Select

Multiplexing using select

Chapters 6 of Textbook
Based on I/O multiplexing useing Select

n = select ( length, readset, writeset, exceptset, time)

values of time:

       NULL      indifinate wait
          0          no-wait (poll)
          T           wait T (sec + micro-sec)

Under what conditions a descriptor is ready?

ready for reading:

§     Receive buffer has data > low-water mark

(default is 1, set SO_RCVLOWWAT socket option to change).

§     Receive FIN from the other side.

§     A listen socket has a completed connection.

§     An error condition (e.g., receive RST ).

ready for writing:

§     Send buffer has space > low-water mark

   (default is 2048, set SO_SNDLOWWAT socket option to change).

§     Sent FIN out (close write end).

§     An error condition.

ready for exception condition:

E.g., out-of-band date is received

Example1: echo client:

  strcliselect01.c   & it is used in tcpcli01.c

Server: (run an echo server at port 9877)
      % cd /home/cs779/stevens3rd.book/unpv13e/tcpcliserv
      % tcpserv01

Client:
      % cd /home/cs779/stevens3rd.book/unpv13e/select
        %   tcpcli01 127.0.0.1
       % tcpcli01 127.0.0.1 < infile > outfile

      Problem:
                   infile is NOT the same as outfile,
                   since we exit when stdin is done:

                   if (Fgets(sendline, MAXLINE, fp) == NULL)

                        return;



Example2:

strcliselect02.c   & it is used in tcpcli02.c


The Problem above is fixed (use of shutdown):
         % tcpcli02 127.0.0.1 < infile > outfile

          if (Fgets(sendline, MAXLINE, fp) == NULL) {

                    stdineof = 1;

                    Shutdown(sockfd, SHUT_WR); /* send FIN */

TCP Echo Server using Select (instead of fork):

Example3: tcpservselect01.c

Server:

% cd /home/cs779/stevens3rd.book/unpv13e/tcpcliserv
% tcpservselect01

Client:

      % cd /home/cs779/stevens3rd.book/unpv13e/tcpcliserv
      %   tcpcli01 127.0.0.1
      (repeat for as many clients as you like)

Problem: denial of service attack

% tcpcli012 127.0.0.1

The user sends data without newline,

The server will hung (blocked) forever!

tcpcli012 uses

str_cli12.c (this program dos not send newline!)

· Poll call:

n = poll (struct pollfd fds[], nfds_t nfds, int timeout);

values of timeout:

       INFTIM     indifinate wait
          0          no-wait (poll)
         T          T millisec

TCP Echo Server using Poll:

Example3: tcpservpoll01.c

Server:

% cd /home/cs779/stevens3rd.book/unpv13e/tcpcliserv
% tcpservpoll01

Client:

      % cd /home/cs779/stevens3rd.book/unpv13e/tcpcliserv
        %   tcpcli01 127.0.0.1 OR telnet 127.0.0.1 9877
      (repeat for as many clients as you like)

Select in Java