Question

I am migrating an applciation from windows to linux. I am facing problem with respect to WaitForSingleObject and WaitForMultipleObjects interfaces.

In my application I spawn multiple threads where all threads wait for events from parent process or periodically run for every t seconds.

I have checked pthread_cond_timedwait, but we have to specify absolute time for this.

How can I implement this in Unix?

Answer 1

Stick to pthread_cond_timedwait and use clock_gettime. For example:

struct timespec ts;
clock_gettime(CLOCK_REALTIME, &ts);
ts.tv_sec += 10; // ten seconds
while (!some_condition && ret == 0)
    ret = pthread_cond_timedwait(&cond, &mutex, &ts);

Wrap it in a function if you wish.

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UPDATE: complementing the answer based on our comments.

POSIX doesn’t have a single API to wait for “all types” of events/objects as Windows does. Each one has its own functions. The simplest way to notify a thread for termination is using atomic variables/operations. For example:

Main thread:

// Declare it globally (argh!) or pass by argument when the thread is created
atomic_t must_terminate = ATOMIC_INIT(0);

// "Signal" termination by changing the initial value
atomic_inc(&must_terminate); 

Secondary thread:

// While it holds the default value
while (atomic_read(&must_terminate) == 0) {
    // Keep it running...
}
// Do proper cleanup, if needed
// Call pthread_exit() providing the exit status

Another alternative is to send a cancellation request using pthread_cancel. The thread being cancelled must have called pthread_cleanup_push to register any necessary cleanup handler. These handlers are invoked in the reverse order they were registered. Never call pthread_exit from a cleanup handler, because it’s undefined behaviour. The exit status of a cancelled thread is PTHREAD_CANCELED. If you opt for this alternative, I recommend you to read mainly about cancellation points and types.

And last but not least, calling pthread_join will make the current thread block until the thread passed by argument terminates. As bonus, you’ll get the thread’s exit status.