lib/ignore-value.h

   1 /* ignore a function return without a compiler warning.  -*- coding: utf-8 -*-
   2
   3    Copyright (C) 2008-2017 Free Software Foundation, Inc.
   4
   5    This program is free software: you can redistribute it and/or modify
   6    it under the terms of the GNU General Public License as published by
   7    the Free Software Foundation; either version 3 of the License, or
   8    (at your option) any later version.
   9
  10    This program is distributed in the hope that it will be useful,
  11    but WITHOUT ANY WARRANTY; without even the implied warranty of
  12    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  13    GNU General Public License for more details.
  14
  15    You should have received a copy of the GNU General Public License
  16    along with this program.  If not, see <https://www.gnu.org/licenses/>.  */
  17
  18 /* Written by Jim Meyering, Eric Blake and Pádraig Brady.  */
  19
  20 /* Use "ignore_value" to avoid a warning when using a function declared with
  21    gcc's warn_unused_result attribute, but for which you really do want to
  22    ignore the result.  Traditionally, people have used a "(void)" cast to
  23    indicate that a function's return value is deliberately unused.  However,
  24    if the function is declared with __attribute__((warn_unused_result)),
  25    gcc issues a warning even with the cast.
  26
  27    Caution: most of the time, you really should heed gcc's warning, and
  28    check the return value.  However, in those exceptional cases in which
  29    you're sure you know what you're doing, use this function.
  30
  31    For the record, here's one of the ignorable warnings:
  32    "copy.c:233: warning: ignoring return value of 'fchown',
  33    declared with attribute warn_unused_result".  */
  34
  35 #ifndef _GL_IGNORE_VALUE_H
  36 #define _GL_IGNORE_VALUE_H
  37
  38 /* Normally casting an expression to void discards its value, but GCC
  39    versions 3.4 and newer have __attribute__ ((__warn_unused_result__))
  40    which may cause unwanted diagnostics in that case.  Use __typeof__
  41    and __extension__ to work around the problem, if the workaround is
  42    known to be needed.  */
  43 #if 3 < __GNUC__ + (4 <= __GNUC_MINOR__)
  44 # define ignore_value(x) \
  45     (__extension__ ({ __typeof__ (x) __x = (x); (void) __x; }))
  46 #else
  47 # define ignore_value(x) ((void) (x))
  48 #endif
  49
  50 #endif