How to communicate between combined tasks

• version

  1.1.1

• scope

  Example.

  This code is provided as example code for a user to base their code on.

• description

  How to communicate between combined tasks

• boards

  Unless otherwise specified, this example runs on the SliceKIT Core Board, but can easily be run on any XMOS device by using a different XN file.

When tasks are combined, they cannot communicate over channels due to deadlock issues but can communicate over interfaces. The method and results of communication are exactly the same as if the tasks were not combined. So the client can call interface functions:

[[combinable]]
void task1(interface my_interface client c)
{
  timer tmr;
  int time;
  int count = 0;
  tmr :> time;
  while (1) {
    select {
    case tmr when timerafter(time) :> int now:
      count++;
      if (count > 5) {
        c.finish();
        return;
      }
      // c is the client end of the connection,
      // let's send a message to the other end.
      c.msgA(count, 10);
      time += 1000;
      break;
    }
  }
}

and the server end can receive these messages:

[[combinable]]
void task2(interface my_interface server c)
{
  while (1) {
    select {
    case c.msgA(int x, int y):
      printf("Received msgA: %d, %d\n", x, y);
      break;
    case c.msgB(float x):
      // handle the message
      printf("Received msgB: %f\n", x);
      break;
    case c.finish():
      return;
    }
  }
}

The tasks can then be combined and communication will work as expected.

int main(void)
{
  interface my_interface c;
  [[combine]]
  par {
    task1(c);
    task2(c);
  }
  return 0;
}

In this case the compiler will change the code to not use xCONNECT channels to communicate, but will instead replace the communication with simple function calls that switch context between the tasks for the duration of the interface call.