Getting Started with QNX Neutrino - QNX Software Systems

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More on synchronization © 2009, QNX Software Systems GmbH & Co. KG. State CONDVAR MUTEX READY INTERRUPT Meaning Waiting for the underlying condition variable associated with the sleepon Waiting for a mutex Capable of using, or already using, the CPU Waiting for an interrupt from the hardware Action Mutex owner Consumer state Producer state Consumer locks mutex Consumer READY INTERRUPT Consumer examines data_ready Consumer READY INTERRUPT Consumer calls pthread_sleepon_wait() Consumer READY INTERRUPT pthread_sleepon_wait() unlocks mutex Free READY INTERRUPT pthread_sleepon_wait() blocks Free CONDVAR INTERRUPT Time passes Free CONDVAR INTERRUPT Hardware generates data Free CONDVAR READY Producer locks mutex Producer CONDVAR READY Producer sets data_ready Producer CONDVAR READY Producer calls pthread_sleepon_signal() Producer CONDVAR READY Consumer wakes up, pthread_sleepon_wait() tries to lock mutex Producer MUTEX READY Producer releases mutex Free MUTEX READY Consumer gets mutex Consumer READY READY Consumer processes data Consumer READY READY Producer waits for more data Consumer READY INTERRUPT Time passes (consumer processing) Consumer READY INTERRUPT Consumer finishes processing, unlocks mutex Free READY INTERRUPT Consumer loops back to top, locks mutex Consumer READY INTERRUPT The last entry in the table is a repeat of the first entry — we’ve gone around one complete cycle. What’s the purpose of the data_ready variable? It actually serves two purposes: 62 Chapter 1 • Processes and Threads April 30, 2009
© 2009, QNX Software Systems GmbH & Co. KG. More on synchronization Condition variables • It’s the status flag between the consumer and the producer that indicates the state of the system. If it’s set to a 1, it means that data is available for processing; if it’s set to a 0, it means that no data is available, and the consumer should block. • It serves as “the place where sleepon synchronization occurs.” More formally, the address of data_ready is used as a unique identifier, that serves as the rendezvous object for sleepon locks. We just as easily could have used “(void *) 12345” instead of “&data_ready” — so long as the identifier is unique and used consistently, the sleepon library really doesn’t care. Actually, using the address of a variable in a process is a guaranteed way to generate a process-unique number — after all, no two variables in a process will have the same address! We’ll defer the discussion of “What’s the difference between pthread_sleepon_signal() and pthread_sleepon_broadcast() ” to the discussion of condition variables next. Condition variables (or “condvars”) are remarkably similar to the sleepon locks we just saw above. In fact, sleepon locks are built on top of condvars, which is why we had a state of CONDVAR in the explanation table for the sleepon example. It bears repeating that the pthread_cond_wait() function releases the mutex, waits, and then reacquires the mutex, just like the pthread_sleepon_wait() function did. Let’s skip the preliminaries and redo the example of the producer and consumer from the sleepon section, using condvars instead. Then we’ll discuss the calls. /* * cp1.c */ #include #include int data_ready = 0; pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER; pthread_cond_t condvar = PTHREAD_COND_INITIALIZER; void * consumer (void *notused) { printf ("In consumer thread...\n"); while (1) { pthread_mutex_lock (&mutex); while (!data_ready) { pthread_cond_wait (&condvar, &mutex); } // process data printf ("consumer: got data from producer\n"); data_ready = 0; pthread_cond_signal (&condvar); pthread_mutex_unlock (&mutex); } } void * April 30, 2009 Chapter 1 • Processes and Threads 63
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Contents About This Guide xi What y
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© 2009, QNX Software Systems GmbH
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List of Figures A process as a cont
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About This Guide April 30, 2009 Abo
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Typographical conventions © 2009,
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Using timers © 2009, QNX Software
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Chapter 4 Interrupts In this chapte
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Neutrino and interrupts © 2009, QN
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Writing interrupt handlers © 2009,
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TIME TIME Writing interrupt handler
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Summary © 2009, QNX Software Syste
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Appendix B Calling 911 In this appe
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Seeking professional help © 2009,
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Seeking professional help © 2009,
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Appendix C Sample Programs In this
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atoz.c © 2009, QNX Software System
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time1.c © 2009, QNX Software Syste
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