123 lines
3.1 KiB
Groff
123 lines
3.1 KiB
Groff
.\" $NetBSD: __USE.3,v 1.2 2013/10/17 19:37:56 jnemeth Exp $
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.\"
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.\" Copyright (c) 2013 The NetBSD Foundation, Inc.
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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 October 17, 2013
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.Dt __USE 3
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.Os
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.Sh NAME
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.Nm __USE
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.Nd compile time macro that marks a variable as being used
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.Sh SYNOPSIS
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.In sys/cdefs.h
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.Ft void
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.Fn __USE x
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.Sh DESCRIPTION
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The
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.Nm __USE
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macro can be used to omit warnings produced by certain compilers when
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variables are being set, but not used in a function.
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.Pp
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There are cases where it is simpler to mark a variable as used, as opposed
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to ifdef out its use:
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.Bd -literal -offset indent
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#ifdef DEBUG_FOO
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#define DPRINTF(a) printf a
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#else
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#define DPRINTF(a)
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void
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foo(void) {
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int var;
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var = getval();
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DPRINTF(("val is %d\n", var));
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}
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.Ed
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.Pp
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In this case, ifdefing the code would make it:
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.Bd -literal -offset indent
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void
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foo(void) {
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#ifdef DEBUG_FOO
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int var;
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var = getval();
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DPRINTF(("val is %d\n", var));
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#else
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(void)getval();
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#endif
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}
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.Ed
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.Pp
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This is not desirable because it duplicates code.
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With the
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.Nm __USE
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macro this can be written as:
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.Bd -literal -offset indent
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void
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foo(void) {
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int var;
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var = getval();
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#ifdef DEBUG_FOO
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DPRINTF(("val is %d\n", var));
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#else
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__USE(var);
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#endif
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}
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.Ed
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.Pp
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without producing compiler warnings.
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.Pp
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Although it is simple to write:
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.Bd -literal -offset indent
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(void)var;
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.Ed
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.Pp
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abstracting this into the macro allows for alternate implementations,
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as well as changing it to an empty implementation so that the liveness
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of the variable can be re-evaluated.
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.Sh IMPLEMENTATION NOTES
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.Nm
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is implemented as:
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.Bd -literal -offset indent
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#define __USE(a) ((void)(a))
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.Ed
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.Sh SEE ALSO
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.Xr cc 1 ,
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.Xr cdefs 3
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.Sh CAVEATS
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.Nm
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should be used sparingly as it can cause valid warnings to be hidden.
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.Pp
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Use of this macro is non-portable; this is part of the implementation
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namespace and should only be used in
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.Nx
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code.
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