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

[Christophe Fergeau]

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);
             } else {
                 FREE_BUF(device_buf);
                 WANT_ID(device_id);
                 WANT_REST(device);
                 SAVE_BUF(device_buf);
-
-                /* Trick gcc set-by-not-read-vars warning */
-                if (device)
-                    device = device;
+                ignore_value(device);
 
                 gchar *id = g_strdup_printf("%s/%s/%s",
                                             baseURI, vendor_id, device_id);
@@ -1521,10 +1517,7 @@ osinfo_loader_process_file_reg_ids(OsinfoLoader *loader,
             WANT_ID(vendor_id);
             WANT_REST(vendor);
             SAVE_BUF(vendor_buf);
-
-            /* Trick gcc set-by-not-read-vars warning */
-            if (vendor)
-                vendor = vendor;
+            ignore_value(vendor);
         }
 
     done:
-- 
1.8.1