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 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 static pid_t initialised;
  32 static int select_pipe[2];
  33 static VOLATILE unsigned pipe_written, pipe_read;
  34
  35
  36 /*******************************************************************
  37 call this from all Samba signal handlers if you want to avoid a
  38 nasty signal race condition
  39 ********************************************************************/
  40 void sys_select_signal(void)
  41 {
  42         char c = 1;
  43         if (!initialised) return;
  44
  45         if (pipe_written > pipe_read+256) return;
  46
  47         if (write(select_pipe[1], &c, 1) == 1) pipe_written++;
  48 }
  49
  50 /*******************************************************************
  51 like select() but avoids the signal race using a pipe
  52 it also guuarantees that fds on return only ever contains bits set
  53 for file descriptors that were readable
  54 ********************************************************************/
  55 int sys_select(int maxfd, fd_set *fds,struct timeval *tval)
  56 {
  57         int ret, saved_errno;
  58
  59         if (initialised != sys_getpid()) {
  60                 pipe(select_pipe);
  61
  62                 /*
  63                  * These next two lines seem to fix a bug with the Linux
  64                  * 2.0.x kernel (and probably other UNIXes as well) where
  65                  * the one byte read below can block even though the
  66                  * select returned that there is data in the pipe and
  67                  * the pipe_written variable was incremented. Thanks to
  68                  * HP for finding this one. JRA.
  69                  */
  70
  71                 if(set_blocking(select_pipe[0],0)==-1)
  72                         smb_panic("select_pipe[0]: O_NONBLOCK failed.\n");
  73                 if(set_blocking(select_pipe[1],0)==-1)
  74                         smb_panic("select_pipe[1]: O_NONBLOCK failed.\n");
  75
  76                 initialised = sys_getpid();
  77         }
  78
  79         maxfd = MAX(select_pipe[0]+1, maxfd);
  80         FD_SET(select_pipe[0], fds);
  81         errno = 0;
  82         ret = select(maxfd,fds,NULL,NULL,tval);
  83
  84         if (ret <= 0) {
  85                 FD_ZERO(fds);
  86         }
  87
  88         if (FD_ISSET(select_pipe[0], fds)) {
  89                 FD_CLR(select_pipe[0], fds);
  90                 ret--;
  91                 if (ret == 0) {
  92                         ret = -1;
  93                         errno = EINTR;
  94                 }
  95         }
  96
  97         saved_errno = errno;
  98
  99         while (pipe_written != pipe_read) {
 100                 char c;
 101                 /* Due to the linux kernel bug in 2.0.x, we
 102                  * always increment here even if the read failed... */
 103                 read(select_pipe[0], &c, 1);
 104                 pipe_read++;
 105         }
 106
 107         errno = saved_errno;
 108
 109         return ret;
 110 }
 111
 112 /*******************************************************************
 113 similar to sys_select() but catch EINTR and continue
 114 this is what sys_select() used to do in Samba
 115 ********************************************************************/
 116 int sys_select_intr(int maxfd, fd_set *fds,struct timeval *tval)
 117 {
 118         int ret;
 119         fd_set fds2;
 120
 121         do {
 122                 fds2 = *fds;
 123                 ret = sys_select(maxfd, &fds2, tval);
 124         } while (ret == -1 && errno == EINTR);
 125
 126         *fds = fds2;
 127
 128         return ret;
 129 }