--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,19 @@
+2010-12-21  Bruno Haible  <bruno@clisp.org>
+
+	New module 'trunc-ieee'.
+	* modules/trunc-ieee: New file.
+	* m4/trunc.m4 (gl_FUNC_TRUNC): If gl_FUNC_TRUNC_IEEE is also used, test
+	whether trunc works according to ISO C 99 with IEC 60559.
+	* m4/trunc-ieee.m4: New file.
+	* lib/math.in.h (trunc): Replace if REPLACE_TRUNC is set.
+	* m4/math_h.m4 (gl_MATH_H_DEFAULTS): Initialize REPLACE_TRUNC.
+	* modules/math (Makefile.am): Substitute REPLACE_TRUNC.
+	* modules/trunc-ieee-tests: New file.
+	* tests/test-trunc-ieee.c: New file, based on tests/test-truncf-ieee.c.
+	* tests/test-trunc1.c (main): Remove signbit tests.
+	* modules/trunc-tests (Depends-on): Remove 'signbit'.
+	* doc/posix-functions/trunc.texi: Mention the new module.
+
 2010-12-21  Bruno Haible  <bruno@clisp.org>
 
 	New module 'ceil-ieee'.

--- a/doc/posix-functions/trunc.texi
+++ b/doc/posix-functions/trunc.texi
@@ -4,9 +4,9 @@
 
 POSIX specification:@* @url{http://www.opengroup.org/onlinepubs/9699919799/functions/trunc.html}
 
-Gnulib module: trunc
+Gnulib module: trunc or trunc-ieee
 
-Portability problems fixed by Gnulib:
+Portability problems fixed by either Gnulib module @code{trunc} or @code{trunc-ieee}:
 @itemize
 @item
 This function is missing on some platforms:
@@ -16,6 +16,14 @@
 glibc 2.8.
 @end itemize
 
+Portability problems fixed by Gnulib module @code{trunc-ieee}:
+@itemize
+@item
+This function returns a positive zero for a minus zero argument
+on some platforms:
+OSF/1 5.1.
+@end itemize
+
 Portability problems not fixed by Gnulib:
 @itemize
 @end itemize

--- a/lib/math.in.h
+++ b/lib/math.in.h
@@ -541,10 +541,18 @@
 #endif
 
 #if @GNULIB_TRUNC@
-# if !@HAVE_DECL_TRUNC@
+# if @REPLACE_TRUNC@
+#  if !(defined __cplusplus && defined GNULIB_NAMESPACE)
+#   define trunc rpl_trunc
+#  endif
+_GL_FUNCDECL_RPL (trunc, double, (double x));
+_GL_CXXALIAS_RPL (trunc, double, (double x));
+# else
+#  if !@HAVE_DECL_TRUNC@
 _GL_FUNCDECL_SYS (trunc, double, (double x));
+#  endif
+_GL_CXXALIAS_SYS (trunc, double, (double x));
 # endif
-_GL_CXXALIAS_SYS (trunc, double, (double x));
 _GL_CXXALIASWARN (trunc);
 #elif defined GNULIB_POSIXCHECK
 # undef trunc

--- a/m4/math_h.m4
+++ b/m4/math_h.m4
@@ -1,4 +1,4 @@
-# math_h.m4 serial 24
+# math_h.m4 serial 25
 dnl Copyright (C) 2007-2010 Free Software Foundation, Inc.
 dnl This file is free software; the Free Software Foundation
 dnl gives unlimited permission to copy and/or distribute it,
@@ -140,6 +140,7 @@
   REPLACE_ROUNDL=0;            AC_SUBST([REPLACE_ROUNDL])
   REPLACE_SIGNBIT=0;           AC_SUBST([REPLACE_SIGNBIT])
   REPLACE_SIGNBIT_USING_GCC=0; AC_SUBST([REPLACE_SIGNBIT_USING_GCC])
+  REPLACE_TRUNC=0;             AC_SUBST([REPLACE_TRUNC])
   REPLACE_TRUNCF=0;            AC_SUBST([REPLACE_TRUNCF])
   REPLACE_TRUNCL=0;            AC_SUBST([REPLACE_TRUNCL])
 ])

new file mode 100644
--- /dev/null
+++ b/m4/trunc-ieee.m4
@@ -0,0 +1,15 @@
+# trunc-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 trunc.m4 and not inlined in the
+dnl module description), so that gl_FUNC_TRUNC can test whether 'aclocal' has
+dnl found uses of this macro.
+
+AC_DEFUN([gl_FUNC_TRUNC_IEEE],
+[
+  m4_divert_text([INIT_PREPARE], [gl_trunc_required=ieee])
+  AC_REQUIRE([gl_FUNC_TRUNC])
+])

--- a/m4/trunc.m4
+++ b/m4/trunc.m4
@@ -1,4 +1,4 @@
-# trunc.m4 serial 4
+# trunc.m4 serial 5
 dnl Copyright (C) 2007, 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_TRUNC],
 [
+  m4_divert_text([DEFAULTS], [gl_trunc_required=plain])
   AC_REQUIRE([gl_MATH_H_DEFAULTS])
   dnl Persuade glibc <math.h> to declare trunc().
   AC_REQUIRE([gl_USE_SYSTEM_EXTENSIONS])
@@ -40,8 +41,44 @@
     if test "$TRUNC_LIBM" = "?"; then
       TRUNC_LIBM=
     fi
+    m4_ifdef([gl_FUNC_TRUNC_IEEE], [
+      if test $gl_trunc_required = ieee && test $REPLACE_TRUNC = 0; then
+        AC_CACHE_CHECK([whether trunc works according to ISO C 99 with IEC 60559],
+          [gl_cv_func_trunc_ieee],
+          [
+            save_LIBS="$LIBS"
+            LIBS="$LIBS $TRUNC_LIBM"
+            AC_RUN_IFELSE(
+              [AC_LANG_SOURCE([[
+#ifndef __NO_MATH_INLINES
+# define __NO_MATH_INLINES 1 /* for glibc */
+#endif
+#include <math.h>
+]gl_DOUBLE_MINUS_ZERO_CODE[
+]gl_DOUBLE_SIGNBIT_CODE[
+int main()
+{
+  /* Test whether trunc (-0.0) is -0.0.  */
+  if (signbitd (minus_zerod) && !signbitd (trunc (minus_zerod)))
+    return 1;
+  return 0;
+}
+              ]])],
+              [gl_cv_func_trunc_ieee=yes],
+              [gl_cv_func_trunc_ieee=no],
+              [gl_cv_func_trunc_ieee="guessing no"])
+            LIBS="$save_LIBS"
+          ])
+        case "$gl_cv_func_trunc_ieee" in
+          *yes) ;;
+          *) REPLACE_TRUNC=1 ;;
+        esac
+      fi
+    ])
   else
     HAVE_DECL_TRUNC=0
+  fi
+  if test $HAVE_DECL_TRUNC = 0 || test $REPLACE_TRUNC = 1; then
     AC_LIBOBJ([trunc])
     TRUNC_LIBM=
   fi

--- a/modules/math
+++ b/modules/math
@@ -111,6 +111,7 @@
 	      -e 's|@''REPLACE_ROUNDL''@|$(REPLACE_ROUNDL)|g' \
 	      -e 's|@''REPLACE_SIGNBIT''@|$(REPLACE_SIGNBIT)|g' \
 	      -e 's|@''REPLACE_SIGNBIT_USING_GCC''@|$(REPLACE_SIGNBIT_USING_GCC)|g' \
+	      -e 's|@''REPLACE_TRUNC''@|$(REPLACE_TRUNC)|g' \
 	      -e 's|@''REPLACE_TRUNCF''@|$(REPLACE_TRUNCF)|g' \
 	      -e 's|@''REPLACE_TRUNCL''@|$(REPLACE_TRUNCL)|g' \
 	      -e '/definitions of _GL_FUNCDECL_RPL/r $(CXXDEFS_H)' \

new file mode 100644
--- /dev/null
+++ b/modules/trunc-ieee
@@ -0,0 +1,27 @@
+Description:
+trunc() function according to ISO C 99 with IEC 60559.
+
+Files:
+m4/trunc-ieee.m4
+m4/minus-zero.m4
+m4/signbit.m4
+
+Depends-on:
+trunc
+
+configure.ac:
+gl_FUNC_TRUNC_IEEE
+
+Makefile.am:
+
+Include:
+<math.h>
+
+Link:
+$(TRUNC_LIBM)
+
+License:
+LGPL
+
+Maintainer:
+Bruno Haible

new file mode 100644
--- /dev/null
+++ b/modules/trunc-ieee-tests
@@ -0,0 +1,15 @@
+Files:
+tests/test-trunc-ieee.c
+tests/minus-zero.h
+tests/macros.h
+
+Depends-on:
+float
+signbit
+
+configure.ac:
+
+Makefile.am:
+TESTS += test-trunc-ieee
+check_PROGRAMS += test-trunc-ieee
+test_trunc_ieee_LDADD = $(LDADD) @TRUNC_LIBM@

--- a/modules/trunc-tests
+++ b/modules/trunc-tests
@@ -9,7 +9,6 @@
 Depends-on:
 float
 isnand-nolibm
-signbit
 stdbool
 stdint
 

new file mode 100644
--- /dev/null
+++ b/tests/test-trunc-ieee.c
@@ -0,0 +1,32 @@
+/* Test of rounding towards zero.
+   Copyright (C) 2010 Free Software Foundation, Inc.
+
+   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 <http://www.gnu.org/licenses/>.  */
+
+#include <config.h>
+
+#include <math.h>
+
+#include "minus-zero.h"
+#include "macros.h"
+
+int
+main ()
+{
+  /* Zero.  */
+  ASSERT (!signbit (trunc (0.0)));
+  ASSERT (!!signbit (trunc (minus_zerod)) == !!signbit (minus_zerod));
+
+  return 0;
+}

--- a/tests/test-trunc1.c
+++ b/tests/test-trunc1.c
@@ -33,9 +33,7 @@
 {
   /* Zero.  */
   ASSERT (trunc (0.0) == 0.0);
-  ASSERT (!signbit (trunc (0.0)));
   ASSERT (trunc (minus_zerod) == 0.0);
-  ASSERT (!!signbit (minus_zerod) == !!signbit (trunc (minus_zerod)));
   /* Positive numbers.  */
   ASSERT (trunc (0.3) == 0.0);
   ASSERT (trunc (0.7) == 0.0);