CODEWITHSUNDEEP
clinuxcondition-variable
Lipika Deka
Lipika Deka

Reputation: 3884

Signalling a particular thread among many waiting on a condition variable

This question follows from Breaking a condition variable deadlock. A number of threads may be waiting on a condition variable, I need to signal only a particular thread say thread 1 and kill it as it was a participant of a deadlock scenario. Is there a way i could signal just a partipular thread amoung the lot.

Would be gratefull for some help

thanks

An Edit; Respecting Nemo's comments. I understand this is a bad idea. But, is there a way to do it

Upvotes: 0

Views: 112

Answers (2)

David Schwartz
David Schwartz

Reputation: 182753

Just write code to do exactly what you need. There's no shortcut since condition variables don't provide this behavior. So just write it. There's nothing difficult about it. For example, you could set a special flag, wake all threads blocked on the condition variable, and then code the threads to check the flag to see if there's supposed to go back to sleep or not.

Upvotes: 0

bdonlan
bdonlan

Reputation: 231093

You can use deferred cancellation points. In your threads, use pthread_setcanceltype(PTHREAD_CANCEL_DEFERRED, &oldstate); (this is the default, but it never hurts to be explicit); then disable cancellation with pthread_setcancelstate except for over the condition variable waits you want to be cancellable. Be sure that you use pthread_cleanup_push to set up cancellation cleanup handlers; this will NOT play nicely with RAII.

And now you can just pthread_cancel your thread. Cancellation cleanup handlers are executed, in reverse order of registration, TLS data destructors are called, and the thread exits (without returning from the condition variable wait).

Of course, this is a rather ugly design. Ideally you should avoid deadlocking at all; if that isn't possible, if it were me, I would arrange for only one thread to ever block on a single cvar at a time, and build a higher level (explicit waiter list) construct based on these cvars in order to handle multiple waiters, while still allowing for threads to be individually addressable.

Upvotes: 1

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