Process Communication - Unix Signals

7/28/2019 Process Communication - Unix Signals

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notifications sent to a process to notify certainevents (errors or other).

interrupt the normal execution of the process.

force it to handle them immediately. A signal has an integer representation (1, 2,)

A signal has also a symbolic name ex.: SIGKILL,SIDUSR1.

signal names are constants usually defined in/usr/include/signal.h


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kill -l unix command.It lists all the signals on the system (names and codes).

Most signals symbolize a certain abnormal event or behavior.

Example: SIGFPE : arithmetical error. SIGKILL : Terminating signal. The process receiving this signal terminates

immediately.

SIGINT : execution interruption (CTRL + C). SIGUSR1 & SIGUSR2 : used for user processes. SIGSTP : suspends execution (CTRL +Z). SIGCONT: signal to continue the execution of a stopped process.fg after a CTRL +Z. SIGSEGV : Segmentation Violation (like wrong addresses).


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In the Shell: Ctrl-C (SIGINT) : Terminates the process

Ctrl-Z (SIGTSTOP) : stops the process

Ctrl-\ (SIGABRT) : Terminates the process From the Command Line kill -

Send the specified signal to the process having PID asprocess id.

The parameter signal is an integer (a number or aconstant)

Ex.:

Kill -9 2345

Kill -KILL 2345


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#include /* signal name macros, and the kill() prototype */

kill( pid, SIG#)

Sending the signal having sig# to the specified process.

Ex.:pid_t my_pid = getpid();

Kill(mypid, SIGSTOP); /* now that i got my PID, send myselfthe STOP signal. */


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Each signal may have a signal handler.

Its a function called when the processreceives that signal.

signal sent to the process OS stops theprocess execution & calls the signal handler.

After the signal handler, process continuesexecution.

In general, signal reception causes thetermination of the process


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Most signals could be caught by the process.We can define the action to do when receivingthis signal (signal handler).

Some could not like SIGKILL (-9 ).

To define a signal handler for a certain process:#include /* signal name macros, and thesignal() prototype */

signal(SIG#, handler_func);

Now the function handler_func will be executed when theprocess receives the signal SIG#.

pause(): causes the process to halt execution,until a signal is received.


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The signal handler is valid for one and only one signal

reception

To let it valid for future signal receptions re-set it usingsignal.

Signal handler format:

void sighandler(int sig_num)/* the parameter sig_num is the number of the signal

received */

{/* re-set the signal handler for next time */

signal(SIGINT, catch_int);

- - -}

The same handler function could be set to more than onesignal.


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SIG_IGN: if this handler is set the processignores the specified signal.Ex.:signal(SIG#, SIG_IGN);

SIG_DFL: its the default signal handlerdefined by the system.Ex.:

signal(SIG#, SIG_DFL);


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#include

#include

/* signal handler */

void catch_int(int sig_num)

{

/* re-set the signal handler again to catch_int, for next time */


/*print the message */printf("Don't do that\n");

}

Void main(){

/* set the (Ctrl-C) signal handler to 'catch_int' */


/* wait for signals */for ( ;; )

pause();

}


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SIGALRM: signal used for alarmfunctionnalities.

#include

alarm(int n);

/*asks the sys. To send SIGALRM to the callingprocess after n senconds.*/

if n = 0 annulate the last alarm call.

Exercise:Write a program displaying the alphabetical letters

such that each second it displays one letter.


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Exercise 1: Write a program ignoring all the signals (if possible.

Then try to send it several signals from the shell.

What happens?

Exercise 2: Write a program that terminates after the 3rd reception of the Ctrl-

C signal.

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Process Communication - Unix Signals