--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,8 @@
 2007-01-28  Paul Eggert  <eggert@cs.ucla.edu>
 
+	* MODULES.html.sh: New module mpsort.
+	* lib/mpsort.c, lib/mpsort.h, m4/mpsort.m4, modules/mpsort: New files.
+
 	* lib/regex.h (_Restrict_): Renamed from __restrict, to avoid
 	a circularity problem with HP-UX ia64 reported by Bob Proulx in
 	<http://lists.gnu.org/archive/html/bug-gnulib/2007-01/msg00394.html>.

--- a/MODULES.html.sh
+++ b/MODULES.html.sh
@@ -1540,6 +1540,16 @@
   func_module pagealign_alloc
   func_end_table
 
+  element="Sorting functions <stdlib.h>"
+  element=`printf "%s" "$element" | sed -e "$sed_lt" -e "$sed_gt"`
+  func_section_wrap ansic_enh_stdlib_sorting
+  func_wrap H3
+  func_echo "$element"
+
+  func_begin_table
+  func_module mpsort
+  func_end_table
+
   element="Date and time <time.h>"
   element=`printf "%s" "$element" | sed -e "$sed_lt" -e "$sed_gt"`
   func_section_wrap ansic_enh_time_datetime

new file mode 100644
--- /dev/null
+++ b/lib/mpsort.c
@@ -0,0 +1,157 @@
+/* Sort a vector of pointers to data.
+
+   Copyright (C) 2007 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 2, 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, write to the Free Software Foundation,
+   Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.  */
+
+/* Written by Paul Eggert.  */
+
+#include <config.h>
+
+#include "mpsort.h"
+
+#include <string.h>
+
+/* The type of qsort-style comparison functions.  */
+
+typedef int (*comparison_function) (void const *, void const *);
+
+static void mpsort_with_tmp (void const **restrict, size_t,
+			     void const **restrict, comparison_function);
+
+/* Sort a vector BASE containing N pointers, placing the sorted array
+   into TMP.  Compare pointers with CMP.  N must be at least 2.  */
+
+static void
+mpsort_into_tmp (void const **restrict base, size_t n,
+		 void const **restrict tmp,
+		 comparison_function cmp)
+{
+  size_t n1 = n / 2;
+  size_t n2 = n - n1;
+  size_t a = 0;
+  size_t alim = n1;
+  size_t b = n1;
+  size_t blim = n;
+  void const *ba;
+  void const *bb;
+
+  mpsort_with_tmp (base + n1, n2, tmp, cmp);
+  mpsort_with_tmp (base, n1, tmp, cmp);
+
+  ba = base[a];
+  bb = base[b];
+
+  for (;;)
+    if (cmp (ba, bb) <= 0)
+      {
+	*tmp++ = ba;
+	a++;
+	if (a == alim)
+	  {
+	    a = b;
+	    alim = blim;
+	    break;
+	  }
+	ba = base[a];
+      }
+    else
+      {
+	*tmp++ = bb;
+	b++;
+	if (b == blim)
+	  break;
+	bb = base[b];
+      }
+
+  memcpy (tmp, base + a, (alim - a) * sizeof *base);
+}
+
+/* Sort a vector BASE containing N pointers, in place.  Use TMP
+   (containing N / 2 pointers) for temporary storage.  Compare
+   pointers with CMP.  */
+
+static void
+mpsort_with_tmp (void const **restrict base, size_t n,
+		 void const **restrict tmp,
+		 comparison_function cmp)
+{
+  if (n <= 2)
+    {
+      if (n == 2)
+	{
+	  void const *p0 = base[0];
+	  void const *p1 = base[1];
+	  if (! (cmp (p0, p1) <= 0))
+	    {
+	      base[0] = p1;
+	      base[1] = p0;
+	    }
+	}
+    }
+  else
+    {
+      size_t n1 = n / 2;
+      size_t n2 = n - n1;
+      size_t i;
+      size_t t = 0;
+      size_t tlim = n1;
+      size_t b = n1;
+      size_t blim = n;
+      void const *bb;
+      void const *tt;
+
+      mpsort_with_tmp (base + n1, n2, tmp, cmp);
+
+      if (n1 < 2)
+	tmp[0] = base[0];
+      else
+	mpsort_into_tmp (base, n1, tmp, cmp);
+
+      tt = tmp[t];
+      bb = base[b];
+
+      for (i = 0; ; )
+	if (cmp (tt, bb) <= 0)
+	  {
+	    base[i++] = tt;
+	    t++;
+	    if (t == tlim)
+	      break;
+	    tt = tmp[t];
+	  }
+	else
+	  {
+	    base[i++] = bb;
+	    b++;
+	    if (b == blim)
+	      {
+		memcpy (base + i, tmp + t, (tlim - t) * sizeof *base);
+		break;
+	      }
+	    bb = base[b];
+	  }
+    }
+}
+
+/* Sort a vector BASE containing N pointers, in place.  BASE must
+   contain enough storage to hold N + N / 2 vectors; the trailing
+   vectors are used for temporaries.  Compare pointers with CMP.  */
+
+void
+mpsort (void const **base, size_t n, comparison_function cmp)
+{
+  return mpsort_with_tmp (base, n, base + n, cmp);
+}

new file mode 100644
--- /dev/null
+++ b/lib/mpsort.h
@@ -0,0 +1,2 @@
+#include <stddef.h>
+void mpsort (void const **, size_t, int (*) (void const *, void const *));

new file mode 100644
--- /dev/null
+++ b/m4/mpsort.m4
@@ -0,0 +1,13 @@
+# Sort a vector of pointers to data.
+
+# Copyright (C) 2007 Free Software Foundation, Inc.
+
+# This file is free software; the Free Software Foundation
+# gives unlimited permission to copy and/or distribute it,
+# with or without modifications, as long as this notice is preserved.
+
+AC_DEFUN([gl_MPSORT],
+[
+  AC_REQUIRE([AC_C_RESTRICT])
+  AC_LIBOBJ([mpsort])
+])

new file mode 100644
--- /dev/null
+++ b/modules/mpsort
@@ -0,0 +1,23 @@
+Description:
+Sort a vector of pointers to data.
+
+Files:
+lib/mpsort.h
+lib/mpsort.c
+m4/mpsort.m4
+
+Depends-on:
+
+configure.ac:
+gl_MPSORT
+
+Makefile.am:
+
+Include:
+"mpsort.h"
+
+License:
+GPL
+
+Maintainer:
+Paul Eggert