On Sun, 02 Jan 2005 10:22:04 +0100, Peter Karlsson <email@hidden> wrote:
> Dear list!
>
> 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.
OK, I'm assuming you have something like the following psuedo-code:
int sysexcounter;
SomeCallbackFunction()
{
sysexcounter = ...;
}
SomeOtherFunction()
{
//Running in some other thread
while(sysexcounter != predefined)
;
//Do something
...;
}
If so, you need two things:
1) sysexcounter needs to be volatile, otherwise, the compiler can
assume that it won't be changed by other threads, and the complier
will be perfectly free to optimize the above while loop to something
like:
if(sysexcounter != predefined)
{
while(1)
;
}
2) Some locking (as you suspected)
Try something like:
volatile int sysexcounter;
NSLock *sysexLock;
SomeInitializationFunction()
{
//Must be called before second thread is spawned
//Maybe from some class' +initialize method, or from main()
sysexLock = [[NSLock alloc] initialize];
}
SomeCallbackFunction()
{
[sysexLock lock];
sysexcounter = ...;
[sysexLock unlock];
}
SomeOtherFunction()
{
//Running in some other thread
BOOL keepGoing = YES;
while(keepGoing)
{
[sysexLock lock];
keepGoing = (sysexcounter != predefined);
[sysexLock unlock];
}
//Do something
...;
}
Essentially, the idea is that you *only* access sysexcounter between
calls to [sysexLock lock] and [sysexLock unlock].
--
Clark S. Cox III
email@hidden
http://www.livejournal.com/users/clarkcox3/http://homepage.mac.com/clarkcox3/
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