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# ===========================================================================
# https://www.gnu.org/software/autoconf-archive/ax_cc_attrcommon.html
# ===========================================================================
#
# SYNOPSIS
#
# AX_CC_ATTRCOMMON
#
# DESCRIPTION
#
# Determine whether the C compiler supports C tentative definitions with
# __attribute((__common__)) See K&R book Appendix A10.2 on 'extern' and
# tentative definitions. Some compilers use a 'strict definition-reference
# model' Traditionally most UNIX C compilers support C tentative
# definitions. GCC 10 switched from default fcommon to fnocommon and will
# need __attribute__ ((__common__)) for tentative definitions.
#
# The $ac_cv_attrcommon variable will be either yes or no, and can be
# overridden on the command line with using --with-attrcommon
#
# See also: ax_cc_tentdef.m4 for a test on C tentative definitions
#
# LICENSE
#
# Copyright (c) 1996-2023 David Stes
#
# This program is free software: you can redistribute it and/or modify it
# under the terms of the GNU General Public License as published by the
# Free Software Foundation, either version 3 of the License, or (at your
# option) any later version.
#
# This program is distributed in the hope that it will be useful, but
# WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General
# Public License for more details.
#
# You should have received a copy of the GNU General Public License along
# with this program. If not, see <https://www.gnu.org/licenses/>.
#serial 4
AC_ARG_WITH(attrcommon,
[ --with-attrcommon use __attribute__ for C tentative definitions],
[ac_cv_attrcommon=$withval]
)
AC_DEFUN([AX_CC_ATTRCOMMON],
[
AC_MSG_CHECKING(whether compiler supports __attribute__((__common__)))
AC_CACHE_VAL(ac_cv_attrcommon,[
t1="ac$$.c"
o1="ac$$.o"
ac_clean_files="$t1 $o1"
cat >$t1 <<EOF
__attribute__((__common__)) int a = 7;
EOF
# tentative definitions are resolved using 'common storage'
# this is the traditional case for most C compilers (K&R book Appendix A10.2)
# this case is supported by setting CC to 'gcc -fcommon'
# which is since gcc 10 not the default (it defaults now to -fno-common)
echo "$CC -c $t1" >&5
if $CC -c $t1 2>&5 1>&5
then
echo " (no)" >&5
ac_cv_attrcommon=yes;
else
echo " (yes)" >&5
ac_cv_attrcommon=no;
fi
rm -rf $ac_clean_files
],ac_cv_attrcommon=yes,ac_cv_attrcommon=no)
AC_MSG_RESULT($ac_cv_attrcommon)
])
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