POSIX Timers: Asynchronous Notification

Basics

• POSIX timers: timer_create() (and timer_delete(), unshown in examples)

  • delivery/expiry: (async) signal, thread (-> later)

  • choose signal to use (never queued, not even rt signals)

  • setup handler for it (see Asynchronous Signal Handling)

  • timer_create() to send that signal on expiry

    • man -s 3 sigevent

    event.sigev_notify = SIGEV_SIGNAL;    // <-- alternatives?
    event.sigev_signo = SIGRTMIN;
    

One-shot Timers

• one shot (code/oneshot.cpp)

  • it_interval is 0 -> oneshot

  • expiry at it_value

  • nothing new regarding signals

Periodic Timers

• periodic (code/periodic.cpp)

  • note it_interval not 0

  • wrap in infinite loop

sigaction(SA_SIGINFO)

• link to sync handling/sigwaitinfo()

• multiplexing multiple timers on one signal via siginfo_t

  • jjj link from signals/async

  • only 32 rt signals

  • might want to use more timers

  • continue from posix-periodic.cpp

  • add 2nd timer (and arm it differently)

  • remove “interrupted” in main loop

  • -> handler cannot say which

  • code/two-timers-problem.cpp

  solution

  • code/two-timers-siginfo.cpp

  • step 1: sigaction -> SA_SIGINFO, sa_sigaction (more info in handler)

    • handler signature: void(int sig, siginfo_t* info, void*)

    • in handler, use info->si_value

    • make timer1, timer2 visible in signal handler (-> global)

  • event notification setup

    • event.sival_ptr = &timer{1,2} (-> handler’s info->si_ptr)