1  /* stdopen.c - ensure that the three standard file descriptors are in use
       2  
       3     Copyright (C) 2005-2006, 2019-2023 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  }