4 years, 7 months ago.

What is the expected behaviour with Semaphores?

I'm getting a bit confused by the behaviour of Semaphores. Could I get some clarifications on the expected behaviour? I'm replacing the mutex in the UARTSerial driver to prevent errors getting thrown due to mutex failures when 2 threads access printf.

So what I'm expecting is that the Semaphore holds a thread until it can acquire the resource.

/** Wait until a Semaphore resource becomes available.
      @note You cannot call this function from ISR context.
    */
void Semaphore::acquire()
{
    osStatus_t status = osSemaphoreAcquire(_id, osWaitForever);
    if (status != osOK) {
        MBED_ERROR1(MBED_MAKE_ERROR(MBED_MODULE_KERNEL, MBED_ERROR_CODE_SEMAPHORE_LOCK_FAILED), "Semaphore acquire failed", status);
    }
}

However osSemaphoreAcquire seems to not do this behaviour, in fact it returns a timeout error even if the timeout period was 'osWaitForever'. So Semaphore::acquire() will actually throw an exception.

/// Acquire a Semaphore token or timeout if no tokens are available.
osStatus_t osSemaphoreAcquire (osSemaphoreId_t semaphore_id, uint32_t timeout) {

The now deprecated wait function returns a value I can use in my case with a while loop. But more importantly to me, doesn't throw an error.

/** Wait until a Semaphore resource becomes available.
    */
int32_t Semaphore::wait(uint32_t millisec = osWaitForever)
{
    osStatus_t stat = osSemaphoreAcquire(_id, millisec);
    switch (stat) {
        case osOK:
            return osSemaphoreGetCount(_id) + 1;
        case osErrorTimeout:
        case osErrorResource:
            return 0;
        case osErrorParameter:
        default:
            return -1;
}

My current work around is this:

rtos::Semaphore _sema_lock; // This is a private member in the UARTSerial Class
void UARTSerial::api_lock(void)
{
	while(_sema_lock.wait() <= 0) {
		wait_ms(1);
	}
}

So what is the actual expected behaviour?