Fix getdtablesize() on mingw.

[gnulib.git] / lib / getdtablesize.c

diff --git a/lib/getdtablesize.c b/lib/getdtablesize.c
index a348cdd..5be1e83 100644 (file)
--- 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