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Diffstat (limited to 'man3/offsetof.3')
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diff --git a/man3/offsetof.3 b/man3/offsetof.3 new file mode 100644 index 0000000..5dbccc8 --- /dev/null +++ b/man3/offsetof.3 @@ -0,0 +1,110 @@ +.\" Copyright (C) 2006 Justin Pryzby <pryzbyj@justinpryzby.com> +.\" and Copyright (C) 2006 Michael Kerrisk <mtk.manpages@gmail.com> +.\" +.\" %%%LICENSE_START(PERMISSIVE_MISC) +.\" Permission is hereby granted, free of charge, to any person obtaining +.\" a copy of this software and associated documentation files (the +.\" "Software"), to deal in the Software without restriction, including +.\" without limitation the rights to use, copy, modify, merge, publish, +.\" distribute, sublicense, and/or sell copies of the Software, and to +.\" permit persons to whom the Software is furnished to do so, subject to +.\" the following conditions: +.\" +.\" The above copyright notice and this permission notice shall be +.\" included in all copies or substantial portions of the Software. +.\" +.\" THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, +.\" EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF +.\" MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. +.\" IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY +.\" CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, +.\" TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE +.\" SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE. +.\" %%%LICENSE_END +.\" +.\" References: +.\" /usr/lib/gcc/i486-linux-gnu/4.1.1/include/stddef.h +.\" glibc-doc +.TH offsetof 3 2023-05-03 "Linux man-pages 6.05.01" +.SH NAME +offsetof \- offset of a structure member +.SH LIBRARY +Standard C library +.RI ( libc ", " \-lc ) +.SH SYNOPSIS +.nf +.B #include <stddef.h> +.PP +.BI "size_t offsetof(" type ", " member ); +.fi +.SH DESCRIPTION +The macro +.BR offsetof () +returns the offset of the field +.I member +from the start of the structure +.IR type . +.PP +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. +.PP +A compiler error will result if +.I member +is not aligned to a byte boundary +(i.e., it is a bit field). +.SH RETURN VALUE +.BR offsetof () +returns the offset of the given +.I member +within the given +.IR type , +in units of bytes. +.SH STANDARDS +C11, POSIX.1-2008. +.SH HISTORY +POSIX.1-2001, C89. +.SH EXAMPLES +On a Linux/i386 system, when compiled using the default +.BR gcc (1) +options, the program below produces the following output: +.PP +.in +4n +.EX +.RB "$" " ./a.out" +offsets: i=0; c=4; d=8 a=16 +sizeof(struct s)=16 +.EE +.in +.SS Program source +\& +.\" SRC BEGIN (offsetof.c) +.EX +#include <stddef.h> +#include <stdio.h> +#include <stdlib.h> +\& +int +main(void) +{ + struct s { + int i; + char c; + double d; + char a[]; + }; +\& + /* Output is compiler dependent */ +\& + printf("offsets: i=%zu; c=%zu; d=%zu a=%zu\en", + offsetof(struct s, i), offsetof(struct s, c), + offsetof(struct s, d), offsetof(struct s, a)); + printf("sizeof(struct s)=%zu\en", sizeof(struct s)); +\& + exit(EXIT_SUCCESS); +} +.EE +.\" SRC END |