lib/getdtablesize.c

   1 /* getdtablesize() function for platforms that don't have it.
   2    Copyright (C) 2008-2013 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 #include "msvc-inval.h"
  28
  29 #if HAVE_MSVC_INVALID_PARAMETER_HANDLER
  30 static int
  31 _setmaxstdio_nothrow (int newmax)
  32 {
  33   int result;
  34
  35   TRY_MSVC_INVAL
  36     {
  37       result = _setmaxstdio (newmax);
  38     }
  39   CATCH_MSVC_INVAL
  40     {
  41       result = -1;
  42     }
  43   DONE_MSVC_INVAL;
  44
  45   return result;
  46 }
  47 # define _setmaxstdio _setmaxstdio_nothrow
  48 #endif
  49
  50 /* Cache for the previous getdtablesize () result.  */
  51 static int dtablesize;
  52
  53 int
  54 getdtablesize (void)
  55 {
  56   if (dtablesize == 0)
  57     {
  58       /* We are looking for the number N such that the valid file descriptors
  59          are 0..N-1.  It can be obtained through a loop as follows:
  60            {
  61              int fd;
  62              for (fd = 3; fd < 65536; fd++)
  63                if (dup2 (0, fd) == -1)
  64                  break;
  65              return fd;
  66            }
  67          On Windows XP, the result is 2048.
  68          The drawback of this loop is that it allocates memory for a libc
  69          internal array that is never freed.
  70
  71          The number N can also be obtained as the upper bound for
  72          _getmaxstdio ().  _getmaxstdio () returns the maximum number of open
  73          FILE objects.  The sanity check in _setmaxstdio reveals the maximum
  74          number of file descriptors.  This too allocates memory, but it is
  75          freed when we call _setmaxstdio with the original value.  */
  76       int orig_max_stdio = _getmaxstdio ();
  77       unsigned int bound;
  78       for (bound = 0x10000; _setmaxstdio (bound) < 0; bound = bound / 2)
  79         ;
  80       _setmaxstdio (orig_max_stdio);
  81       dtablesize = bound;
  82     }
  83   return dtablesize;
  84 }
  85
  86 #endif