external/gpl2/xcvs/dist/lib/sunos57-select.c

   1 /* Work around the bug in Solaris 7 whereby a fd that is opened on
   2    /dev/null will cause select/poll to hang when given a NULL timeout.
   3
   4    Copyright (C) 2004 Free Software Foundation, Inc.
   5
   6    This program is free software; you can redistribute it and/or modify
   7    it under the terms of the GNU General Public License as published by
   8    the Free Software Foundation; either version 2, or (at your option)
   9    any later version.
  10
  11    This program is distributed in the hope that it will be useful,
  12    but WITHOUT ANY WARRANTY; without even the implied warranty of
  13    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  14    GNU General Public License for more details.
  15
  16    You should have received a copy of the GNU General Public License
  17    along with this program; if not, write to the Free Software Foundation,
  18    Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.  */
  19
  20 /* written by Mark D. Baushke */
  21
  22 /*
  23  * Observed on Solaris 7:
  24  *   If /dev/null is in the readfds set, it will never be marked as
  25  *   ready by the OS. In the case of a /dev/null fd being the only fd
  26  *   in the select set and timeout == NULL, the select will hang.
  27  *   If /dev/null is in the exceptfds set, it will not be set on
  28  *   return from select().
  29  */
  30 #ifdef HAVE_CONFIG_H
  31 # include <config.h>
  32 #endif /* HAVE_CONFIG_H */
  33
  34 /* The rpl_select function calls the real select. */
  35 #undef select
  36
  37 #include <stdbool.h>
  38 #include <stdio.h>
  39 #include <sys/types.h>
  40 #include <sys/stat.h>
  41 #include <errno.h>
  42
  43 #ifdef HAVE_UNISTD_H
  44 # include <unistd.h>
  45 #endif /* HAVE_UNISTD_H */
  46
  47 #include "minmax.h"
  48 #include "xtime.h"
  49
  50 static struct stat devnull;
  51 static int devnull_set = -1;
  52 int
  53 rpl_select (int nfds, fd_set *readfds, fd_set *writefds, fd_set *exceptfds,
  54             struct timeval *timeout)
  55 {
  56   int ret = 0;
  57
  58   /* Argument checking */
  59   if (nfds < 1 || nfds > FD_SETSIZE)
  60     {
  61       errno = EINVAL;
  62       return -1;
  63     }
  64
  65   /* Perform the initial stat on /dev/null */
  66   if (devnull_set == -1)
  67     devnull_set = stat ("/dev/null", &devnull);
  68
  69   if (devnull_set >= 0)
  70     {
  71       int fd;
  72       int maxfd = -1;
  73       fd_set null_rfds, null_wfds;
  74       bool altered = false;     /* Whether we have altered the caller's args.
  75                                  */
  76
  77       FD_ZERO (&null_rfds);
  78       FD_ZERO (&null_wfds);
  79
  80       for (fd = 0; fd < nfds; fd++)
  81         {
  82           /* Check the callers bits for interesting fds */
  83           bool isread = (readfds && FD_ISSET (fd, readfds));
  84           bool isexcept = (exceptfds && FD_ISSET (fd, exceptfds));
  85           bool iswrite = (writefds && FD_ISSET (fd, writefds));
  86
  87           /* Check interesting fds against /dev/null */
  88           if (isread || iswrite || isexcept)
  89             {
  90               struct stat sb;
  91
  92               /* Equivalent to /dev/null ? */
  93               if (fstat (fd, &sb) >= 0
  94                   && sb.st_dev == devnull.st_dev
  95                   && sb.st_ino == devnull.st_ino
  96                   && sb.st_mode == devnull.st_mode
  97                   && sb.st_uid == devnull.st_uid
  98                   && sb.st_gid == devnull.st_gid
  99                   && sb.st_size == devnull.st_size
 100                   && sb.st_blocks == devnull.st_blocks
 101                   && sb.st_blksize == devnull.st_blksize)
 102                 {
 103                   /* Save the interesting bits for later use. */
 104                   if (isread)
 105                     {
 106                       FD_SET (fd, &null_rfds);
 107                       FD_CLR (fd, readfds);
 108                       altered = true;
 109                     }
 110                   if (isexcept)
 111                     /* Pass exception bits through.
 112                      *
 113                      * At the moment, we only know that this bug
 114                      * exists in Solaris 7 and so this file should
 115                      * only be compiled on Solaris 7. Since Solaris 7
 116                      * never returns ready for exceptions on
 117                      * /dev/null, we probably could assume this too,
 118                      * but since Solaris 9 is known to always return
 119                      * ready for exceptions on /dev/null, pass this
 120                      * through in case any other systems turn out to
 121                      * do the same. Besides, this will cause the
 122                      * timeout to be processed as it would have been
 123                      * otherwise.
 124                      */
 125                     maxfd = MAX (maxfd, fd);
 126                   if (iswrite)
 127                     {
 128                       /* We know of no bugs involving selecting /dev/null
 129                        * writefds, but we also know that /dev/null is always
 130                        * ready for write.  Therefore, since we have already
 131                        * performed all the necessary processing, avoid calling
 132                        * the system select for this case.
 133                        */
 134                       FD_SET (fd, &null_wfds);
 135                       FD_CLR (fd, writefds);
 136                       altered = true;
 137                     }
 138                 }
 139               else
 140                 /* A non-/dev/null fd is present. */
 141                 maxfd = MAX (maxfd, fd);
 142             }
 143         }
 144
 145       if (maxfd >= 0)
 146         {
 147           /* we need to call select, one way or another.  */
 148           if (altered)
 149             {
 150               /* We already have some ready bits set, so timeout immediately
 151                * if no bits are set.
 152                */
 153               struct timeval ztime;
 154               ztime.tv_sec = 0;
 155               ztime.tv_usec = 0;
 156               ret = select (maxfd + 1, readfds, writefds, exceptfds, &ztime);
 157               if (ret == 0)
 158                 {
 159                   /* Timeout.  Zero the sets since the system select might
 160                    * not have.
 161                    */
 162                   if (readfds)
 163                     FD_ZERO (readfds);
 164                   if (exceptfds)
 165                     FD_ZERO (exceptfds);
 166                   if (writefds)
 167                     FD_ZERO (writefds);
 168                 }
 169             }
 170           else
 171             /* No /dev/null fds.  Call select just as the user specified.  */
 172             ret = select (maxfd + 1, readfds, writefds, exceptfds, timeout);
 173         }
 174
 175       /*
 176        * Borrowed from the Solaris 7 man page for select(3c):
 177        *
 178        *   On successful completion, the objects pointed to by the
 179        *   readfds, writefds, and exceptfds arguments are modified to
 180        *   indicate which file descriptors are ready for reading,
 181        *   ready for writing, or have an error condition pending,
 182        *   respectively. For each file descriptor less than nfds, the
 183        *   corresponding bit will be set on successful completion if
 184        *   it was set on input and the associated condition is true
 185        *   for that file descriptor.
 186        *
 187        *   On failure, the objects pointed to by the readfds,
 188        *   writefds, and exceptfds arguments are not modified. If the
 189        *   timeout interval expires without the specified condition
 190        *   being true for any of the specified file descriptors, the
 191        *   objects pointed to by the readfs, writefs, and errorfds
 192        *   arguments have all bits set to 0.
 193        *
 194        *   On successful completion, select() returns the total number
 195        *   of bits set in the bit masks. Otherwise, -1 is returned,
 196        *   and errno is set to indicate the error.
 197        */
 198
 199       /* Fix up the fd sets for any changes we may have made. */
 200       if (altered)
 201         {
 202           /* Tell the caller that nothing is blocking the /dev/null fds */
 203           for (fd = 0; fd < nfds; fd++)
 204             {
 205               /* If ret < 0, then we still need to restore the fd sets.  */
 206               if (FD_ISSET (fd, &null_rfds))
 207                 {
 208                   FD_SET (fd, readfds);
 209                   if (ret >= 0)
 210                     ret++;
 211                 }
 212               if (FD_ISSET (fd, &null_wfds))
 213                 {
 214                   FD_SET (fd, writefds);
 215                   if (ret >= 0)
 216                     ret++;
 217                 }
 218             }
 219         }
 220     }
 221   else
 222     ret = select (nfds, readfds, writefds, exceptfds, timeout);
 223
 224   return ret;
 225 }