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Date: Mon, 17 Sep 2007 14:27:31 -0600
From: Martin Sebor <sebor@roguewave.com>
Organization: Rogue Wave Software, Inc.
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To: stdcxx-dev@incubator.apache.org
Subject: Re: [PATCH] std::messages thread safety
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Travis Vitek wrote:
> Martin Sebor wrote:
>> Travis Vitek wrote: 
>>> Martin Sebor wrote:
>>>
>>>> AFAIK, catopen() and catgets() are not required to be thread
>>>> safe so we need to guard calls to them across all instances of
>>>> the facet.
>>>>
>>> Yes, but the affected functions are doing more than just calling
>>> catgets() and catopen(). Since they are using shared data, they
>>> need to lock that shared data with a shared mutex.
>> And unless I'm missing something, they are.
>>
>>     _RWSTD_MT_STATIC_GUARD (__rw_open_cat_data);
>>
>> This provides mutual exclusion across all othe such guards
>> (i.e, those with the same type as an argument).
>>
> 
> Nope. See _defs.h...

Ouch! You're right. My bad for misremembering, and for misnaming
the macros in the first place.

I withdraw my objection to the patch :)

Martin

> 
>   // synchronizes access by all objects holding the same mutex
>   #  define _RWSTD_MT_GUARD(mutex)  \
>             _RW::__rw_guard _RWSTD_PASTE (__guard, __LINE__) (mutex)
> 
>   // synchronizes access by all threads holding the same mutex 
>   #  define _RWSTD_MT_STATIC_GUARD(type)                           \
>             typedef _RW::__rw_type<type,__LINE__> _UniqueType;     \
>             _RWSTD_MT_CLASS_GUARD(_UniqueType)
> 
>   // synchronizes access by all objects of the same type
>   #  define _RWSTD_MT_CLASS_GUARD(type)                            \
>             _RWSTD_MT_GUARD (_RW::__rw_get_static_mutex ((type*)0))
> 
> _RWSTD_MT_CLASS_GUARD locks a static mutex that unique for the type
> provided. This provides the behavior that you are describing above.
> 
> _RWSTD_MT_STATIC_GUARD creates a unique type given a type and a line
> number. That new type is then used to get a handle to a static mutex.
> The only time two _RWSTD_MT_STATIC_GUARD lines are guarding the same
> mutex is if they happen to be on the same line number and have the
> same type argument.
> 
> Travis