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select.c

/* 
   Unix SMB/Netbios implementation.
   Version 3.0
   Samba select/poll implementation
   Copyright (C) Andrew Tridgell 1992-1998
   
   This program is free software; you can redistribute it and/or modify
   it under the terms of the GNU General Public License as published by
   the Free Software Foundation; either version 2 of the License, or
   (at your option) any later version.
   
   This program is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
   GNU General Public License for more details.
   
   You should have received a copy of the GNU General Public License
   along with this program; if not, write to the Free Software
   Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
*/

#include "includes.h"

/* This is here because it allows us to avoid a nasty race in signal handling. 
   We need to guarantee that when we get a signal we get out of a select immediately
   but doing that involves a race condition. We can avoid the race by getting the 
   signal handler to write to a pipe that is in the select/poll list 

   This means all Samba signal handlers should call sys_select_signal().
*/

static pid_t initialised;
static int select_pipe[2];
static VOLATILE unsigned pipe_written, pipe_read;

/*******************************************************************
 Call this from all Samba signal handlers if you want to avoid a 
 nasty signal race condition.
********************************************************************/

void sys_select_signal(void)
{
      char c = 1;
      if (!initialised) return;

      if (pipe_written > pipe_read+256) return;

      if (write(select_pipe[1], &c, 1) == 1) pipe_written++;
}

/*******************************************************************
 Like select() but avoids the signal race using a pipe
 it also guuarantees that fds on return only ever contains bits set
 for file descriptors that were readable.
********************************************************************/

int sys_select(int maxfd, fd_set *readfds, fd_set *writefds, fd_set *errorfds, struct timeval *tval)
{
      int ret, saved_errno;
      fd_set *readfds2, readfds_buf;

      if (initialised != sys_getpid()) {
            pipe(select_pipe);

            /*
             * These next two lines seem to fix a bug with the Linux
             * 2.0.x kernel (and probably other UNIXes as well) where
             * the one byte read below can block even though the
             * select returned that there is data in the pipe and
             * the pipe_written variable was incremented. Thanks to
             * HP for finding this one. JRA.
             */

            if(set_blocking(select_pipe[0],0)==-1)
                  smb_panic("select_pipe[0]: O_NONBLOCK failed.\n");
            if(set_blocking(select_pipe[1],0)==-1)
                  smb_panic("select_pipe[1]: O_NONBLOCK failed.\n");

            initialised = sys_getpid();
      }

      maxfd = MAX(select_pipe[0]+1, maxfd);

      /* If readfds is NULL we need to provide our own set. */
      if (readfds) {
            readfds2 = readfds;
      } else {
            readfds2 = &readfds_buf;
            FD_ZERO(readfds2);
      }
      FD_SET(select_pipe[0], readfds2);

      errno = 0;
      ret = select(maxfd,readfds2,writefds,errorfds,tval);

      if (ret <= 0) {
            FD_ZERO(readfds2);
            if (writefds)
                  FD_ZERO(writefds);
            if (errorfds)
                  FD_ZERO(errorfds);
      }

      if (FD_ISSET(select_pipe[0], readfds2)) {
            char c;
            saved_errno = errno;
            if (read(select_pipe[0], &c, 1) == 1) {
                  pipe_read++;
            }
            errno = saved_errno;
            FD_CLR(select_pipe[0], readfds2);
            ret--;
            if (ret == 0) {
                  ret = -1;
                  errno = EINTR;
            }
      }

      return ret;
}

/*******************************************************************
 Similar to sys_select() but catch EINTR and continue.
 This is what sys_select() used to do in Samba.
********************************************************************/

int sys_select_intr(int maxfd, fd_set *readfds, fd_set *writefds, fd_set *errorfds, struct timeval *tval)
{
      int ret;
      fd_set *readfds2, readfds_buf, *writefds2, writefds_buf, *errorfds2, errorfds_buf;
      struct timeval tval2, *ptval;

      readfds2 = (readfds ? &readfds_buf : NULL);
      writefds2 = (writefds ? &writefds_buf : NULL);
      errorfds2 = (errorfds ? &errorfds_buf : NULL);
      ptval = (tval ? &tval2 : NULL);

      do {
            if (readfds)
                  readfds_buf = *readfds;
            if (writefds)
                  writefds_buf = *writefds;
            if (errorfds)
                  errorfds_buf = *errorfds;
            if (tval)
                  tval2 = *tval;

            ret = sys_select(maxfd, readfds2, writefds2, errorfds2, ptval);
      } while (ret == -1 && errno == EINTR);

      if (readfds)
            *readfds = readfds_buf;
      if (writefds)
            *writefds = writefds_buf;
      if (errorfds)
            *errorfds = errorfds_buf;

      return ret;
}

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