--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,18 @@
 2010-12-22  Bruno Haible  <bruno@clisp.org>
 
+	trunc: Implement result sign according to IEEE 754.
+	* lib/trunc.c (MIN, MINUS_ZERO): New macros.
+	(FUNC): Return +0.0 for 0 < x < 1 and -0.0 for -1 < x < 0.
+	* tests/test-trunc2.c: Include minus-zero.h.
+	(MINUS_ZERO): New macro.
+	(trunc_reference): Keep in sync with lib/trunc.c.
+	* tests/test-truncf2.c: Include minus-zero.h.
+	(MINUS_ZERO): New macro.
+	(truncf_reference): Keep in sync with lib/trunc.c.
+	* tests/test-truncf-ieee.c (main): Test also values between -1 and 1.
+	* tests/test-trunc-ieee.c (main): Likewise.
+	* tests/test-truncl-ieee.c (main): Likewise.
+
 	ceil: Implement result sign according to IEEE 754.
 	* lib/ceil.c (MIN, MINUS_ZERO): New macros.
 	(FUNC): Return -0.0 for -1 < x < 0.

--- a/lib/trunc.c
+++ b/lib/trunc.c
@@ -1,5 +1,5 @@
 /* Round towards zero.
-   Copyright (C) 2007, 2009, 2010 Free Software Foundation, Inc.
+   Copyright (C) 2007, 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
@@ -27,19 +27,29 @@
 # define FUNC truncl
 # define DOUBLE long double
 # define MANT_DIG LDBL_MANT_DIG
+# define MIN LDBL_MIN
 # define L_(literal) literal##L
 #elif ! defined USE_FLOAT
 # define FUNC trunc
 # define DOUBLE double
 # define MANT_DIG DBL_MANT_DIG
+# define MIN DBL_MIN
 # define L_(literal) literal
 #else /* defined USE_FLOAT */
 # define FUNC truncf
 # define DOUBLE float
 # define MANT_DIG FLT_MANT_DIG
+# define MIN FLT_MIN
 # define L_(literal) literal##f
 #endif
 
+/* -0.0.  See minus-zero.h.  */
+#if defined __hpux || defined __sgi || defined __ICC
+# define MINUS_ZERO (-MIN * MIN)
+#else
+# define MINUS_ZERO L_(-0.0)
+#endif
+
 /* 2^(MANT_DIG-1).  */
 static const DOUBLE TWO_MANT_DIG =
   /* Assume MANT_DIG <= 5 * 31.
@@ -65,8 +75,12 @@
 
   if (z > L_(0.0))
     {
+      /* For 0 < x < 1, return +0.0 even if the current rounding mode is
+         FE_DOWNWARD.  */
+      if (z < L_(1.0))
+        z = L_(0.0);
       /* Avoid rounding errors for values near 2^k, where k >= MANT_DIG-1.  */
-      if (z < TWO_MANT_DIG)
+      else if (z < TWO_MANT_DIG)
         {
           /* Round to the next integer (nearest or up or down, doesn't matter).  */
           z += TWO_MANT_DIG;
@@ -78,8 +92,12 @@
     }
   else if (z < L_(0.0))
     {
+      /* For -1 < x < 0, return -0.0 regardless of the current rounding
+         mode.  */
+      if (z > L_(-1.0))
+        z = MINUS_ZERO;
       /* Avoid rounding errors for values near -2^k, where k >= MANT_DIG-1.  */
-      if (z > - TWO_MANT_DIG)
+      else if (z > - TWO_MANT_DIG)
         {
           /* Round to the next integer (nearest or up or down, doesn't matter).  */
           z -= TWO_MANT_DIG;

--- a/tests/test-trunc-ieee.c
+++ b/tests/test-trunc-ieee.c
@@ -24,9 +24,20 @@
 int
 main ()
 {
+  /* See IEEE 754, section 6.3:
+       "the sign of the result of the round floating-point number to
+        integral value operation is the sign of the operand. These rules
+        shall apply even when operands or results are zero or infinite."  */
+
   /* Zero.  */
   ASSERT (!signbit (trunc (0.0)));
   ASSERT (!!signbit (trunc (minus_zerod)) == !!signbit (minus_zerod));
+  /* Positive numbers.  */
+  ASSERT (!signbit (trunc (0.3)));
+  ASSERT (!signbit (trunc (0.7)));
+  /* Negative numbers.  */
+  ASSERT (!!signbit (trunc (-0.3)) == !!signbit (minus_zerod));
+  ASSERT (!!signbit (trunc (-0.7)) == !!signbit (minus_zerod));
 
   return 0;
 }

--- a/tests/test-trunc2.c
+++ b/tests/test-trunc2.c
@@ -29,6 +29,7 @@
 #include <stdio.h>
 
 #include "isnand-nolibm.h"
+#include "minus-zero.h"
 #include "macros.h"
 
 
@@ -38,6 +39,9 @@
 #define MANT_DIG DBL_MANT_DIG
 #define L_(literal) literal
 
+/* -0.0.  See minus-zero.h.  */
+#define MINUS_ZERO minus_zerod
+
 /* 2^(MANT_DIG-1).  */
 static const DOUBLE TWO_MANT_DIG =
   /* Assume MANT_DIG <= 5 * 31.
@@ -63,8 +67,12 @@
 
   if (z > L_(0.0))
     {
+      /* For 0 < x < 1, return +0.0 even if the current rounding mode is
+         FE_DOWNWARD.  */
+      if (z < L_(1.0))
+        z = L_(0.0);
       /* Avoid rounding errors for values near 2^k, where k >= MANT_DIG-1.  */
-      if (z < TWO_MANT_DIG)
+      else if (z < TWO_MANT_DIG)
         {
           /* Round to the next integer (nearest or up or down, doesn't matter).  */
           z += TWO_MANT_DIG;
@@ -76,8 +84,12 @@
     }
   else if (z < L_(0.0))
     {
+      /* For -1 < x < 0, return -0.0 regardless of the current rounding
+         mode.  */
+      if (z > L_(-1.0))
+        z = MINUS_ZERO;
       /* Avoid rounding errors for values near -2^k, where k >= MANT_DIG-1.  */
-      if (z > - TWO_MANT_DIG)
+      else if (z > - TWO_MANT_DIG)
         {
           /* Round to the next integer (nearest or up or down, doesn't matter).  */
           z -= TWO_MANT_DIG;

--- a/tests/test-truncf-ieee.c
+++ b/tests/test-truncf-ieee.c
@@ -24,9 +24,20 @@
 int
 main ()
 {
+  /* See IEEE 754, section 6.3:
+       "the sign of the result of the round floating-point number to
+        integral value operation is the sign of the operand. These rules
+        shall apply even when operands or results are zero or infinite."  */
+
   /* Zero.  */
   ASSERT (!signbit (truncf (0.0f)));
   ASSERT (!!signbit (truncf (minus_zerof)) == !!signbit (minus_zerof));
+  /* Positive numbers.  */
+  ASSERT (!signbit (truncf (0.3f)));
+  ASSERT (!signbit (truncf (0.7f)));
+  /* Negative numbers.  */
+  ASSERT (!!signbit (truncf (-0.3f)) == !!signbit (minus_zerof));
+  ASSERT (!!signbit (truncf (-0.7f)) == !!signbit (minus_zerof));
 
   return 0;
 }

--- a/tests/test-truncf2.c
+++ b/tests/test-truncf2.c
@@ -29,6 +29,7 @@
 #include <stdio.h>
 
 #include "isnanf-nolibm.h"
+#include "minus-zero.h"
 #include "macros.h"
 
 
@@ -38,6 +39,9 @@
 #define MANT_DIG FLT_MANT_DIG
 #define L_(literal) literal##f
 
+/* -0.0.  See minus-zero.h.  */
+#define MINUS_ZERO minus_zerof
+
 /* 2^(MANT_DIG-1).  */
 static const DOUBLE TWO_MANT_DIG =
   /* Assume MANT_DIG <= 5 * 31.
@@ -63,8 +67,12 @@
 
   if (z > L_(0.0))
     {
+      /* For 0 < x < 1, return +0.0 even if the current rounding mode is
+         FE_DOWNWARD.  */
+      if (z < L_(1.0))
+        z = L_(0.0);
       /* Avoid rounding errors for values near 2^k, where k >= MANT_DIG-1.  */
-      if (z < TWO_MANT_DIG)
+      else if (z < TWO_MANT_DIG)
         {
           /* Round to the next integer (nearest or up or down, doesn't matter).  */
           z += TWO_MANT_DIG;
@@ -76,8 +84,12 @@
     }
   else if (z < L_(0.0))
     {
+      /* For -1 < x < 0, return -0.0 regardless of the current rounding
+         mode.  */
+      if (z > L_(-1.0))
+        z = MINUS_ZERO;
       /* Avoid rounding errors for values near -2^k, where k >= MANT_DIG-1.  */
-      if (z > - TWO_MANT_DIG)
+      else if (z > - TWO_MANT_DIG)
         {
           /* Round to the next integer (nearest or up or down, doesn't matter).  */
           z -= TWO_MANT_DIG;

--- a/tests/test-truncl-ieee.c
+++ b/tests/test-truncl-ieee.c
@@ -29,9 +29,20 @@
 
   BEGIN_LONG_DOUBLE_ROUNDING ();
 
+  /* See IEEE 754, section 6.3:
+       "the sign of the result of the round floating-point number to
+        integral value operation is the sign of the operand. These rules
+        shall apply even when operands or results are zero or infinite."  */
+
   /* Zero.  */
   ASSERT (!signbit (truncl (0.0L)));
   ASSERT (!!signbit (truncl (minus_zerol)) == !!signbit (minus_zerol));
+  /* Positive numbers.  */
+  ASSERT (!signbit (truncl (0.3L)));
+  ASSERT (!signbit (truncl (0.7L)));
+  /* Negative numbers.  */
+  ASSERT (!!signbit (truncl (-0.3L)) == !!signbit (minus_zerol));
+  ASSERT (!!signbit (truncl (-0.7L)) == !!signbit (minus_zerol));
 
   return 0;
 }