85 lines
2.8 KiB
Groff
85 lines
2.8 KiB
Groff
.\" $NetBSD: __alignof__.3,v 1.5 2011/04/14 06:56:28 wiz Exp $
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.\"
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.\" Copyright (c) 2010 Jukka Ruohonen <jruohonen@iki.fi>
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.\" All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS
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.\" ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
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.\" TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
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.\" PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS
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.\" BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
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.\" CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
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.\" SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
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.\" INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
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.\" CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
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.\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
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.\" POSSIBILITY OF SUCH DAMAGE.
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.\"
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.Dd December 20, 2010
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.Dt __ALIGNOF__ 3
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.Os
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.Sh NAME
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.Nm __alignof__
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.Nd GNU extension for alignment of an object
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.Sh SYNOPSIS
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.Ft int
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.Fn __alignof__ "void x"
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.Sh DESCRIPTION
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The
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.Fn __alignof__
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operator returns the alignment of its operand.
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The operand can be a type or an expression.
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If the operand is a
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.Sq lvalue ,
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the return value represents the required alignment of the underlying type,
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not the actual alignment of the specified
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.Sq lvalue .
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.Pp
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The returned value is specific to the architecture and the
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.Tn ABI .
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If the architecture does not impose strict alignment requirements,
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.Fn __alignof__
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returns the minimum required alignment.
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If
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.Xr __aligned 3
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is used to increase the alignment,
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.Fn __alignof__
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returns the specified alignment.
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.Sh EXAMPLES
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The syntax is comparable to the
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.Fn sizeof
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operator.
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If the architecture aligns integers along 32-bit address boundaries,
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the following should print the value 4.
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.Bd -literal -offset indent
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(void)printf("%d\en", __alignof__(int));
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.Ed
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.Pp
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On the other hand, the following example should print the value 1,
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even though this is unlikely to be the actual alignment of the
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structure member.
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.Bd -literal -offset indent
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struct align {
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int x;
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char y;
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} a;
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(void)printf("%d\en", __alignof__(a.y));
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.Ed
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.Sh SEE ALSO
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.Xr gcc 1 ,
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.Xr attribute 3 ,
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.Xr offsetof 3 ,
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.Xr typeof 3
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.Sh CAVEATS
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This is a non-standard, compiler-specific extension.
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