source/lib/select.c

   1 /*
   2    Unix SMB/Netbios implementation.
   3    Version 3.0
   4    Samba select/poll implementation
   5    Copyright (C) Andrew Tridgell 1992-1998
   6
   7    This program is free software; you can redistribute it and/or modify
   8    it under the terms of the GNU General Public License as published by
   9    the Free Software Foundation; either version 3 of the License, or
  10    (at your option) any later version.
  11
  12    This program is distributed in the hope that it will be useful,
  13    but WITHOUT ANY WARRANTY; without even the implied warranty of
  14    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  15    GNU General Public License for more details.
  16
  17    You should have received a copy of the GNU General Public License
  18    along with this program.  If not, see <http://www.gnu.org/licenses/>.
  19 */
  20
  21 #include "includes.h"
  22
  23 /* This is here because it allows us to avoid a nasty race in signal handling.
  24    We need to guarantee that when we get a signal we get out of a select immediately
  25    but doing that involves a race condition. We can avoid the race by getting the
  26    signal handler to write to a pipe that is in the select/poll list
  27
  28    This means all Samba signal handlers should call sys_select_signal().
  29 */
  30
  31 static pid_t initialised;
  32 static int select_pipe[2];
  33 static VOLATILE unsigned pipe_written, pipe_read;
  34
  35 /*******************************************************************
  36  Call this from all Samba signal handlers if you want to avoid a
  37  nasty signal race condition.
  38 ********************************************************************/
  39
  40 void sys_select_signal(char c)
  41 {
  42         if (!initialised) return;
  43
  44         if (pipe_written > pipe_read+256) return;
  45
  46         if (write(select_pipe[1], &c, 1) == 1) pipe_written++;
  47 }
  48
  49 /*******************************************************************
  50  Like select() but avoids the signal race using a pipe
  51  it also guuarantees that fds on return only ever contains bits set
  52  for file descriptors that were readable.
  53 ********************************************************************/
  54
  55 int sys_select(int maxfd, fd_set *readfds, fd_set *writefds, fd_set *errorfds, struct timeval *tval)
  56 {
  57         int ret, saved_errno;
  58         fd_set *readfds2, readfds_buf;
  59
  60         if (initialised != sys_getpid()) {
  61                 if (pipe(select_pipe) == -1)
  62                         smb_panic("Could not create select pipe");
  63
  64                 /*
  65                  * These next two lines seem to fix a bug with the Linux
  66                  * 2.0.x kernel (and probably other UNIXes as well) where
  67                  * the one byte read below can block even though the
  68                  * select returned that there is data in the pipe and
  69                  * the pipe_written variable was incremented. Thanks to
  70                  * HP for finding this one. JRA.
  71                  */
  72
  73                 if(set_blocking(select_pipe[0],0)==-1)
  74                         smb_panic("select_pipe[0]: O_NONBLOCK failed");
  75                 if(set_blocking(select_pipe[1],0)==-1)
  76                         smb_panic("select_pipe[1]: O_NONBLOCK failed");
  77
  78                 initialised = sys_getpid();
  79         }
  80
  81         maxfd = MAX(select_pipe[0]+1, maxfd);
  82
  83         /* If readfds is NULL we need to provide our own set. */
  84         if (readfds) {
  85                 readfds2 = readfds;
  86         } else {
  87                 readfds2 = &readfds_buf;
  88                 FD_ZERO(readfds2);
  89         }
  90         FD_SET(select_pipe[0], readfds2);
  91
  92         errno = 0;
  93         ret = select(maxfd,readfds2,writefds,errorfds,tval);
  94
  95         if (ret <= 0) {
  96                 FD_ZERO(readfds2);
  97                 if (writefds)
  98                         FD_ZERO(writefds);
  99                 if (errorfds)
 100                         FD_ZERO(errorfds);
 101         } else if (FD_ISSET(select_pipe[0], readfds2)) {
 102                 char c;
 103                 saved_errno = errno;
 104                 if (read(select_pipe[0], &c, 1) == 1) {
 105                         pipe_read++;
 106                         /* Mark Weaver <mark-clist@npsl.co.uk> pointed out a critical
 107                            fix to ensure we don't lose signals. We must always
 108                            return -1 when the select pipe is set, otherwise if another
 109                            fd is also ready (so ret == 2) then we used to eat the
 110                            byte in the pipe and lose the signal. JRA.
 111                         */
 112                         ret = -1;
 113 #if 0
 114                         /* JRA - we can use this to debug the signal messaging... */
 115                         DEBUG(0,("select got %u signal\n", (unsigned int)c));
 116 #endif
 117                         errno = EINTR;
 118                 } else {
 119                         FD_CLR(select_pipe[0], readfds2);
 120                         ret--;
 121                         errno = saved_errno;
 122                 }
 123         }
 124
 125         return ret;
 126 }
 127
 128 /*******************************************************************
 129  Similar to sys_select() but catch EINTR and continue.
 130  This is what sys_select() used to do in Samba.
 131 ********************************************************************/
 132
 133 int sys_select_intr(int maxfd, fd_set *readfds, fd_set *writefds, fd_set *errorfds, struct timeval *tval)
 134 {
 135         int ret;
 136         fd_set *readfds2, readfds_buf, *writefds2, writefds_buf, *errorfds2, errorfds_buf;
 137         struct timeval tval2, *ptval, end_time;
 138
 139         readfds2 = (readfds ? &readfds_buf : NULL);
 140         writefds2 = (writefds ? &writefds_buf : NULL);
 141         errorfds2 = (errorfds ? &errorfds_buf : NULL);
 142         if (tval) {
 143                 GetTimeOfDay(&end_time);
 144                 end_time.tv_sec += tval->tv_sec;
 145                 end_time.tv_usec += tval->tv_usec;
 146                 end_time.tv_sec += end_time.tv_usec / 1000000;
 147                 end_time.tv_usec %= 1000000;
 148                 errno = 0;
 149                 tval2 = *tval;
 150                 ptval = &tval2;
 151         } else {
 152                 ptval = NULL;
 153         }
 154
 155         do {
 156                 if (readfds)
 157                         readfds_buf = *readfds;
 158                 if (writefds)
 159                         writefds_buf = *writefds;
 160                 if (errorfds)
 161                         errorfds_buf = *errorfds;
 162                 if (ptval && (errno == EINTR)) {
 163                         struct timeval now_time;
 164                         SMB_BIG_INT tdif;
 165
 166                         GetTimeOfDay(&now_time);
 167                         tdif = usec_time_diff(&end_time, &now_time);
 168                         if (tdif <= 0) {
 169                                 ret = 0; /* time expired. */
 170                                 break;
 171                         }
 172                         ptval->tv_sec = tdif / 1000000;
 173                         ptval->tv_usec = tdif % 1000000;
 174                 }
 175
 176                 /* We must use select and not sys_select here. If we use
 177                    sys_select we'd lose the fact a signal occurred when sys_select
 178                    read a byte from the pipe. Fix from Mark Weaver
 179                    <mark-clist@npsl.co.uk>
 180                 */
 181                 ret = select(maxfd, readfds2, writefds2, errorfds2, ptval);
 182         } while (ret == -1 && errno == EINTR);
 183
 184         if (readfds)
 185                 *readfds = readfds_buf;
 186         if (writefds)
 187                 *writefds = writefds_buf;
 188         if (errorfds)
 189                 *errorfds = errorfds_buf;
 190
 191         return ret;
 192 }