New page for offsetof()

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+.\" Copyright (C) 2006 Justin Pryzby <pryzbyj@justinpryzby.com>
+.\"     and Copyright (C) 2006 Michael Kerrisk <mtk-manpages@gmx.net>
+.\"
+.\" 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.
+.\"
+.\" References:
+.\"   /usr/lib/gcc/i486-linux-gnu/4.1.1/include/stddef.h
+.\"   glibc-doc
+.TH OFFSETOF 3 "2006-05-23" GNU "Linux Programmer's Manual"
+.SH NAME
+offsetof \- offset of a structure member
+.SH SYNOPSIS
+\fB#include <stddef.h>
+
+\fBsize_t offsetof(\fItype\fP, \fPmember\fP);
+.SH DESCRIPTION
+The macro \fBoffsetof\fP() returns the offset of the field
+\fImember\fP from the start of the structure \fItype\fP.
+
+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.
+
+A compiler error will result if 
+\fImember\fP is not aligned to a byte boundary 
+(i.e., it is a bit field).
+.SH "RETURN VALUE"
+\fBoffsetof\fP() returns the offset of the given element within the
+given type, in units of bytes.
+.SH EXAMPLE
+.nf
+.fi
+#include <stddef.h>
+#include <stdio.h>
+#include <stdlib.h>
+
+int main()
+{
+    struct s {
+        int i;
+        char c;
+        double d;
+       char a[];
+    };
+
+    /* Output is compiler-dependent */
+    printf("offsets: i=%ld; c=%ld; d=%ld a=%ld\\n",
+            (long) offsetof(struct s, i), 
+           (long) offsetof(struct s, c),
+            (long) offsetof(struct s, d), 
+           (long) offsetof(struct s, a));
+    printf("sizeof(struct s)=%ld\\n", (long) sizeof(struct s));
+    exit(EXIT_SUCCESS);
+}
+.\" .SH NOTES
+.\" \fBoffsetof\fP can be implemented as:
+.\" .sp
+.\" .nf
+.\"        \fB#define offsetof(\fItype\fP, \fPmember\fP) \\
+.\"            \fB&((type *)0)->\fImember\fP - (char *)((\fPtype\fP *)0)
+.\" .fi
+.SH "CONFORMING TO"
+POSIX.1-2001.
+.\" .SH SEE ALSO
+.\" FIXME . When one day readdir_r(3) is documented, it should have 
+.\" a SEE ALSO that refers to this page.