Signals & Out-of-Band Data

Ø Signal Handling

Ø Handling SIG_CHLD

Ø Handling SIG_ALRM

Ø Termination of Server Process

Ø Out-of-Band Data Using SIG_URG

Ø Heartbeat

Ø Siganl Handling:

siganl ( sig, handler )

ü sig: SIGCHLD, SIGKILL, etc ...

(use % kill –l to list all signals)

ü handler: SIG-IGN, SIG-DFL, etc., or specific-function

Ø Handling SIGCHLD:

Example:

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

ü Run serever: tcpserv01.c  (it uses  str_echo.c )

% tcpserv01

ü Run  client:    tcpcli01.c     (it uses  str_cli.c )

% tcpcli01 127.0.0.1

ü Use ps -a to find the pid of the child process P serving the client.

ü kill  the client process tcpcli01

ü Use ps -a and  you will find the pid of P is <defunct>  i.e., child process is zombie.

Fix:

In tcpserv02.c  add a line:

Signal (SIGCHLD, sig_chld) ;

ü This is a Bad Fix !

This fix KILLS the server!  (since Accept return EINTR error).

ü Good Fix:

Use accept (instead of the wrapper function Accept) in tcpserv03.c to check for EINTR and continue.

Multiple childern:

§ tcpcli04.c  creates 50 connections to the tcpserv03.c   (it use: sigchldwait.c)

killing the client may cause zomies leftover.

§ To solve this use: tcpserv04.c  (it uses: sigchldwaitpid.c)

Ø Handling SIGALRM:

Example: connect_timeo.c

Example: timeoDaytimeClient.c (uses) tcpDaytimeServer.c

% cd /home/cs779/stevens3rd.book/unpv13e/nonblock/example

% tcpDaytimeServer 1234 0 25
% timeoDaytimeClient  127.0.0.1 1234  0  & (will get the time)
% timeoDaytimeClient  127.0.0.1 1234  10  & (will timeout)
% timeoDaytimeClient  127.0.0.1  1234  50  & (will get the time)

Example: ChatClientAlarm.c

Ø Termination of Server Process:

• A server may terminate if:

à readline error: Connection reset by peer.
Occures when you read from a socket that has received FIN.

à writen error: Broken pipe.
Occures when you write to a socket that has recieved RST.

§ Crashing Server Host:  Client gets Timeout.

§ Crashing & Reboot Host: Client gets RST

§ Normal Shutdown of Host:  Client gets FIN

Ø Out-of-Band data Using  SIG_URG:

Each out-of-band data sent generates SIGURG at the receiver.

Example:

% cd /home/cs779/stevens3rd.book/unpv13e/oob

ü Run serever: tcprecv01.c

% tcprecv01 10234

ü Run client: tcpsend01.c

% tcpsend01 127.0.0.1 10234

Stopped Flow:

If TCP flow control causes the sender to stop sending data:

TCP still allows out-of-band notification to be sent.

Example:

% cd /home/cs779/stevens3rd.book/unpv13e/oob

ü Run serever: tcprecv05.c

% tcprecv05 10234

ü Run client: tcpsend05.c

% tcpsend05 127.0.0.1 10234

Ø Heartbeat:

To detect the early failure of either the peer host or the communication path to the peer.

ü heartbeatcli.c  &

ü heartbeatserv.c

ü tcpcli02.c it uses:

strcliselect02.c  and it uses:

heartbeat_cli(sockfd, 1, 5);

ü tcpserv02.c it uses:

strecho02.c and it uses:

heartbeat_serv(sockfd, 1, 5);

Run serever & client as follows:

% tcpserv02 10111

% tcpcli02 127.0.0.1 10111

Back Doors:

Type: CTRL_C for tcpcli02,

This will slow down the client response and causes the server to disconnect.