lib/isapipe.c

   1 /* Test whether a file descriptor is a pipe.
   2
   3    Copyright (C) 2006, 2008-2025 Free Software Foundation, Inc.
   4
   5    This file is free software: you can redistribute it and/or modify
   6    it under the terms of the GNU Lesser General Public License as
   7    published by the Free Software Foundation, either version 3 of the
   8    License, or (at your option) any later version.
   9
  10    This file 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 Lesser General Public License for more details.
  14
  15    You should have received a copy of the GNU Lesser 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 <sys/types.h>
  57 # include <sys/stat.h>
  58 # include <unistd.h>
  59
  60 /* The maximum link count for pipes; (nlink_t) -1 if not known.  */
  61 # ifndef PIPE_LINK_COUNT_MAX
  62 #  define PIPE_LINK_COUNT_MAX ((nlink_t) (-1))
  63 # endif
  64
  65 /* Return 1 if FD is a pipe, 0 if not, -1 (setting errno) on error.
  66
  67    Test fairly strictly whether FD is a pipe.  lseek and checking for
  68    ESPIPE does not suffice, since many non-pipe files cause lseek to
  69    fail with errno == ESPIPE.  */
  70
  71 int
  72 isapipe (int fd)
  73 {
  74   nlink_t pipe_link_count_max = PIPE_LINK_COUNT_MAX;
  75   bool check_for_fifo = (HAVE_FIFO_PIPES == 1);
  76   struct stat st;
  77   int fstat_result = fstat (fd, &st);
  78
  79   if (fstat_result != 0)
  80     return fstat_result;
  81
  82   /* We want something that succeeds only for pipes, but on
  83      POSIX-conforming hosts S_ISFIFO succeeds for both FIFOs and pipes
  84      and we know of no portable, reliable way to distinguish them in
  85      general.  However, in practice pipes always have a link count <=
  86      PIPE_LINK_COUNT_MAX (unless someone attaches them to the file
  87      system name space using fattach, in which case they're not really
  88      pipes any more), so test for that as well.
  89
  90      On Darwin 7.7, pipes are sockets, so check for those instead.  */
  91
  92   if (! ((HAVE_FIFO_PIPES == 0 || HAVE_FIFO_PIPES == 1)
  93          && PIPE_LINK_COUNT_MAX != (nlink_t) -1)
  94       && (S_ISFIFO (st.st_mode) | S_ISSOCK (st.st_mode)))
  95     {
  96       int fd_pair[2];
  97       int pipe_result = pipe (fd_pair);
  98       if (pipe_result != 0)
  99         return pipe_result;
 100       else
 101         {
 102           struct stat pipe_st;
 103           int fstat_pipe_result = fstat (fd_pair[0], &pipe_st);
 104           int fstat_pipe_errno = errno;
 105           close (fd_pair[0]);
 106           close (fd_pair[1]);
 107           if (fstat_pipe_result != 0)
 108             {
 109               errno = fstat_pipe_errno;
 110               return fstat_pipe_result;
 111             }
 112           check_for_fifo = (S_ISFIFO (pipe_st.st_mode) != 0);
 113           pipe_link_count_max = pipe_st.st_nlink;
 114         }
 115     }
 116
 117   return
 118     (st.st_nlink <= pipe_link_count_max
 119      && (check_for_fifo ? S_ISFIFO (st.st_mode) : S_ISSOCK (st.st_mode)));
 120 }
 121
 122 #endif