--- a/ChangeLog	Mon Apr 05 23:46:02 2010 +0200
+++ b/ChangeLog	Tue Apr 06 00:30:28 2010 +0200
@@ -1,3 +1,10 @@
+2010-04-05  Bruno Haible  <bruno@clisp.org>
+
+	btowc: Work around Cygwin 1.7.2 bug.
+	* m4/btowc.m4 (gl_FUNC_BTOWC): Set REPLACE_BTOWC to 1 if the function
+	does not map NUL to 0.
+	* doc/posix-functions/btowc.texi: Mention the Cygwin bug.
+
 2010-04-05  Bruno Haible  <bruno@clisp.org>
 
 	Make the multithread modules work on Cygwin 1.7.2.

--- a/doc/posix-functions/btowc.texi	Mon Apr 05 23:46:02 2010 +0200
+++ b/doc/posix-functions/btowc.texi	Tue Apr 06 00:30:28 2010 +0200
@@ -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 returns WEOF for a NUL argument on some platforms:
+Cygwin 1.7.2.
+@item
 This function does not return WEOF for an EOF argument on some platforms:
 IRIX 6.5.
 @end itemize

--- a/m4/btowc.m4	Mon Apr 05 23:46:02 2010 +0200
+++ b/m4/btowc.m4	Tue Apr 06 00:30:28 2010 +0200
@@ -1,4 +1,4 @@
-# btowc.m4 serial 5
+# btowc.m4 serial 6
 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,
@@ -18,10 +18,39 @@
     HAVE_BTOWC=0
   else
 
-    dnl IRIX 6.5 btowc(EOF) is 0xFF, not WEOF.
     AC_REQUIRE([AC_PROG_CC])
     AC_REQUIRE([gt_LOCALE_FR])
     AC_REQUIRE([AC_CANONICAL_HOST]) dnl for cross-compiles
+
+    dnl Cygwin 1.7.2 btowc('\0') is WEOF, not 0.
+    AC_CACHE_CHECK([whether btowc(0) is correct],
+      [gl_cv_func_btowc_nul],
+      [
+        AC_TRY_RUN([
+#include <stdio.h>
+#include <string.h>
+#include <wchar.h>
+int main ()
+{
+  if (btowc ('\0') != 0)
+    return 1;
+  return 0;
+}],
+          [gl_cv_func_btowc_nul=yes],
+          [gl_cv_func_btowc_nul=no],
+          [
+changequote(,)dnl
+           case "$host_os" in
+                      # Guess no on Cygwin.
+             cygwin*) gl_cv_func_btowc_nul="guessing no" ;;
+                      # Guess yes otherwise.
+             *)       gl_cv_func_btowc_nul="guessing yes" ;;
+           esac
+changequote([,])dnl
+          ])
+      ])
+
+    dnl IRIX 6.5 btowc(EOF) is 0xFF, not WEOF.
     AC_CACHE_CHECK([whether btowc(EOF) is correct],
       [gl_cv_func_btowc_eof],
       [
@@ -55,6 +84,11 @@
             [:])
         fi
       ])
+
+    case "$gl_cv_func_btowc_nul" in
+      *yes) ;;
+      *) REPLACE_BTOWC=1 ;;
+    esac
     case "$gl_cv_func_btowc_eof" in
       *yes) ;;
       *) REPLACE_BTOWC=1 ;;