lib/stdopen.c

   1 /* stdopen.c - ensure that the three standard file descriptors are in use
   2
   3    Copyright (C) 2005-2006, 2019-2024 Free Software Foundation, Inc.
   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 <https://www.gnu.org/licenses/>.  */
  17
  18 /* Written by Paul Eggert and Jim Meyering.  */
  19
  20 #include <config.h>
  21
  22 #include "stdopen.h"
  23
  24 #include <sys/stat.h>
  25 #include <fcntl.h>
  26 #include <unistd.h>
  27 #include <errno.h>
  28
  29 /* Try to ensure that all of the standard file numbers (0, 1, 2)
  30    are in use.  Without this, each application would have to guard
  31    every call to open, dup, fopen, etc. with tests to ensure they
  32    don't use one of the special file numbers when opening a file.
  33    Return zero if successful, an errno value if at least one of
  34    the file descriptors is initially closed and could not be opened.  */
  35
  36 int
  37 stdopen (void)
  38 {
  39   int fd;
  40   for (fd = STDIN_FILENO; fd <= STDERR_FILENO; fd++)
  41     {
  42       if (fcntl (fd, F_GETFD) < 0)
  43         {
  44           /* Open /dev/null with the contrary mode so that the typical
  45              read (stdin) or write (stdout, stderr) operation will fail.
  46              With descriptor 0, we can do even better on systems that
  47              have /dev/full, by opening that write-only instead of
  48              /dev/null.  The only drawback is that a write-provoked
  49              failure comes with a misleading errno value, ENOSPC.  */
  50           int mode = fd == STDIN_FILENO ? O_WRONLY : O_RDONLY;
  51           int full_fd = fd == STDIN_FILENO ? open ("/dev/full", mode) : -1;
  52           int new_fd = full_fd < 0 ? open ("/dev/null", mode) : full_fd;
  53           if (new_fd < 0)
  54             return errno;
  55           if (STDERR_FILENO < new_fd)
  56             {
  57               /* 0, 1, and 2 are already open somehow.
  58                  Our is not to reason why.  */
  59               close (new_fd);
  60               return 0;
  61             }
  62         }
  63     }
  64
  65   return 0;
  66 }