On Apr 13, 2007, at 10:45 AM, Mehedi Bakht wrote:
What happens if I declare a Queue of messages instead of a Queue of
message pointers ?
I am trying to find out the potential problems of declaring QueueC
(message_t, 8) instead of QueueC(message_t*, 8). Then, when I get a
packet from the lower layer, I instantiate a new message_t
variable, copy the contents of the received message there, queue it
and return the buffer to radio Layer.
event message_t* Receive.receive(message_t* bufPtr, void* payload,
uint8_t len) {
message_t tempMsg;
memcpy(&tempMsg,bufPtr,sizeof(message_t);
SendQueue.enqueue(tempMsg);
return bufPtr;
}
Then you are passing the structure by value. This will cause there to
be up to six copies:
1. From pointer to stack: memcpy(&tempMsg,bufPtr,sizeof(message_t);
2. From stack to stack (making tempMsg a by-value parameter to enqueue);
3. From stack to data (assigning the parameter of enqueue to an
element of the queue)
dequeueing:
4. From data to stack (assigning element of the queue to the local
stack variable in dequeue)
5. From stack to stack (copying the local stack variable to the
return value)
6. From stack to data (copying the return value to a pointer you can
do something with).
Furthermore, passing large structures on the stack is notoriously
inefficient in C. That's why you rarely see it done.
Phil
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