--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,9 @@
+2009-08-23  Bruno Haible  <bruno@clisp.org>
+
+	Fix getdtablesize() on mingw.
+	* lib/getdtablesize.c (getdtablesize): Implement differently.
+	* lib/unistd.in.h (getdtablesize): Improve comment.
+
 2009-08-23  Bruno Haible  <bruno@clisp.org>
 
 	New module 'mkostemp'.

--- a/lib/getdtablesize.c
+++ b/lib/getdtablesize.c
@@ -1,5 +1,5 @@
 /* getdtablesize() function for platforms that don't have it.
-   Copyright (C) 2008 Free Software Foundation, Inc.
+   Copyright (C) 2008-2009 Free Software Foundation, Inc.
    Written by Bruno Haible <bruno@clisp.org>, 2008.
 
    This program is free software: you can redistribute it and/or modify
@@ -24,10 +24,40 @@
 
 #include <stdio.h>
 
+/* Cache for the previous getdtablesize () result.  */
+static int dtablesize;
+
 int
 getdtablesize (void)
 {
-  return _getmaxstdio ();
+  if (dtablesize == 0)
+    {
+      /* We are looking for the number N such that the valid file descriptors
+	 are 0..N-1.  It can be obtained through a loop as follows:
+	   {
+	     int fd;
+	     for (fd = 3; fd < 65536; fd++)
+	       if (dup2 (0, fd) == -1)
+	         break;
+	     return fd;
+	   }
+	 On Windows XP, the result is 2048.
+	 The drawback of this loop is that it allocates memory for a libc
+	 internal array that is never freed.
+
+	 The number N can also be obtained as the upper bound for
+	 _getmaxstdio ().  _getmaxstdio () returns the maximum number of open
+	 FILE objects.  The sanity check in _setmaxstdio reveals the maximum
+	 number of file descriptors.  This too allocates memory, but it is
+	 freed when we call _setmaxstdio with the original value.  */
+      int orig_max_stdio = _getmaxstdio ();
+      unsigned int bound;
+      for (bound = 0x10000; _setmaxstdio (bound) < 0; bound = bound / 2)
+	;
+      _setmaxstdio (orig_max_stdio);
+      dtablesize = bound;
+    }
+  return dtablesize;
 }
 
 #endif

--- a/lib/unistd.in.h
+++ b/lib/unistd.in.h
@@ -328,7 +328,8 @@
 
 #if @GNULIB_GETDTABLESIZE@
 # if !@HAVE_GETDTABLESIZE@
-/* Return the maximum number of file descriptors in the current process.  */
+/* Return the maximum number of file descriptors in the current process.
+   In POSIX, this is same as sysconf (_SC_OPEN_MAX).  */
 extern int getdtablesize (void);
 # endif
 #elif defined GNULIB_POSIXCHECK