April 24, 2006

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Quicktime Warm-Up :-)

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• review of cpufraction.c
• review of timechild.c
  • why don't we get the same result when we pass it ls as we do with the time shell command?
    • here we always get 0 clock ticks

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Don't Get Caught sleeping

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• sleep function
  • this is old hat by now
  • takes seconds and blocks the calling thread until time up or signal caught
  • interacts with SIGALRM
    • therefore, avoid using them in the same process
  • beeper.c: beeps every n seconds
• nanosleep function
  • causes calling thread to block until time interval in rqtp (struct timespec) expired or until the thread receives a signal
  • allows nanosecond resolution
  • not affected by SIGALRM
  • nanotest.c: tests the resolution of nanosleep

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POSIX:XSI Interval Timers

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• a timer is the complement of a clock
• while a clock increments time, a timer decrements its value and generates a signal when that value reaches zero
• computers have a few hardware interval timers
• the operating system implements multiple software timers using those hardware timers
• timers have a variety of uses, e.g,
  • to keep track of time since last reboot
  • for process scheduling (quantum)
  • for increasing priority of a ready process
• struct itimerval (we have seen this before)
  • struct timeval it_value: time remaining before expiration
  • struct timeval it_interval: value to reload into timer
• POSIX:XSI provides each process the following three timers
  • ITIMER_REAL: generates a SIGALRM
  • ITIMER_VIRTUAL: generates a SIGVTALRM
  • ITIMER_PROF: decrements in virtual time and system time for the process and generates a SIGPROF
• functions for manipulating these interval timers
  • getitimer: retrieves the current time interval from a user interval timer
  • setitimer: starts and stops a user interval timer
• periodicasterisk.c: prints an asterisk for each two seconds of CPU time used
  • uses an ITIMER_PROF
  • uses a signal handler for SIGPROF to print an asterisk
• xsitimer.c: uses a POSIX:XSI interval timer to measure the execution time of a function

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Realtime Signals

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• remember the limitations of signal handling?
  • blocked signals of the same type are not queued
  • handlers can only take the signal number as a parameter
• POSIX:XSI and POSIX:RTS Realtime Signal Extensions address these limitations
• sa_sigaction member of sigaction
  • alternative type of signal handler
  • sa_flags must be nonzero
• handler must have the form: void func (int signo, siginfo_t *info, void *context);
  • siginfo_t is a struct containing fields (e.g., int si_code) to store information such as the cause of the signal
    • SI_USER: kill, raise, abort
    • SI_QUEUE: sigqueue
    • SI_TIMER: timer expired
    • SI_ASYNCIO: asynchronous I/O completed
    • SI_MESGQ: arrival of a message
• sigqueue function
  • extension to kill which permits signals to be queued
• examples:
  • sendsigqueue.c: program which behaves like kill, but sends a queued signal to a process
  • sigqueuehandler.c
    • program which receives SIGUSR1 signals and displays their values
    • notice the use of fprintf in handler

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References

[USP] K. A. Robbins and S. Robbins. UNIX Systems Programming: Concurrency, Communication, and Threads. Prentice Hall, 2003