Re: [boost] scoped_lock weirdness?

From: John Zorko (jmzorko_at_[hidden])
Date: 2007-06-26 16:23:43

Giovanni,

Thanks for the quick response!  However, i'm still confused, since  
the Boost documentation seems to say that what i'm seeing will not  
happen.  Specifically, in the Threading Concepts documentation at  
http://www.boost.org/doc/html/thread/concepts.html, the following  
statements appear:

"Unchecked Locking Strategy
With an unchecked locking strategy, when a thread attempts to acquire  
a lock on a mutex object for which the thread already owns a lock the  
operation will deadlock. In general this locking strategy is less  
safe than a checked or recursive strategy, but it's also a faster  
strategy and so is employed by many libraries.

Boost.Thread does not currently provide any mutex objects that use  
this strategy."

Is there something that i'm just not understanding?

On Jun 26, 2007, at 1:08 PM, Giovanni Piero Deretta wrote:

> On 6/26/07, John Zorko <jmzorko_at_[hidden]> wrote:
>>
>> Hello, all ...
>> [...]
>> boost::mutex g_Mutex;
>>
>> void f1()
>> {
>>          boost::mutex::scoped_lock lock( g_Mutex );
>>
>>          std::cout << "1\n";
>> }
>>
>> int main(int argc, char* argv[])
>> {
>>          boost::mutex::scoped_lock lock( g_Mutex );
>>
>>          std::cout << "main\n";
>>
>>          f1();
>>
>>          return 0;
>> }
>>
>> ... under Windows XP SP2 (using VC 8.0), the output is this:
>>
>> main
>> 1
>>
>> ... however, under Mac OSX 10.4.10 (using Apple's gcc4.0.1), the
>> output is:
>>
>> main
>> (program hangs)
>>
>
> Trying to lock a mutex from a thread that already owns it in
> Boost.Threads is undefined behavior. Under Windows (because of the
> underlying implementation) it happens to work, but it deadlocks on
> other platforms.
>
> If you really want a recursive mutex you should use boost.thread
> recursive_mutex instead of plain mutexes. Anyways, consider that
> recursive mutexes are generally "Considered Harmful" [1]
>
> Also, probably this question belongs to the boost.users mailing list.
>
> HTH,
>
> gpd
>
> [1]:  http://zaval.org/resources/library/butenhof1.html
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Regards,

John

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