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switching from mutex to binary semaphore does not work

Posted by williamjsell on August 12, 2015

I made an error as I was taking a mutex from one task and giving it back in another task. This is not the correct way to signal tasks, so I replaced the mutex with a binary sempaphore:

    //wait forever for a message.  This updates the global variable "device"
    xQueueReceive(calibrationQueue, &calDev, portMAX_DELAY);
    //wait forever for the resource
    xSemaphoreTake(calibrationSmphr, portMAX_DELAY);
    //now update the public device
    memcpy(&calibrateDevice, calDev, sizeof(_calibration_type_t));
    //call the calibrate task
    xEventGroupSetBits(xCalibrationGroup, calibrateDevice.calType);

The problem is the xSempahoreTake never returns on the first call to the function, so my system never gets started. Basically this task queues requests and waits for the semaphore to be available, before setting another event. When the calibration event is complete it gives back the semaphore. What am I doing wrong? I checked the semaphore has been properly allocated from CreateBinarySemaphore. When I substiture the mutex rather than binary semaphore, everything works.

switching from mutex to binary semaphore does not work

Posted by rtel on August 12, 2015

Which function are you using to create the semaphore? If you are using xSemaphoreCreateMutex() (which is the correct function to use in newer versions of FreeRTOS), then the semaphore is created empty. Whereas if you create a mutex, then the mutrex is created full. Hence the difference in the behaviour you observed.

If you want the semaphore to be available the first time you call the function, then give the semaphore immediately after you have created it. Then, inside the function where it is used, the first 'take' operation will pass.

See the comments in the source code example at the bottom of the following page: http://www.freertos.org/xSemaphoreCreateBinary.html


switching from mutex to binary semaphore does not work

Posted by williamjsell on August 12, 2015

Thanks, I missed the part about giving before using the semaphore...!


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