--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,9 @@
+2010-05-02  Bruno Haible  <bruno@clisp.org>
+
+	wctob: Work around nasty Cygwin 1.7.2 bug.
+	* m4/wctob.m4 (gl_FUNC_WCTOB): Detect the Cygwin bug.
+	* doc/posix-functions/wctob.texi: Mention the Cygwin bug.
+
 2010-05-01  Bruno Haible  <bruno@clisp.org>
 
 	fpurge: Sharper test.

--- a/doc/posix-functions/wctob.texi
+++ b/doc/posix-functions/wctob.texi
@@ -12,6 +12,9 @@
 This function is missing on some platforms:
 HP-UX 11.00, IRIX 5.3, Solaris 2.6, mingw, Interix 3.5.
 @item
+This function clobbers caller-owned registers on some platforms:
+Cygwin 1.7.2.
+@item
 This function does not work on some platforms:
 Solaris 9.
 @item

--- a/m4/wctob.m4
+++ b/m4/wctob.m4
@@ -1,4 +1,4 @@
-# wctob.m4 serial 4
+# wctob.m4 serial 5
 dnl Copyright (C) 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,
@@ -17,6 +17,8 @@
   else
 
     dnl Solaris 9 has the wctob() function but it does not work.
+    dnl Cygwin 1.7.2 has the wctob() function but it clobbers caller-owned
+    dnl registers, see <http://cygwin.com/ml/cygwin/2010-05/msg00015.html>.
     AC_REQUIRE([AC_PROG_CC])
     AC_REQUIRE([gt_LOCALE_FR])
     AC_REQUIRE([AC_CANONICAL_HOST]) dnl for cross-compiles
@@ -27,14 +29,35 @@
         dnl is present.
 changequote(,)dnl
         case "$host_os" in
-            # Guess no on Solaris <= 9.
-          solaris2.[1-9] | solaris2.[1-9].*)
+            # Guess no on Solaris <= 9 and Cygwin.
+          solaris2.[1-9] | solaris2.[1-9].* | cygwin*)
             gl_cv_func_wctob_works="guessing no" ;;
             # Guess yes otherwise.
           *) gl_cv_func_wctob_works="guessing yes" ;;
         esac
 changequote([,])dnl
-        if test $LOCALE_FR != none; then
+        case "$host_os" in
+          cygwin*)
+            AC_TRY_RUN([
+#include <locale.h>
+#include <wchar.h>
+
+register long global __asm__ ("%ebx");
+
+int main ()
+{
+  setlocale (LC_ALL, "en_US.UTF-8");
+
+  global = 0x12345678;
+  if (wctob (0x00FC) != -1)
+    return 1;
+  if (global != 0x12345678)
+    return 2;
+  return 0;
+}], [:], [gl_cv_func_wctob_works=no], [:])
+            ;;
+        esac
+        if test "$gl_cv_func_wctob_works" != no && test $LOCALE_FR != none; then
           AC_TRY_RUN([
 #include <locale.h>
 #include <string.h>