SIGNALS (7.11)

WHAT IS A "SIGNAL"??

A SIGNAL IS A MESSAGE SENT TO A PROGRAM UNDER EXECUTION IN 2 OCCASIONS:

1)UNDER SOME ERROR CONDITIONS OR THE USER INTERRUPTIONS,THE KERNEL GENERATES SIGNALS.

2)DURING INTER PROCESS COMMUNICATION BETWEEN 2 OR MORE PROCESSES ,THE PARTICIPATING PROCESSES GENERATES THE SIGNALS.

EG: CHILD PROCESS SENDS A SIGNAL TO ITS PARENTS PROCESS UPON ITS TERMINATION.

IN UNIX,SIGNALS ARE IDENTIFIED BY INTEGER AND NAMES.THE NAMES ARE USUALLY IN UPPERCASE AND START WITH "SIG". THERE ARE ABOUT 1-30 SIGNALS.

THE TABLE BELOW GIVES THE LIST OF EXIT OR INTERRUPT SIGNALS:

SIGNAL NO:            NAME                           FUNCTIONS

    1                             SIGHUP                        HANGUP;CLOSES PROCESS  
                                                                        COMMUNICATION LINKS.

    2                            SIGINT                          INTERRUPT;TELLS THE PROCESS
                                                                        TO EXIT(ctrl-c)
    
   3                             SIGQUIT                       QUIT;FORCES THE PROCESS TO   
                                                                        QUIT.(ctrl-\)

   9                             SIGKILL                       SURE KILL;CANT BE TRAPPED OR 
                                                                       IGNORED.

  15                            SIGTERM                     SOFTWARE TERMINATION;
                                                                       DEFAULT SIGNAL FOR THE KILL  
                                                                       COMMAND.

  24                            SIGSTOP                      STOP(ctrl-Z)