--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,19 @@
+2011-02-22  Bruno Haible  <bruno@clisp.org>
+
+	New module 'wctomb'.
+	* lib/stdlib.in.h (wctomb): New declaration.
+	* lib/wctomb.c: New file.
+	* lib/wctomb-impl.h: New file.
+	* m4/wctomb.m4: New file.
+	* m4/stdlib_h.m4 (gl_STDLIB_H_DEFAULTS): Initialize GNULIB_WCTOMB,
+	REPLACE_WCTOMB.
+	* modules/stdlib (Makefile.am): Substitute GNULIB_WCTOMB,
+	REPLACE_WCTOMB.
+	* modules/wctomb: New file.
+	* tests/test-stdlib-c++.cc: Test signature of wctomb.
+	* doc/posix-functions/wctomb.texi: Mention the new module.
+	* modules/wctob (Depends-on): Add wctomb.
+
 2011-02-22  Bruno Haible  <bruno@clisp.org>
 
 	New module 'mbtowc'.

--- a/doc/posix-functions/wctomb.texi
+++ b/doc/posix-functions/wctomb.texi
@@ -4,7 +4,7 @@
 
 POSIX specification:@* @url{http://www.opengroup.org/onlinepubs/9699919799/functions/wctomb.html}
 
-Gnulib module: ---
+Gnulib module: wctomb
 
 Portability problems fixed by Gnulib:
 @itemize

--- a/lib/stdlib.in.h
+++ b/lib/stdlib.in.h
@@ -738,6 +738,21 @@
 # endif
 #endif
 
+/* Convert a wide character to a multibyte character.  */
+#if @GNULIB_WCTOMB@
+# if @REPLACE_WCTOMB@
+#  if !(defined __cplusplus && defined GNULIB_NAMESPACE)
+#   undef wctomb
+#   define wctomb rpl_wctomb
+#  endif
+_GL_FUNCDECL_RPL (wctomb, int, (char *s, wchar_t wc));
+_GL_CXXALIAS_RPL (wctomb, int, (char *s, wchar_t wc));
+# else
+_GL_CXXALIAS_SYS (wctomb, int, (char *s, wchar_t wc));
+# endif
+_GL_CXXALIASWARN (wctomb);
+#endif
+
 
 #endif /* _GL_STDLIB_H */
 #endif /* _GL_STDLIB_H */

new file mode 100644
--- /dev/null
+++ b/lib/wctomb-impl.h
@@ -0,0 +1,34 @@
+/* Convert wide character to multibyte character.
+   Copyright (C) 2011 Free Software Foundation, Inc.
+   Written by Bruno Haible <bruno@clisp.org>, 2011.
+
+   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/>.  */
+
+int
+wctomb (char *s, wchar_t wc)
+{
+  if (s == NULL)
+    return 0;
+  else
+    {
+      mbstate_t state;
+      size_t result;
+
+      memset (&state, 0, sizeof (mbstate_t));
+      result = wcrtomb (s, wc, &state);
+      if (result == (size_t)-1)
+        return -1;
+      return result;
+    }
+}

new file mode 100644
--- /dev/null
+++ b/lib/wctomb.c
@@ -0,0 +1,25 @@
+/* Convert wide character to multibyte character.
+   Copyright (C) 2011 Free Software Foundation, Inc.
+   Written by Bruno Haible <bruno@clisp.org>, 2011.
+
+   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 <stdlib.h>
+
+#include <string.h>
+#include <wchar.h>
+
+#include "wctomb-impl.h"

--- a/m4/stdlib_h.m4
+++ b/m4/stdlib_h.m4
@@ -63,6 +63,7 @@
   GNULIB_SYSTEM_POSIX=0;  AC_SUBST([GNULIB_SYSTEM_POSIX])
   GNULIB_UNLOCKPT=0;      AC_SUBST([GNULIB_UNLOCKPT])
   GNULIB_UNSETENV=0;      AC_SUBST([GNULIB_UNSETENV])
+  GNULIB_WCTOMB=0;        AC_SUBST([GNULIB_WCTOMB])
   dnl Assume proper GNU behavior unless another module says otherwise.
   HAVE__EXIT=1;              AC_SUBST([HAVE__EXIT])
   HAVE_ATOLL=1;              AC_SUBST([HAVE_ATOLL])
@@ -100,4 +101,5 @@
   REPLACE_SETENV=0;          AC_SUBST([REPLACE_SETENV])
   REPLACE_STRTOD=0;          AC_SUBST([REPLACE_STRTOD])
   REPLACE_UNSETENV=0;        AC_SUBST([REPLACE_UNSETENV])
+  REPLACE_WCTOMB=0;          AC_SUBST([REPLACE_WCTOMB])
 ])

new file mode 100644
--- /dev/null
+++ b/m4/wctomb.m4
@@ -0,0 +1,23 @@
+# wctomb.m4 serial 1
+dnl Copyright (C) 2011 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.
+
+AC_DEFUN([gl_FUNC_WCTOMB],
+[
+  AC_REQUIRE([gl_STDLIB_H_DEFAULTS])
+
+  if false; then
+    REPLACE_WCTOMB=1
+  fi
+  if test $REPLACE_WCTOMB = 1; then
+    AC_LIBOBJ([wctomb])
+    gl_PREREQ_WCTOMB
+  fi
+])
+
+# Prerequisites of lib/wctomb.c.
+AC_DEFUN([gl_PREREQ_WCTOMB], [
+  :
+])

--- a/modules/stdlib
+++ b/modules/stdlib
@@ -55,6 +55,7 @@
 	      -e 's|@''GNULIB_SYSTEM_POSIX''@|$(GNULIB_SYSTEM_POSIX)|g' \
 	      -e 's|@''GNULIB_UNLOCKPT''@|$(GNULIB_UNLOCKPT)|g' \
 	      -e 's|@''GNULIB_UNSETENV''@|$(GNULIB_UNSETENV)|g' \
+	      -e 's|@''GNULIB_WCTOMB''@|$(GNULIB_WCTOMB)|g' \
 	      < $(srcdir)/stdlib.in.h | \
 	  sed -e 's|@''HAVE__EXIT''@|$(HAVE__EXIT)|g' \
 	      -e 's|@''HAVE_ATOLL''@|$(HAVE_ATOLL)|g' \
@@ -91,6 +92,7 @@
 	      -e 's|@''REPLACE_SETENV''@|$(REPLACE_SETENV)|g' \
 	      -e 's|@''REPLACE_STRTOD''@|$(REPLACE_STRTOD)|g' \
 	      -e 's|@''REPLACE_UNSETENV''@|$(REPLACE_UNSETENV)|g' \
+	      -e 's|@''REPLACE_WCTOMB''@|$(REPLACE_WCTOMB)|g' \
 	      -e '/definitions of _GL_FUNCDECL_RPL/r $(CXXDEFS_H)' \
 	      -e '/definition of _GL_ARG_NONNULL/r $(ARG_NONNULL_H)' \
 	      -e '/definition of _GL_WARN_ON_USE/r $(WARN_ON_USE_H)'; \

--- a/modules/wctob
+++ b/modules/wctob
@@ -9,6 +9,7 @@
 
 Depends-on:
 wchar
+wctomb
 
 configure.ac:
 gl_FUNC_WCTOB

new file mode 100644
--- /dev/null
+++ b/modules/wctomb
@@ -0,0 +1,26 @@
+Description:
+wctomb() function: convert wide character to multibyte character.
+
+Files:
+lib/wctomb.c
+lib/wctomb-impl.h
+m4/wctomb.m4
+
+Depends-on:
+stdlib
+wcrtomb
+
+configure.ac:
+gl_FUNC_WCTOMB
+gl_STDLIB_MODULE_INDICATOR([wctomb])
+
+Makefile.am:
+
+Include:
+<stdlib.h>
+
+License:
+LGPLv2+
+
+Maintainer:
+Bruno Haible

--- a/tests/test-stdlib-c++.cc
+++ b/tests/test-stdlib-c++.cc
@@ -152,6 +152,10 @@
 SIGNATURE_CHECK (GNULIB_NAMESPACE::unsetenv, int, (const char *));
 #endif
 
+#if GNULIB_TEST_WCTOMB
+SIGNATURE_CHECK (GNULIB_NAMESPACE::wctomb, int, (char *, wchar_t));
+#endif
+
 
 int
 main ()