--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,14 @@
+2010-12-25  Bruno Haible  <bruno@clisp.org>
+
+	truncl-ieee: Make it work on OSF/1 5.1 with cc.
+	* doc/posix-functions/truncl.texi: Mention the OSF/1 5.1 bug.
+	* m4/truncl.m4 (gl_FUNC_TRUNCL): If gl_FUNC_TRUNCL_IEEE is also used,
+	test whether truncl works according to ISO C 99 with IEC 60559.
+	* m4/truncl-ieee.m4: New file.
+	* modules/truncl-ieee (Files): Add it and m4/minus-zero.m4,
+	m4/signbit.m4.
+	(configure.ac): Invoke gl_FUNC_TRUNCL_IEEE.
+
 2010-12-25  Bruno Haible  <bruno@clisp.org>
 
 	ceill-ieee: Make it work on OSF/1 5.1 with cc.

--- a/doc/posix-functions/truncl.texi
+++ b/doc/posix-functions/truncl.texi
@@ -6,7 +6,7 @@
 
 Gnulib module: truncl or truncl-ieee
 
-Portability problems fixed by Gnulib:
+Portability problems fixed by either Gnulib module @code{truncl} or @code{truncl-ieee}:
 @itemize
 @item
 This function is missing on some platforms:
@@ -19,6 +19,14 @@
 OSF/1 4.0.
 @end itemize
 
+Portability problems fixed by Gnulib module @code{truncl-ieee}:
+@itemize
+@item
+This function returns a positive zero for an argument between -1 and 0
+on some platforms:
+OSF/1 5.1.
+@end itemize
+
 Portability problems not fixed by Gnulib:
 @itemize
 @end itemize

new file mode 100644
--- /dev/null
+++ b/m4/truncl-ieee.m4
@@ -0,0 +1,15 @@
+# truncl-ieee.m4 serial 1
+dnl Copyright (C) 2010 Free Software Foundation, Inc.
+dnl This file is free software; the Free Software Foundation
+dnl gives unlimited permission to copy and/or distribute it,
+dnl with or without modifications, as long as this notice is preserved.
+
+dnl This macro is in a separate file (not in truncl.m4 and not inlined in the
+dnl module description), so that gl_FUNC_TRUNCL can test whether 'aclocal' has
+dnl found uses of this macro.
+
+AC_DEFUN([gl_FUNC_TRUNCL_IEEE],
+[
+  m4_divert_text([INIT_PREPARE], [gl_truncl_required=ieee])
+  AC_REQUIRE([gl_FUNC_TRUNCL])
+])

--- a/m4/truncl.m4
+++ b/m4/truncl.m4
@@ -1,4 +1,4 @@
-# truncl.m4 serial 5
+# truncl.m4 serial 6
 dnl Copyright (C) 2007-2008, 2010 Free Software Foundation, Inc.
 dnl This file is free software; the Free Software Foundation
 dnl gives unlimited permission to copy and/or distribute it,
@@ -6,6 +6,7 @@
 
 AC_DEFUN([gl_FUNC_TRUNCL],
 [
+  m4_divert_text([DEFAULTS], [gl_truncl_required=plain])
   AC_REQUIRE([gl_MATH_H_DEFAULTS])
   AC_REQUIRE([AC_CANONICAL_HOST]) dnl for cross-compiles
   dnl Persuade glibc <math.h> to declare truncl().
@@ -68,6 +69,40 @@
       *yes) ;;
       *) REPLACE_TRUNCL=1 ;;
     esac
+    m4_ifdef([gl_FUNC_TRUNCL_IEEE], [
+      if test $gl_truncl_required = ieee && test $REPLACE_TRUNCL = 0; then
+        AC_CACHE_CHECK([whether truncl works according to ISO C 99 with IEC 60559],
+          [gl_cv_func_truncl_ieee],
+          [
+            save_LIBS="$LIBS"
+            LIBS="$LIBS $TRUNCL_LIBM"
+            AC_RUN_IFELSE(
+              [AC_LANG_SOURCE([[
+#ifndef __NO_MATH_INLINES
+# define __NO_MATH_INLINES 1 /* for glibc */
+#endif
+#include <math.h>
+]gl_LONG_DOUBLE_MINUS_ZERO_CODE[
+]gl_LONG_DOUBLE_SIGNBIT_CODE[
+int main()
+{
+  /* Test whether truncl (-0.3L) is -0.0L.  */
+  if (signbitl (minus_zerol) && !signbitl (truncl (-0.3L)))
+    return 1;
+  return 0;
+}
+              ]])],
+              [gl_cv_func_truncl_ieee=yes],
+              [gl_cv_func_truncl_ieee=no],
+              [gl_cv_func_truncl_ieee="guessing no"])
+            LIBS="$save_LIBS"
+          ])
+        case "$gl_cv_func_truncl_ieee" in
+          *yes) ;;
+          *) REPLACE_TRUNCL=1 ;;
+        esac
+      fi
+    ])
   else
     HAVE_DECL_TRUNCL=0
   fi

--- a/modules/truncl-ieee
+++ b/modules/truncl-ieee
@@ -2,11 +2,15 @@
 truncl() function according to ISO C 99 with IEC 60559.
 
 Files:
+m4/truncl-ieee.m4
+m4/minus-zero.m4
+m4/signbit.m4
 
 Depends-on:
 truncl
 
 configure.ac:
+gl_FUNC_TRUNCL_IEEE
 
 Makefile.am: