Is mutex needed to synchronize a simple flag between pthreads?

Question

Let\'s imagine that I have a few worker threads such as follows:

while (1) {
    do_something();

    if (flag_isset())
        do_something_else();
}
         


        

Answer 1

Assigning incoming job to worker threads requires no locking. Typical example is webserver, where the request is catched by a main thread, and this main thread selects a worker. I'm trying explain it with some pesudo code.

main task {

  // do forever
  while (true)

    // wait for job
    while (x != null) {
      sleep(some);
      x = grabTheJob(); 
    }

    // select worker
    bool found = false;
    for (n = 0; n < NUM_OF_WORKERS; n++)
     if (workerList[n].getFlag() != AVAILABLE) continue;
     workerList[n].setJob(x);
     workerList[n].setFlag(DO_IT_PLS);
     found = true;
    }

    if (!found) panic("no free worker task! ouch!");

  } // while forever
} // main task


worker task {

  while (true) {
    while (getFlag() != DO_IT_PLS) sleep(some);
    setFlag(BUSY_DOING_THE_TASK);

    /// do it really

    setFlag(AVAILABLE);

  } // while forever 
} // worker task

So, if there are one flag, which one party sets is to A and another to B and C (the main task sets it to DO_IT_PLS, and the worker sets it to BUSY and AVAILABLE), there is no confilct. Play it with "real-life" example, say, when the teacher is giving different tasks to students. The teacher selects a student, gives him/her a task. Then, the teacher looks for next available student. When a student is ready, he/she gets back to the pool of available students.

UPDATE: just clarify, there are only one main() thread and several - configurable number of - worker threads. As main() runs only one instance, there is no need to sync the selection and launc of the workers.