Hi,
I have a quick question. As semaphores are implemented as queues, is there any reason why uxQueueMessagesWaiting() can’t be used to determine is a semaphore is able to be given/taken without actually giving/taking it?
Thanks,
Chris.
Using uxQueueMessagesWaiting with Semaphores
Using uxQueueMessagesWaiting with Semaphores
That should work ok.
Using uxQueueMessagesWaiting with Semaphores
Dave,
Thanks for that. It compiles OK, but doesn’t seem to do what I wanted it to do. Turns out what I really need to know is “has a task already given the semaphore“, not “is the semaphore able to be given“.
Thanks again,
Chris.
Chris.
Using uxQueueMessagesWaiting with Semaphores
Using uxQueueMessagesWaiting to get a semaphore’s count works as expected for me. Here is a demo excerpt:
// uxQueueMessagesWaiting Demo
int dummy_value = 3;
xQueueHandle demo_queue = xQueueCreate(5, sizeof(int));
// Initial size is 0
if (demo_queue != NULL)
{
unsigned int q_size = uxQueueMessagesWaiting(demo_queue);
// Output q_size here; should be 0
}
// Push 3 items
portBASE_TYPE xStatus;
xStatus = xQueueSendToBack(demo_queue, &dummy_value, 0);
xStatus = xQueueSendToBack(demo_queue, &dummy_value, 0);
xStatus = xQueueSendToBack(demo_queue, &dummy_value, 0);
// New size is 3
if (xStatus == pdPASS)
{
unsigned int q_size = uxQueueMessagesWaiting(demo_queue);
// Output q_size here; should be 3
}
Which yields the expected output of 0 and 3.
From my tests, the number of tasks pending on a semaphore does not seem to impact the count.
Note: Using uxQueueMessagesWaiting with semaphores seems to be undocumented, unless I missed something.
Edit: I realized that my sample didn’t actually use a Semaphore. The following sample verifies uxQueueMessagesWaiting for semaphores with both zero and non-zero init_count.
// semaphore queue size demo
SemaphoreHandle_t semaphore_handle1 = xSemaphoreCreateCounting(20, 0);
if (semaphore_handle1 != NULL)
{
unsigned int q_size = uxQueueMessagesWaiting(semaphore_handle1);
// Output q_size here; should be 3
xSemaphoreGive(semaphore_handle1);
xSemaphoreGive(semaphore_handle1);
xSemaphoreGive(semaphore_handle1);
q_size = uxQueueMessagesWaiting(semaphore_handle1);
// Output q_size here; should be 0
}
// semaphore queue size demo (with init_count > 0)
SemaphoreHandle_t semaphore_handle2 = xSemaphoreCreateCounting(20, 5);
if (semaphore_handle2 != NULL)
{
unsigned int q_size = uxQueueMessagesWaiting(semaphore_handle2);
// Output q_size here; should be 5
// Give 3
xSemaphoreGive(semaphore_handle2);
xSemaphoreGive(semaphore_handle2);
xSemaphoreGive(semaphore_handle2);
q_size = uxQueueMessagesWaiting(semaphore_handle2);
// Output q_size here; should be 3
// Take 4
xSemaphoreTake(semaphore_handle2, portMAX_DELAY);
xSemaphoreTake(semaphore_handle2, portMAX_DELAY);
xSemaphoreTake(semaphore_handle2, portMAX_DELAY);
xSemaphoreTake(semaphore_handle2, portMAX_DELAY);
q_size = uxQueueMessagesWaiting(semaphore_handle2);
// Output q_size here; should be 4
}
Output:
Semaphore Size (init_count == 0):0
After giving 3 times:3
Semaphore Size (init_count == 5):5
After giving 3 times:8
After taking 4 times:4