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diff --git a/m4/free.m4 b/m4/free.m4
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+# Check whether free (NULL) is supposed to work.
+
+# Copyright (C) 2003 Free Software Foundation, Inc.
+
+# This file is free software, distributed under the terms of the GNU
+# General Public License.  As a special exception to the GNU General
+# Public License, this file may be distributed as part of a program
+# that contains a configuration script generated by Autoconf, under
+# the same distribution terms as the rest of that program.
+
+# Written by Paul Eggert.
+
+# We can't test for free (NULL) even at runtime, since it might
+# happen to "work" for our test program, but not in general.  So, be
+# conservative and use feature tests for relatively modern hosts,
+# where free (NULL) is known to work.  This costs a bit of
+# performance on some older hosts, but we can fix that later if
+# needed.
+
+AC_DEFUN([gl_FUNC_FREE],
+[
+  AC_CHECK_HEADERS_ONCE(unistd.h)
+  AC_CACHE_CHECK([whether free (NULL) is known to work],
+    [gl_cv_func_free],
+    [AC_COMPILE_IFELSE(
+       [AC_LANG_PROGRAM(
+	  [[@%:@if HAVE_UNISTD_H
+	      @%:@include <unistd.h>
+	    @%:@endif]],
+	  [[@%:@if _POSIX_VERSION < 199009L && \
+	        (defined unix || defined _unix || defined _unix_ \
+	         || defined __unix || defined __unix__)
+	      @%:@error "'free (NULL)' is not known to work"
+	    @%:@endif]])],
+       [gl_cv_func_free=yes],
+       [gl_cv_func_free=no])])
+
+  if test $gl_cv_func_free = yes; then
+    AC_LIBOBJ(free)
+    AC_DEFINE(free, rpl_free,
+      [Define to rpl_free if the replacement function should be used.])
+  fi
+])
+
+# Prerequisites of lib/free.c.
+AC_DEFUN([gl_PREREQ_FREE], [:])