u-tools/apps/tar/gnu/getdtablesize.c

   1 /* -*- buffer-read-only: t -*- vi: set ro: */
   2 /* DO NOT EDIT! GENERATED AUTOMATICALLY! */
   3 /* getdtablesize() function for platforms that don't have it.
   4    Copyright (C) 2008-2011 Free Software Foundation, Inc.
   5    Written by Bruno Haible <bruno@clisp.org>, 2008.
   6
   7    This program is free software: you can redistribute it and/or modify
   8    it under the terms of the GNU General Public License as published by
   9    the Free Software Foundation; either version 3 of the License, or
  10    (at your option) any later version.
  11
  12    This program is distributed in the hope that it will be useful,
  13    but WITHOUT ANY WARRANTY; without even the implied warranty of
  14    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  15    GNU General Public License for more details.
  16
  17    You should have received a copy of the GNU General Public License
  18    along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
  19
  20 #include <config.h>
  21
  22 /* Specification.  */
  23 #include <unistd.h>
  24
  25 #if (defined _WIN32 || defined __WIN32__) && ! defined __CYGWIN__
  26
  27 #include <stdio.h>
  28
  29 /* Cache for the previous getdtablesize () result.  */
  30 static int dtablesize;
  31
  32 int
  33 getdtablesize (void)
  34 {
  35   if (dtablesize == 0)
  36     {
  37       /* We are looking for the number N such that the valid file descriptors
  38          are 0..N-1.  It can be obtained through a loop as follows:
  39            {
  40              int fd;
  41              for (fd = 3; fd < 65536; fd++)
  42                if (dup2 (0, fd) == -1)
  43                  break;
  44              return fd;
  45            }
  46          On Windows XP, the result is 2048.
  47          The drawback of this loop is that it allocates memory for a libc
  48          internal array that is never freed.
  49
  50          The number N can also be obtained as the upper bound for
  51          _getmaxstdio ().  _getmaxstdio () returns the maximum number of open
  52          FILE objects.  The sanity check in _setmaxstdio reveals the maximum
  53          number of file descriptors.  This too allocates memory, but it is
  54          freed when we call _setmaxstdio with the original value.  */
  55       int orig_max_stdio = _getmaxstdio ();
  56       unsigned int bound;
  57       for (bound = 0x10000; _setmaxstdio (bound) < 0; bound = bound / 2)
  58         ;
  59       _setmaxstdio (orig_max_stdio);
  60       dtablesize = bound;
  61     }
  62   return dtablesize;
  63 }
  64
  65 #endif