receiving efficiently messages from queues

bodev wrote on Monday, October 27, 2008:

Hi all,
in many FreeRTOS examples, i have seen code like this:

     while( xQueueReceive( queue, &mess, portMAX_DELAY ) != pdPASS );

i was wondering if this is efficient in the sense that it blocks the thread until a message is posted
or this instruction implies a true active polling and hence consumes CPU time.

is there any efficient way to do this?

the obvious solution that come to mind is to use a binarysemaphore to signal that a message has been posted,
so the receiving thread can stop on the sempahore and only when woken up receive the message.

any other solution?


rtel wrote on Monday, October 27, 2008:

> while( xQueueReceive( queue, &mess, portMAX_DELAY ) != pdPASS );

This code is truely blocking - not polling.

xQueueReceive() is in a while loop to deal with the case where the block time expires, but no data has been received.

Taking the two extremes:

Depending on your configuration (see the configuration section of the WEB docs), portMAX_DELAY may block indefinitely, in which case xQueueReceive() will only return when it has successfully received some data.  In this case the while() loop has no effect as xQueueReceive() will only ever return pdPASS.

If you have not configured your system to allow indefinite blocking, and you are using 16bit ticks, then portMAX_DELAY will expire every 0xffff ticks, so it xQueueReceive() could well return pdFALSE, depending on how often data is written to the queue.  [most systems would use 32bit ticks].