lib/popen.c

   1 /* Open a stream to a sub-process.
   2    Copyright (C) 2009-2020 Free Software Foundation, Inc.
   3
   4    This program is free software: you can redistribute it and/or modify
   5    it under the terms of the GNU General Public License as published by
   6    the Free Software Foundation; either version 3 of the License, or
   7    (at your option) any later version.
   8
   9    This program is distributed in the hope that it will be useful,
  10    but WITHOUT ANY WARRANTY; without even the implied warranty of
  11    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  12    GNU General Public License for more details.
  13
  14    You should have received a copy of the GNU General Public License
  15    along with this program.  If not, see <https://www.gnu.org/licenses/>.  */
  16
  17 /* Written by Eric Blake <ebb9@byu.net>, 2009.  */
  18
  19 #include <config.h>
  20
  21 /* Specification.  */
  22 #include <stdio.h>
  23
  24 #if defined _WIN32 && ! defined __CYGWIN__
  25 /* Native Windows API.  */
  26
  27 # include <string.h>
  28
  29 FILE *
  30 popen (const char *filename, const char *mode)
  31 {
  32   /* Use binary mode by default.  */
  33   if (strcmp (mode, "r") == 0)
  34     mode = "rb";
  35   else if (strcmp (mode, "w") == 0)
  36     mode = "wb";
  37
  38   return _popen (filename, mode);
  39 }
  40
  41 #else
  42
  43 # include <errno.h>
  44 # include <fcntl.h>
  45 # include <stdlib.h>
  46 # include <unistd.h>
  47
  48 # undef popen
  49
  50 FILE *
  51 rpl_popen (const char *filename, const char *mode)
  52 {
  53   /* All other platforms have popen and fcntl.
  54      The bug of the child clobbering its own file descriptors if stdin
  55      or stdout was closed in the parent can be worked around by
  56      opening those two fds as close-on-exec to begin with.  */
  57   /* Cygwin 1.5.x also has a bug where the popen fd is improperly
  58      marked close-on-exec, and if the application undoes this, then
  59      the fd leaks into subsequent popen calls.  We could work around
  60      this by maintaining a list of all fd's opened by popen, and
  61      temporarily marking them cloexec around the real popen call, but
  62      we would also have to override pclose, and the bookkeeping seems
  63      extreme given that cygwin 1.7 no longer has the bug.  */
  64   FILE *result;
  65   int cloexec0 = fcntl (STDIN_FILENO, F_GETFD);
  66   int cloexec1 = fcntl (STDOUT_FILENO, F_GETFD);
  67   int saved_errno;
  68
  69   /* If either stdin or stdout was closed (that is, fcntl failed),
  70      then we open a dummy close-on-exec fd to occupy that slot.  That
  71      way, popen's internal use of pipe() will not contain either fd 0
  72      or 1, overcoming the fact that the child process blindly calls
  73      close() on the parent's end of the pipe without first checking
  74      whether it is clobbering the fd just placed there via dup2(); the
  75      exec will get rid of the dummy fd's in the child.  Fortunately,
  76      closed stderr in the parent does not cause problems in the
  77      child.  */
  78   if (cloexec0 < 0)
  79     {
  80       if (open ("/dev/null", O_RDONLY) != STDIN_FILENO
  81           || fcntl (STDIN_FILENO, F_SETFD,
  82                     fcntl (STDIN_FILENO, F_GETFD) | FD_CLOEXEC) == -1)
  83         abort ();
  84     }
  85   if (cloexec1 < 0)
  86     {
  87       if (open ("/dev/null", O_RDONLY) != STDOUT_FILENO
  88           || fcntl (STDOUT_FILENO, F_SETFD,
  89                     fcntl (STDOUT_FILENO, F_GETFD) | FD_CLOEXEC) == -1)
  90         abort ();
  91     }
  92   result = popen (filename, mode);
  93   /* Now, close any dummy fd's created in the parent.  */
  94   saved_errno = errno;
  95   if (cloexec0 < 0)
  96     close (STDIN_FILENO);
  97   if (cloexec1 < 0)
  98     close (STDOUT_FILENO);
  99   errno = saved_errno;
 100   return result;
 101 }
 102
 103 #endif