fix getdtablesize() on mingw

[Bruno Haible]

On mingw, getdtablesize() currently returns 512 but the dup2 test shows that
file descriptors up until 2047 can be used. (On Windows XP.)

This fixes getdtablesize() so that it returns 2048.


2009-08-23  Bruno Haible  <address@hidden>

        Fix getdtablesize() on mingw.
        * lib/getdtablesize.c (getdtablesize): Implement differently.
        * lib/unistd.in.h (getdtablesize): Improve comment.

--- lib/getdtablesize.c.orig    2009-08-23 21:52:22.000000000 +0200
+++ lib/getdtablesize.c 2009-08-23 21:47:39.000000000 +0200
@@ -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 <address@hidden>, 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
--- lib/unistd.in.h.orig        2009-08-23 21:52:23.000000000 +0200
+++ lib/unistd.in.h     2009-08-23 21:51:22.000000000 +0200
@@ -328,7 +328,8 @@
 
 #if @GNULIB_GETDTABLESIZE@
 # if address@hidden@
-/* 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