1Reference “Introduction To Unix Signals Programming” in the reference material section Man page – sigprocmask, alarm “Understanding the Linux Kernel” (OReilly) in the reference material section

2 Signal: An IPC Mechanism With pipes we communicated data With signal, we can communicate control command The only information is:  The number identifying the signal Interrupts a process and forces it to handle the event immediately kill –l can be used to view all the signals supported by your system

3 Sending signals to process From keyboard  Ctrl-C, Ctrl-Z etc From the command line  kill -  fg :On most shells, using the 'fg' command will resume execution of the process (that was suspended with Ctrl-Z), by sending it a CONT signal. Using the kill() system call Also used by the kill and fg command #include /* signal name macros, and the kill() prototype */ /* first, find my own process ID */ pid_t my_pid = getpid(); /* now that i got my PID, send myself the STOP signal. */ kill(my_pid, SIGSTOP);

4 Actions Performed upon Receiving a Signal There are three ways in which a process can respond to a signal: 1.Explicitly ignore the signal. 2.Execute the default action associated with the signal. 3.Catch the signal by invoking a corresponding signal-handler function.

5 Signal Handler Corresponding to each signal is a signal handler Called when a process receives a signal The function is called “asynchronously” When the signal handler returns the process continues, as if it was never interrupted Signal are different from interrupts as:  Interrupts are sent to OS by H/W  Signals are sent to a process by the OS, or by other processes  Note that signals have nothing to do with software interrupts, which are still sent by the hardware (the CPU itself, in this case).

6 Catching a Signal

7 The signal() System Call Used to set signal handler for a signal type

8Example void main() {/* set the INT (Ctrl-C) signal handler to 'catch_int' */ signal(SIGINT, catch_int); /* now, lets get into an infinite loop of doing nothing. */ for ( ;; ) pause(); } /* first, here is the signal handler */ void catch_int(int sig_num) { /* re-set the signal handler again to catch_int, for next time */ signal(SIGINT, catch_int); /* and print the message */ printf("Don't do that"); }

9 Signal Handlers Pre-defined Signal Handlers SIG_IGN:  Causes the process to ignore the specified signal.  To ignore Ctrl-C completely signal(SIGINT, SIG_IGN); SIG_DFL:  Causes the system to set the default signal handler for the given signal signal(SIGTSTP, SIG_DFL);

10 Race conditions The occurrence of a second signal while the signal handler function executes Might be of a different type then the one being handled Or even of the same type

11sigprocmask() Allows us to specify a set of signals to block Returns the list of signals that were previously blocked int sigprocmask(int how, const sigset_t *set, sigset_t *oldset);

12sigprocmask() int how  SIG_BLOCK Add set to the set of currently blocked signals  SIG_UNBLOCK  The signals in set are removed from the current set of blocked signals.  SIG_SETMASK The set of blocked signals is set to the argument set. const sigset_t *set  The set of signals to be blocked, or to be added to the current mask, or removed from the current mask (depending on the 'how' parameter). sigset_t *oldset  If this parameter is not NULL, then it'll contain the previous mask.  Can be used later restore back the situation, before sigprocmask() was called.

13 Signal functions Define a new mask set sigset_t mask_set; Clear the set (i.e. make it contain no signal numbers) sigemptyset(&mask_set); Add a signal sigaddset(&mask_set, SIGTSTP); Remove the TSTP signal from the set sigdelset(&mask_set, SIGTSTP); Check if the TSTP signal is defined in our set if (sigismember(&mask_set, SIGINT) printf("signal INT is in our set\n"); else printf("signal INT is not in our set - how strange...\n"); Make the set contain ALL signals available on the system sigfillset(&mask_set)

14 Count_CTRL_C.c

15 #include /* standard I/O functions */ #include /* standard unix functions, like getpid()*/ #include /* signal name macros, and the signal() prototype */ /* first, define the Ctrl-C counter, initialize it with zero. */ int ctrl_c_count = 0; #defineCTRL_C_THRESHOLD5 int main(int argc, char* argv[]) { /* set the Ctrl-C and Ctrl-Z signal handlers */ signal(SIGINT, catch_int); signal(SIGTSTP, catch_suspend); /* enter an infinite loop of waiting for signals */ for ( ;; ) pause(); return 0; }

16 /* the Ctrl-C signal handler */ void catch_int(int sig_num) { sigset_t mask_set;/* used to set a signal masking set. */ sigset_t old_set;/* used to store the old mask set. */ /* mask any further signals while we're inside the handler. */ sigfillset(&mask_set); sigprocmask(SIG_SETMASK, &mask_set, &old_set); /* increase count, and check if threshold was reached */ ctrl_c_count++; if (ctrl_c_count >= CTRL_C_THRESHOLD) { char answer[30]; /* prompt the user to tell us if to really exit or not */ printf("\nRealy Exit? [y/N]: "); fflush(stdout);

17 gets(answer); if (answer[0] == 'y' || answer[0] == 'Y') { printf("\nExiting...\n"); fflush(stdout); exit(0); } else { printf("\nContinuing\n"); fflush(stdout); /* reset Ctrl-C counter */ ctrl_c_count = 0; } } /* restore the old signal mask */ sigprocmask(SIG_SETMASK, &old_set, NULL); }

18 /* the Ctrl-Z signal handler */ void catch_suspend(int sig_num) { sigset_t mask_set;/* used to set a signal masking set. */ sigset_t old_set;/* used to store the old mask set. */ /* mask any further signals while we're inside the handler. */ sigfillset(&mask_set); sigprocmask(SIG_SETMASK, &mask_set, &old_set); /* print the current Ctrl-C counter */ printf("\n\nSo far, '%d' Ctrl-C presses were counted\n\n", ctrl_c_count); fflush(stdout); /* restore the old signal mask */ sigprocmask(SIG_SETMASK, &old_set, NULL); }

19 Implementing Timers Using Signals alarm : set a process alarm clock #include unsigned alarm(unsigned sec); Sends the signal SIGALRM to the calling process after the sec number of seconds have elapsed If sec is 0, any previously made alarm request is canceled.

20 use-alarms_c.c

21 #include /* signal name macros, and the signal() prototype */ char user[40];/* buffer to read user name from the user */ int main(int argc, char* argv[]) { /* set a signal handler for ALRM signals */ signal(SIGALRM, catch_alarm); /* prompt the user for input */ printf("Username: "); fflush(stdout); /* start a 30 seconds alarm */ alarm(30); /* wait for user input */ gets(user); /* remove the timer, now that we've got the user's input */ alarm(0); printf("User name: '%s'\n", user); }`

22 /* define an alarm signal handler. */ void catch_alarm(int sig_num) { printf("Operation timed out. Exiting...\n\n"); exit(0); }