Signals and TimersOutlineSMP5 previewReview: signalsProcess Signal MasksThread Signal MasksProblem 1 (ds6-p1.c)Signal Handlerssa_handler vs. sa_sigactionProblem 2 (ds6-p2.c)pause()sigwait()Problem 3 (ds6-p3.c)TimersAccessing the clockSetting a timerResetting (or disabling) a timerUsing timers effectively (SMP5 advice)Problem 4 (ds6-p4.c)Timing your codeProblem 5 (ds6-p5.c)RecapSignals and TimersCS241 Discussion Section Week 62/26/07 – 3/02/07OutlineSMP5 previewSignals and threadspause(), sigwait()POSIX:TMR timersclock_gettime()nanosleep()SMP5 previewMuch like in SMP4, you will be implementing a schedulerHowever, this one’s a real preemptive schedulerIt uses timers and signals to interrupt running threadsReview: signalsAsynchronous notification to a process indicating some action should be takenSending signals to a process:kill -<signal> <pid>int kill(pid_t pid, int sig);We can signal individual threads, too:int pthread_kill(thread_t tid, int sig);Process Signal MasksSetting SIGINT to be blockedif ((sigemptyset(&set) == -1) || (sigaddset(&set, SIGINT) == -1)) perror(“Failed init signal set”);else if(sigprocmask(SIG_BLOCK, &set, &oldset) == -1) perror(“Failed to block SIGINT”);SIG_BLOCK adds set to current maskoldset will store the previous signal maskThread Signal Maskspthread_sigmask():Takes same parameters as sigprocmaskOnly affects the signal mask of a single threadSignal mask is inherited on thread creationProblem 1 (ds6-p1.c)Another “hello world” variantPrints out “Hello” and “World” periodicallyChange the code to stop the hello thread when receiving SIGUSR1 and the world thread when receiving SIGUSR2.Hint 1: you should not have to change the signal handlerHint 2: what about the main thread?Signal HandlersAllow us to change what happens when a signal is receivedvoid handler(int signo) { … }struct sigaction act;act.sa_flags = 0;act.sa_handler = handler;// additional signals blocked in the handlersigemptyset(&act.sa_mask);sigaction(SIGUSR1, &act, NULL);sa_handler vs. sa_sigactionWe can get additional information about the signalvoid handler(int signo, siginfo_t* info, void* context);act.sa_flags = SA_SIGINFO;// fill sa_sigaction instead of sa_handleract.sa_sigaction = handler;Extra information contains, e.g., the source of the signal (info->si_code):SI_USER – user-created signal (with abort, kill, etc.)SI_TIMER – a POSIX:RTS timer expiredetc.Problem 2 (ds6-p2.c)Windows users will sometimes press Ctrl-C (for copy to clipboard), inadvertently killing a UNIX process.Let’s change the SIGINT handler to kill the process only if the user really means it! I.e., presses Ctrl-C three times within a 5-second “tick” And let’s only count signals sent by the kernel (based on keyboard input) info->si_code == SI_KERNELpause()Waits for any signal that is not blocked/ignoredBut if a signal is generated before pause() is called, pause() will never see itIf we use the sigmask to block the signal until pause() is called, it will be queued until we remove itHowever, pause() will just sit there waiting for the signal that is blocked; it will never check the queueIn summary: pause() only returns if called before the signal is generated!sigwait()Takes as parameter a sigset corresponding to which signals it should wait forYou should block the signals firstsigwait() will remove a signal from the queue that is in its sigsetMust also pass a pointer to an integer for it to store signal that was removedsigwait(sigset_t *set, int *signo);Problem 3 (ds6-p3.c)Counting signalsUse sigwait() to count how many times a process receives SIGUSR1 or SIGUSR2Don’t forget to block them first!TimersWe will be using POSIX:TMR timersSend the SIGALRM signal to the processIf we set up a signal handler for SIGALRM, we have a programmable timer!!! Signals sent for timers or interrupts need to be unblocked for the thread that will be receiving them !!Accessing the clockThe POSIX:TMR extension allows us to get and set time from the real-time clockstruct timespec { time_t tv_sec; /* seconds */ long tv_nsec; /* nanoseconds */ }Timers need two of these: one for how long from now to begin, another for how often to generate the interrupt (timer interval)struct itimerspec { struct timespec it_interval; /* period */ struct timespec it_value; }Setting a timerCreate the timertimer_t timerid;timer_create(CLOCK_REALTIME, NULL, &timerid);Set up the structs (fire first at 5 s, then every 2.5 s)struct itimerspec value;value.it_interval.tv_sec = 2;value.it_interval.tv_nsec = 500000000L;value.it_value.tv_sec = 5;value.it_value.tv_nsec = 0;Start the timertimer_settime(timerid, 0, &value, NULL);Resetting (or disabling) a timerHow do we turn off a timer?Simple: just “set” it to 0We can also restart a timerJust call timer_settime with the same parameters as before; the timer will be resetOr pass in a different time to change its behaviorUsing timers effectively (SMP5 advice)POSIX:TMR timers by default send SIGALRMUse struct sigevent to change this if neededSetup a signal handler to catch that signalUse infinite pause or sched_yield loops to put a thread to sleep while it waits for the timerIf a thread terminates while the timer is still active, the signal may be delivered to a different threadProblem 4 (ds6-p4.c)Let’s see how timers are used in this simple example program#include <time.h>gcc –o p4 ds6-p4.c –lrtTiming your codeclock_gettime() fills in a struct timespec with elapsed time in nanoseconds since the epoch (Jan 1, 1970)Difference between two structs can time a function/actionUseful to keep track of how long threads are waiting or executingstruct timespec tend, tstart;clock_gettime(CLOCK_REALTIME, &tstart);function_to_time();clock_gettime(CLOCK_REALTIME, &tend);double timediff = tend.tv_sec – tstart.tv_sec +((double)tend.tv_nsec – tstart.tv_nsec)/1e9;Problem 5 (ds6-p5.c)Time how long 1e5, 1e6, etc. iterations of an empty for loop take to executeTime how long 1e5, 1e6, etc. sched_yields take to executeRecapHow to generate and block signalsHow to modify signal handlersHow to initialize and start a timerHow to time your
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