CODEWITHSUNDEEP
embeddedvxworks
Benoit
Benoit

Reputation: 39064

How can a task wait on multiple vxworks Queues?

We have a vxWorks design which requires one task to process messages from two message queues, Q1 & Q2.

We would like to wait on both Queues and whichever gets a message, process the message from the queue.

We want to avoid either polling the queues or using a timeout value which could cause the other queue to fill up.

Is there a way to do this?

Upvotes: 3

Views: 6343

Answers (2)

JayG
JayG

Reputation: 4459

If you use named pipes (pipeDevCreate(), write(), read()) instead of message queues, you can use select() to block until there are messages in either pipe.

Whenever select() triggers, you process all messages in the high priority pipe. Then you process a single message from the low priority pipe. Then call select again (loop).

Example Code snippets:

 // Initialization: Create high and low priority named pipes
 pipeDrv(); //initialize pipe driver
 int fdHi = pipeDevCreate("/pipe/high",numMsgs,msgSize);
 int fdLo = pipeDevCreate("/pipe/low",numMsgs,msgSize);

 ...

 // Message sending thread: Add messages to pipe
 write(fdHi, buf, sizeof(buf));

 ...

 // Message processing Thread: select loop
 fd_set rdFdSet;

 while(1)
 {
     FD_ZERO(&rdFdSet);
     FD_SET(fdHi, &rdFdSet);
     FD_SET(fdLo, &rdFdSet;

     if (select(FD_SETSIZE, &rdFdSet, NULL, NULL, NULL) != ERROR)
     {
         if (FD_ISSET(fdHi, &rdFdSet))
         {
             // process all high-priority messages
             while(read(fdHi,buf,size) > 0)
             {
                 //process high-priority
             }
         }

         if (FD_ISSET(fdLo, &rdFdSet))
         {
             // process a single low priority message
             if (read(fdLo,buf,size) > 0)
             {
                 // process low priority
             }
         }
     }
 }

Upvotes: 3

Benoit
Benoit

Reputation: 39064

In vxWorks, you can't wait directly on multiple queues. You can however use the OS events (from eventLib) to achieve this result. Here is a simple code snippet:


MSG_Q_ID lowQ, hiQ;

void Init() {
// Task Initialization Code.  This should be called from the task that will
// be receiving the messages
...
hiQ = msgQCreate(...);
lowQ = msgQCreate(...);
msgQEvStart(hiQ, VX_EV01);  // Event 1 sent when hiQ receives message
msgQEvStart(loQ, VX_EV02);  // Event 2 sent when loQ receives message
...
}
void RxMessages() {
...
  UINT32 ev;   // Event received

   // Blocks until we receive Event 1 or 2
   eventReceive(VX_EV01 | VX_EV02, EVENT_WAIT_ANY, WAIT_FOREVER, &ev);
   if(ev & VX_EV01) {
      msgQReceive(hiQ, ...);
   }
   if(ev & VX_EV02) {
      msgQReceive(loQ, ...);
   }
}

Note that you need to modify that code to make sure you drain all your queues in case there is more than one message that was received.

The same mechanism can also be applied to Binary semaphores using the semEvStart() function.

Upvotes: 1

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