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I am trying to synchronise two processes
I guess you mean threads not processes...
But in any case, synchronising two threads isn't really possible. At least not with any great accuracy.
A few scenarios:
The problem here is designing software that requires synchronisation.
The best you can do -- assuming I've understood your application -- is first signal the recorder to start recording; then have it signal the player to start playing; and accept that there will be some small portion of dead space at the lead in of the recording.
You can minimise the dead space by setting the priority of your threads to real time -- assuming your OS supports that possibility -- but that still doesn't guarantee that they will see and react to signals immediately.
Also, the way you are using cond_wait() is wrong. As you are locking the wait variable, before you wait on it -- a requirement of the (crappy*) cond_var mechanism) -- and lock()s are mutually exclusive, only one of your threads will be able to enter the wait-state at a time.
And, unless you cond_signal(), which ever thread gets in first will never wake up, so the second thread will never get a look in (to achieve a lock). I'm not sure how you think what you have should work?
I'm loathed to offer any solution at this point as it is not clear to me that what you are trying to do -- synchronously release two threads from wait states -- is either possible or desirable.
If you are absolutely adamant that is is a requirement, then you'll need to look into using the two var version of the cond_wait() and cond_broadcast(). And you'll need to use separate lock_vars for each thread, but a common sig_var.
Read the docs carefully and expect it to be hard to wrap your brain around. And even when you've got it working, understand that the release of the two signaled threads will not happen either instantaneously or exactly synchronously.
(*Not really Perl/iThreads designer's fault; they simply chose to reflect the underlying pthreads mechanisms. It is they that are crap.)
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