source3/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 2 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, write to the Free Software
  19    Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
  20 */
  21
  22 #include "includes.h"
  23
  24 /* This is here because it allows us to avoid a nasty race in signal handling.
  25    We need to guarantee that when we get a signal we get out of a select immediately
  26    but doing that involves a race condition. We can avoid the race by getting the
  27    signal handler to write to a pipe that is in the select/poll list
  28
  29    This means all Samba signal handlers should call sys_select_signal().
  30 */
  31
  32 static pid_t initialised;
  33 static int select_pipe[2];
  34 static VOLATILE unsigned pipe_written, pipe_read;
  35
  36 /*******************************************************************
  37  Call this from all Samba signal handlers if you want to avoid a
  38  nasty signal race condition.
  39 ********************************************************************/
  40
  41 void sys_select_signal(void)
  42 {
  43         char c = 1;
  44         if (!initialised) return;
  45
  46         if (pipe_written > pipe_read+256) return;
  47
  48         if (write(select_pipe[1], &c, 1) == 1) pipe_written++;
  49 }
  50
  51 /*******************************************************************
  52  Like select() but avoids the signal race using a pipe
  53  it also guuarantees that fds on return only ever contains bits set
  54  for file descriptors that were readable.
  55 ********************************************************************/
  56
  57 int sys_select(int maxfd, fd_set *readfds, fd_set *writefds, fd_set *errorfds, struct timeval *tval)
  58 {
  59         int ret, saved_errno;
  60         fd_set *readfds2, readfds_buf;
  61
  62         if (initialised != sys_getpid()) {
  63                 pipe(select_pipe);
  64
  65                 /*
  66                  * These next two lines seem to fix a bug with the Linux
  67                  * 2.0.x kernel (and probably other UNIXes as well) where
  68                  * the one byte read below can block even though the
  69                  * select returned that there is data in the pipe and
  70                  * the pipe_written variable was incremented. Thanks to
  71                  * HP for finding this one. JRA.
  72                  */
  73
  74                 if(set_blocking(select_pipe[0],0)==-1)
  75                         smb_panic("select_pipe[0]: O_NONBLOCK failed.\n");
  76                 if(set_blocking(select_pipe[1],0)==-1)
  77                         smb_panic("select_pipe[1]: O_NONBLOCK failed.\n");
  78
  79                 initialised = sys_getpid();
  80         }
  81
  82         maxfd = MAX(select_pipe[0]+1, maxfd);
  83
  84         /* If readfds is NULL we need to provide our own set. */
  85         if (readfds) {
  86                 readfds2 = readfds;
  87         } else {
  88                 readfds2 = &readfds_buf;
  89                 FD_ZERO(readfds2);
  90         }
  91         FD_SET(select_pipe[0], readfds2);
  92
  93         errno = 0;
  94         ret = select(maxfd,readfds2,writefds,errorfds,tval);
  95
  96         if (ret <= 0) {
  97                 FD_ZERO(readfds2);
  98                 if (writefds)
  99                         FD_ZERO(writefds);
 100                 if (errorfds)
 101                         FD_ZERO(errorfds);
 102         }
 103
 104         if (FD_ISSET(select_pipe[0], readfds2)) {
 105                 char c;
 106                 saved_errno = errno;
 107                 if (read(select_pipe[0], &c, 1) == 1) {
 108                         pipe_read++;
 109                 }
 110                 errno = saved_errno;
 111                 FD_CLR(select_pipe[0], readfds2);
 112                 ret--;
 113                 if (ret == 0) {
 114                         ret = -1;
 115                         errno = EINTR;
 116                 }
 117         }
 118
 119         return ret;
 120 }
 121
 122 /*******************************************************************
 123  Similar to sys_select() but catch EINTR and continue.
 124  This is what sys_select() used to do in Samba.
 125 ********************************************************************/
 126
 127 int sys_select_intr(int maxfd, fd_set *readfds, fd_set *writefds, fd_set *errorfds, struct timeval *tval)
 128 {
 129         int ret;
 130         fd_set *readfds2, readfds_buf, *writefds2, writefds_buf, *errorfds2, errorfds_buf;
 131         struct timeval tval2, *ptval;
 132
 133         readfds2 = (readfds ? &readfds_buf : NULL);
 134         writefds2 = (writefds ? &writefds_buf : NULL);
 135         errorfds2 = (errorfds ? &errorfds_buf : NULL);
 136         ptval = (tval ? &tval2 : NULL);
 137
 138         do {
 139                 if (readfds)
 140                         readfds_buf = *readfds;
 141                 if (writefds)
 142                         writefds_buf = *writefds;
 143                 if (errorfds)
 144                         errorfds_buf = *errorfds;
 145                 if (tval)
 146                         tval2 = *tval;
 147
 148                 ret = sys_select(maxfd, readfds2, writefds2, errorfds2, ptval);
 149         } while (ret == -1 && errno == EINTR);
 150
 151         if (readfds)
 152                 *readfds = readfds_buf;
 153         if (writefds)
 154                 *writefds = writefds_buf;
 155         if (errorfds)
 156                 *errorfds = errorfds_buf;
 157
 158         return ret;
 159 }