lib/isapipe.c

   1 /* Test whether a file descriptor is a pipe.
   2
   3    Copyright (C) 2006, 2008-2020 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.  */
  19
  20 #include <config.h>
  21
  22 #include "isapipe.h"
  23
  24 #include <errno.h>
  25
  26 #if defined _WIN32 && ! defined __CYGWIN__
  27 /* Windows platforms.  */
  28
  29 /* Get GetFileType.  */
  30 # include <windows.h>
  31
  32 /* Get _get_osfhandle.  */
  33 # if GNULIB_MSVC_NOTHROW
  34 #  include "msvc-nothrow.h"
  35 # else
  36 #  include <io.h>
  37 # endif
  38
  39 int
  40 isapipe (int fd)
  41 {
  42   HANDLE h = (HANDLE) _get_osfhandle (fd);
  43
  44   if (h == INVALID_HANDLE_VALUE)
  45     {
  46       errno = EBADF;
  47       return -1;
  48     }
  49
  50   return (GetFileType (h) == FILE_TYPE_PIPE);
  51 }
  52
  53 #else
  54 /* Unix platforms.  */
  55
  56 # include <stdbool.h>
  57 # include <sys/types.h>
  58 # include <sys/stat.h>
  59 # include <unistd.h>
  60
  61 /* The maximum link count for pipes; (nlink_t) -1 if not known.  */
  62 # ifndef PIPE_LINK_COUNT_MAX
  63 #  define PIPE_LINK_COUNT_MAX ((nlink_t) (-1))
  64 # endif
  65
  66 /* Return 1 if FD is a pipe, 0 if not, -1 (setting errno) on error.
  67
  68    Test fairly strictly whether FD is a pipe.  lseek and checking for
  69    ESPIPE does not suffice, since many non-pipe files cause lseek to
  70    fail with errno == ESPIPE.  */
  71
  72 int
  73 isapipe (int fd)
  74 {
  75   nlink_t pipe_link_count_max = PIPE_LINK_COUNT_MAX;
  76   bool check_for_fifo = (HAVE_FIFO_PIPES == 1);
  77   struct stat st;
  78   int fstat_result = fstat (fd, &st);
  79
  80   if (fstat_result != 0)
  81     return fstat_result;
  82
  83   /* We want something that succeeds only for pipes, but on
  84      POSIX-conforming hosts S_ISFIFO succeeds for both FIFOs and pipes
  85      and we know of no portable, reliable way to distinguish them in
  86      general.  However, in practice pipes always have a link count <=
  87      PIPE_LINK_COUNT_MAX (unless someone attaches them to the file
  88      system name space using fattach, in which case they're not really
  89      pipes any more), so test for that as well.
  90
  91      On Darwin 7.7, pipes are sockets, so check for those instead.  */
  92
  93   if (! ((HAVE_FIFO_PIPES == 0 || HAVE_FIFO_PIPES == 1)
  94          && PIPE_LINK_COUNT_MAX != (nlink_t) -1)
  95       && (S_ISFIFO (st.st_mode) | S_ISSOCK (st.st_mode)))
  96     {
  97       int fd_pair[2];
  98       int pipe_result = pipe (fd_pair);
  99       if (pipe_result != 0)
 100         return pipe_result;
 101       else
 102         {
 103           struct stat pipe_st;
 104           int fstat_pipe_result = fstat (fd_pair[0], &pipe_st);
 105           int fstat_pipe_errno = errno;
 106           close (fd_pair[0]);
 107           close (fd_pair[1]);
 108           if (fstat_pipe_result != 0)
 109             {
 110               errno = fstat_pipe_errno;
 111               return fstat_pipe_result;
 112             }
 113           check_for_fifo = (S_ISFIFO (pipe_st.st_mode) != 0);
 114           pipe_link_count_max = pipe_st.st_nlink;
 115         }
 116     }
 117
 118   return
 119     (st.st_nlink <= pipe_link_count_max
 120      && (check_for_fifo ? S_ISFIFO (st.st_mode) : S_ISSOCK (st.st_mode)));
 121 }
 122
 123 #endif