Re: Semaphore strangeness
Re: Semaphore strangeness
- Subject: Re: Semaphore strangeness
- From: Greg Parker <email@hidden>
- Date: Wed, 17 Apr 2013 17:06:28 -0700
On Apr 17, 2013, at 4:14 PM, Quincey Morris <email@hidden> wrote:
> I'm not sure if this is the right list for this, but then I'm not sure what is the right list.
>
> I'm seeing a consistent problem disposing of GCD semaphores. For example, if I create a semaphore like this:
>
> semaphore = dispatch_semaphore_create (10);
>
> and then use if for a while, then try to destroy it (using ARC):
>
> semaphore = nil;
>
> If the semaphore's count is less than the original value (10 in the example), I get a EXC_BAD_INSTRUCTION crash.
>
> It's not clear to me why it should matter whether the original count has been restored. There's nothing waiting on the semaphore -- the operations that decremented the count have themselves be disposed of already.
>
> If I forcibly increment the count to 10 *or more*, there's no crash.
dispatch assumes you are using the semaphore in a lock-like pattern, where all waiters are expected to signal when they are done with their work. In that pattern, destroying a semaphore whose value is less than its original value indicates a bug somewhere (because somebody should have signaled the semaphore but has not yet done so). _dispatch_semaphore_dispose() is enforcing that assumption and deliberately halting your process if it fails.
--
Greg Parker email@hidden Runtime Wrangler
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