fP is not aligned to a byte boundary | |
12e41662 | 52 | (i.e., it is a bit field). |
47297adb | 53 | .SH RETURN VALUE |
60a90ecd | 54 | .BR offsetof () |
d7e02b73 MK |
55 | returns the offset of the given |
56 | .I member | |
57 | within the given | |
58 | .IR type , | |
59 | in units of bytes. | |
47297adb | 60 | .SH CONFORMING TO |
2b2581ee | 61 | C89, C99, POSIX.1-2001. |
12e41662 | 62 | .SH EXAMPLE |
34ccb744 | 63 | On a Linux/i386 system, when compiled using the default |
60a90ecd | 64 | .BR gcc (1) |
e15f6322 | 65 | options, the program below produces the following output: |
624c0456 | 66 | .in +4n |
c8250206 | 67 | .nf |
e15f6322 | 68 | |
b43a3b30 | 69 | .RB "$" " ./a.out" |
624c0456 MK |
70 | offsets: i=0; c=4; d=8 a=16 |
71 | sizeof(struct s)=16 | |
c8250206 | 72 | .fi |
9c330504 | 73 | .SS Program source |
d84d0300 | 74 | \& |
624c0456 | 75 | .nf |
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76 | #include <stddef.h> |
77 | #include <stdio.h> | |
78 | #include <stdlib.h> | |
79 | ||
c13182ef | 80 | int |
cf0a9ace | 81 | main(void) |
12e41662 MK |
82 | { |
83 | struct s { | |
84 | int i; | |
85 | char c; | |
86 | double d; | |
e15f6322 | 87 | char a[]; |
12e41662 MK |
88 | }; |
89 | ||
e15f6322 | 90 | /* Output is compiler dependent */ |
c13182ef | 91 | |
12e41662 | 92 | printf("offsets: i=%ld; c=%ld; d=%ld a=%ld\\n", |
c13182ef | 93 | (long) offsetof(struct s, i), |
e15f6322 | 94 | (long) offsetof(struct s, c), |
c13182ef | 95 | (long) offsetof(struct s, d), |
e15f6322 | 96 | (long) offsetof(struct s, a)); |
12e41662 | 97 | printf("sizeof(struct s)=%ld\\n", (long) sizeof(struct s)); |
c13182ef | 98 | |
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99 | exit(EXIT_SUCCESS); |
100 | } | |
e15f6322 | 101 | .fi |