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Message Passing

Message Passing. Akos Ledeczi EECE 354, Fall 2012 Vanderbilt University. Message Queues. Inter-task communication Usually better practice than using global variables Message queues are used to manage messages Standalone queues Each task has a built-in queue

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Message Passing

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  1. Message Passing Akos Ledeczi EECE 354, Fall 2012 Vanderbilt University

  2. Message Queues • Inter-task communication • Usually better practice than using global variables • Message queues are used to manage messages • Standalone queues • Each task has a built-in queue • Messages are passed by reference, that is, no copy is ever made!!! Must remain in scope!!! • Default is FIFO, but optionally LIFO can be used for important messaged to bypass waiting messages in the queue • Multiple tasks can Pend() on a queue. Post() can optionally broadcast to all waiting tasks

  3. Bilateral Rendezvous • Using tasks queues in this case • Just like semaphores, but can pass data as well

  4. Flow Control • Message producer may produce faster than consumer can consume • To prevent overflowing the message queue, a semaphore helps manage the resource • Using a task semaphore and a task queue in this case

  5. Clients and servers • Different tasks/ISRs (clients) report different error conditions • Single error handler (“server”) processes the reports

  6. Example • Measure RPM using a hole in the disk • A free running counter is used to measure the time between two hole detections • ISR shouldn’t be used to compute average, maximum, etc. • Task may be low priority in the system, so a message queue is used to store measurements until they can be processed

  7. Usage OS_Q MyQ; MyMsgDataMyMsg; OSQCreate(&MyQ, “My Queue”, 10, /* max queue size */ &err); … OSQPost(&MyQ, (void *)&MyMsg, /* actual data to send */ /* DANGER: using a global */ sizeof(MyMsg), OS_OPT_POST_FIFO, &err); … OSQPend(&MyQ, 1000, /* timeout */ OS_OPT_PEND_BLOCKING, &size, &ts, &err); if (err == OS_ERR_TIMEOUT) { … API: OSQCreate() OSQPend() OSQPost() OSQFlush() OSQPendAbort() OSQDel()

  8. Internals

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