[Libosinfo] [libosinfo] Use gnulib's ignore-value.h to silence gcc

[Michal Privoznik]

On 31.01.2013 13:43, Christophe Fergeau wrote:
> osinfo_loader.c contains a few local hacks to avoid gcc warnings
> about set-but-not-read variables. While they are good to silence
> gcc, they also cause warnings from the Coverity checker.
> Use the ignore_value() macro from gnulib to avoid these warnings
> as they don't cause Coverity warnings.
> ---
>  osinfo/ignore-value.h  | 47 +++++++++++++++++++++++++++++++++++++++++++++++
>  osinfo/osinfo_loader.c | 19 ++++++-------------
>  2 files changed, 53 insertions(+), 13 deletions(-)
>  create mode 100644 osinfo/ignore-value.h
> 
> diff --git a/osinfo/ignore-value.h b/osinfo/ignore-value.h
> new file mode 100644
> index 0000000..6bab971
> --- /dev/null
> +++ b/osinfo/ignore-value.h
> @@ -0,0 +1,47 @@
> +/* ignore a function return without a compiler warning
> +
> +   Copyright (C) 2008-2013 Free Software Foundation, Inc.
> +
> +   This program is free software: you can redistribute it and/or modify
> +   it under the terms of the GNU Lesser General Public License as published by
> +   the Free Software Foundation; either version 2.1 of the License, or
> +   (at your option) any later version.
> +
> +   This program is distributed in the hope that it will be useful,
> +   but WITHOUT ANY WARRANTY; without even the implied warranty of
> +   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
> +   GNU Lesser General Public License for more details.
> +
> +   You should have received a copy of the GNU Lesser General Public License
> +   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
> +
> +/* Written by Jim Meyering, Eric Blake and Pádraig Brady.  */
> +
> +/* Use "ignore_value" to avoid a warning when using a function declared with
> +   gcc's warn_unused_result attribute, but for which you really do want to
> +   ignore the result.  Traditionally, people have used a "(void)" cast to
> +   indicate that a function's return value is deliberately unused.  However,
> +   if the function is declared with __attribute__((warn_unused_result)),
> +   gcc issues a warning even with the cast.
> +
> +   Caution: most of the time, you really should heed gcc's warning, and
> +   check the return value.  However, in those exceptional cases in which
> +   you're sure you know what you're doing, use this function.
> +
> +   For the record, here's one of the ignorable warnings:
> +   "copy.c:233: warning: ignoring return value of 'fchown',
> +   declared with attribute warn_unused_result".  */
> +
> +#ifndef _GL_IGNORE_VALUE_H
> +# define _GL_IGNORE_VALUE_H
> +
> +/* The __attribute__((__warn_unused_result__)) feature
> +   is available in gcc versions 3.4 and newer,
> +   while the typeof feature has been available since 2.7 at least.  */
> +# if __GNUC__ < 3 || (__GNUC__ == 3 && __GNUC_MINOR__ < 4)
> +#  define ignore_value(x) ((void) (x))
> +# else
> +#  define ignore_value(x) (({ __typeof__ (x) __x = (x); (void) __x; }))
> +# endif
> +
> +#endif
> diff --git a/osinfo/osinfo_loader.c b/osinfo/osinfo_loader.c
> index d713767..76e9bc2 100644
> --- a/osinfo/osinfo_loader.c
> +++ b/osinfo/osinfo_loader.c
> @@ -34,6 +34,7 @@
>  #include <libxml/tree.h>
>  #include <libxml/xpath.h>
>  #include <libxml/xmlreader.h>
> +#include "ignore-value.h"
>  #include "osinfo_install_script_private.h"
>  #include "osinfo_device_driver_private.h"
>  
> @@ -1486,20 +1487,15 @@ osinfo_loader_process_file_reg_ids(OsinfoLoader *loader,
>                  WANT_ID(subvendor_id);
>                  WANT_ID(subdevice_id);
>                  WANT_REST(subsystem);
> -
> -                /* Pretend we 'use' these variables to get around
> -                 * 'gcc' warnings about set-but-not-read vars */
> -                if (subvendor_id || subdevice_id || subsystem)
> -                    subsystem = subsystem;
> +                ignore_value(subvendor_id);
> +                ignore_value(subdevice_id);
> +                ignore_value(subsystem);

The code looks okay, but why do we have these variable if they are not
used anywhere?

Michal