I was wondering if there is a way to post to the beginning of the queue. In the event of an emergency i want to post to the beginning of the queue.

Is this possible?

There is no way of doing this with the standard code but I think it would be easy to do with the addition of another function (xQueueSendEmergency() or something).

Queues are simply a block of RAM with a couple of pointers. One pointer points to the next block that can be written (the end of the queue), and another the next block that should be read (the front of the queue).

When you write to a queue the data is copied to the "next write" block, then the next write pointer is incremented past this block to again point to the next free block. This is done by the macro prvCopyQueueData().

If you wanted to write to the start of the queue then you could instead decrement the next read pointer (so it points to a free block at the front of the queue), then copy the data being queued into that position. This way the next time the queue was read the read pointer would point to the data you just wrote.

This is just theory - I have not actually tried it. You would basically copy the xQueueSend() function, but replay the prvCopyQueueData() macro with one that wrote to the other end of the queue.

Thanks for pointing out to th epart where i need to make the changes i try it out.