Can some please explain how I use NSLock to protect a variable from
being
used or destroyed by another thread?
My app have 2 threads and a readProc. The normal thread that is always
there and a second thread that starts when I want to request a sysex
dump
from my synthesizer.
The variable 'sysexcounter' is used in 2 places.
1 - 'sysexcounter' is incremented In my readProc that get the MIDI
bytes
2 - In my second thread my app waits for 'sysexcounter' to get a
predefined
value that is the size of my sysex message before the code flow
continues.
But it seems that something happens that makes 'sysexcounter' useless.
So I
want to protect it from being used by 2 places at the same time. I
think
that is the right way to explain my problem.
Clark Cox's solution will work, but if I understand you correctly, you
are implementing producer/consumer threads and so you really want to
sleep one thread while waiting for a sysex message. In this case, you
should use NSConditionLock. The code should look something like this:
enum {
kEmpty,
kFull
};
// Thread 1
while(1) {
...get more data here
[lock lock];
status = (sysexcounter == sizeof_sysex_message) ? kFull : kEmpty;
[lock unlockWithCondition:status];
}
Though I should point out that simply protecting sysexcounter is not
sufficient if I understand your needs correctly. You also need to
protect whatever buffer you're reading/writing into as well. In that
case, it's probably easier to make a queue out of an NSMutableArray and
a NSConditionLock which just contains NSData's from the MIDI thread.
Then you know that all the data is only owned by one thread at a time.
Brendan Younger
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