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Re: Semaphore strangeness
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Re: Semaphore strangeness


  • Subject: Re: Semaphore strangeness
  • From: Charles Srstka <email@hidden>
  • Date: Wed, 17 Apr 2013 19:44:57 -0500

On Apr 17, 2013, at 7:06 PM, Greg Parker <email@hidden> wrote:

> 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.

Then why not use something like an assertion or an exception which could actually let the user / developer know why you crashed, instead of just EXC_BAD_INSTRUCTION?

Charles

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  • Follow-Ups:
    • Re: Semaphore strangeness
      • From: Greg Parker <email@hidden>
References: 
 >Semaphore strangeness (From: Quincey Morris <email@hidden>)
 >Re: Semaphore strangeness (From: Greg Parker <email@hidden>)

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