--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,10 @@
+2012-02-04  Bruno Haible  <bruno@clisp.org>
+
+	isatty: Fix test failure of ptsname_r on native Windows.
+	* lib/isatty.c (_isatty_nothrow): Upon exception, return 0, not -1,
+	and don't set errno.
+	(isatty): Test first whether fd is valid. Set errno when returning 0.
+
 2012-02-04  Bruno Haible  <bruno@clisp.org>
 
 	spawn-pipe tests: Fix a NULL program name in a diagnostic.

--- a/lib/isatty.c
+++ b/lib/isatty.c
@@ -48,8 +48,7 @@
     }
   CATCH_MSVC_INVAL
     {
-      result = -1;
-      errno = EBADF;
+      result = 0;
     }
   DONE_MSVC_INVAL;
 
@@ -59,15 +58,24 @@
 # define _isatty_nothrow _isatty
 #endif
 
+/* Determine whether FD refers to a console device.  Return 1 if yes.
+   Return 0 and set errno if no. (ptsname_r relies on the errno value.)  */
 int
 isatty (int fd)
 {
-  /* _isatty (fd) tests whether GetFileType of the handle is FILE_TYPE_CHAR.  */
+  HANDLE h = (HANDLE) _get_osfhandle (fd);
+  if (h == INVALID_HANDLE_VALUE)
+    {
+      errno = EBADF;
+      return 0;
+    }
+  /* _isatty (fd) tests whether GetFileType of the handle is FILE_TYPE_CHAR.
+     But it does not set errno when it returns 0.  */
   if (_isatty_nothrow (fd))
     {
-      HANDLE h = (HANDLE) _get_osfhandle (fd);
-      return IsConsoleHandle (h);
+      if (IsConsoleHandle (h))
+        return 1;
     }
-  else
-    return 0;
+  errno = ENOTTY;
+  return 0;
 }