Is this busy waiting?

Is this loop busy waiting, I would think the wait call takes care of that. If it is, how can it be fixed to not busy wait?

id = fork();
for (i = 0; i < 20; i++)
{
    switch (id)
    {
             case 0:
                /* do stuff with child */
                exit(0);
             default:
             {
                 if (children>=3) {
                     int s;
                     wait(&s);
            开发者_如何学JAVA         children--;
                 }
                 children++;
                 id = fork();
             }
    }
}

wait will cause the kernel to select other jobs that are not marked as blocked, so this isn't a case of busy-waiting. Also isn't switch a bit excessive for a fork(), why not use a simple if statement?

You are right, wait waits non-busy by handing over the CPU to the kernel until the child has exited.

It's not really buys waiting (it doesn't check the status of the children in a loop; instead if blocks inside the wait() call).

Nonetheless, the code can hog the CPU depending on what happens in do stuff with child. That will look like busy waiting (CPU usage 100%) even though it's really several processes doing actual work.

I agree with you, waiting on the death of a child (even inside a loop) would not be busy waiting. OTOH you can get in real trouble if one or more of the child processes are compute intensive. The compute-intensive children will always be ready to run and it is not guaranteed that the parent will get the CPU.