gl/getdtablesize.c

   1 /* getdtablesize() function for platforms that don't have it.
   2    Copyright (C) 2008-2010 Free Software Foundation, Inc.
   3    Written by Bruno Haible <bruno@clisp.org>, 2008.
   4
   5    This program is free software: you can redistribute it and/or modify
   6    it under the terms of the GNU General Public License as published by
   7    the Free Software Foundation; either version 3 of the License, or
   8    (at your option) any later version.
   9
  10    This program is distributed in the hope that it will be useful,
  11    but WITHOUT ANY WARRANTY; without even the implied warranty of
  12    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  13    GNU General Public License for more details.
  14
  15    You should have received a copy of the GNU General Public License
  16    along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
  17
  18 #include <config.h>
  19
  20 /* Specification.  */
  21 #include <unistd.h>
  22
  23 #if (defined _WIN32 || defined __WIN32__) && ! defined __CYGWIN__
  24
  25 #include <stdio.h>
  26
  27 /* Cache for the previous getdtablesize () result.  */
  28 static int dtablesize;
  29
  30 int
  31 getdtablesize (void)
  32 {
  33   if (dtablesize == 0)
  34     {
  35       /* We are looking for the number N such that the valid file descriptors
  36          are 0..N-1.  It can be obtained through a loop as follows:
  37            {
  38              int fd;
  39              for (fd = 3; fd < 65536; fd++)
  40                if (dup2 (0, fd) == -1)
  41                  break;
  42              return fd;
  43            }
  44          On Windows XP, the result is 2048.
  45          The drawback of this loop is that it allocates memory for a libc
  46          internal array that is never freed.
  47
  48          The number N can also be obtained as the upper bound for
  49          _getmaxstdio ().  _getmaxstdio () returns the maximum number of open
  50          FILE objects.  The sanity check in _setmaxstdio reveals the maximum
  51          number of file descriptors.  This too allocates memory, but it is
  52          freed when we call _setmaxstdio with the original value.  */
  53       int orig_max_stdio = _getmaxstdio ();
  54       unsigned int bound;
  55       for (bound = 0x10000; _setmaxstdio (bound) < 0; bound = bound / 2)
  56         ;
  57       _setmaxstdio (orig_max_stdio);
  58       dtablesize = bound;
  59     }
  60   return dtablesize;
  61 }
  62
  63 #endif