Re: [HACKERS] [PATCHES] Compiling libpq with

Magnus Hagander wrote:

>>>>What is the recommended way to create mutex objects 
>>>>        
>>>>
>>(CreateMutex) from
>>    
>>
>>>>Win32 libraries?  There must be a clean way like there is 
>>>>        
>>>>
>>in pthreads.
>>    
>>
>>>>   
>>>>
>>>>        
>>>>
>>>A mutex is inherently a global object. CreateMutex(NULL, 
>>>      
>>>
>>FALSE, NULL) 
>>    
>>
>>>will return a handle to an unowned mutex.
>>>
>>> 
>>>
>>>      
>>>
>>That's not the problem. Under pthread, it's possible to 
>>initialize a mutex from compile time:
>>
>>    static pthread_mutex_t init_mutex = PTHREAD_MUTEX_INITIALIZER;
>>
>>This means that the mutex is immediately valid, no races with 
>>the initialization. I couldn't find an equivalent Win32 feature.
>>    
>>
>
>AFAIK, there is no such thing on Win32. The clean way is probably to
>rqeuire the library to export a function InitialyzeFooLibrary() that
>does it (like Winsock does with requiring WSAStartup()).
>
>To do something like it though, you can use a named mutex. Then doing,
>in pseudocode:
>
>if (CreateMutex(...,"my_unique_mutex_name") == ERROR_ALREADY_EXISTS)
>   OpenMutex(...,"my_unique_mutex_name")
>
>Assuming nobody closes the mutex between your attempt to create and open
>(which shouldn't happen if you just ignore closing it until process
>exit), this should be safe.
>
>Store the HANDLE to the Mutex in TLS, and have each thread do the
>create/open when it needs the mutex (e.g. wrap the wait on the mutex in
>a function/macro that will create/open the mutex if it's
>INVALID_HANDLE_VALUE, which you assign it to by default).
>
>
>You need a unique name for the mutex, since it's not per-process but
>per-sessino. But that can easily be constructed from the pid.
>  
>

A libpq patch avoiding the InitializeFooLibrary() creating the mutex 
on-demand is in pgsql-patches already.

+#ifndef WIN32
     static pthread_mutex_t singlethread_lock = PTHREAD_MUTEX_INITIALIZER;
+#else
+    static pthread_mutex_t singlethread_lock;
+        static long mutex_initialized = 0;
+        if (!InterlockedExchange(&mutex_initialized, 1L))
+                pthread_mutex_init(&singlethread_lock, NULL); // wraps 
CreateMutex(NULL,FALSE,NULL)
+#endif

Regards,
Andreas


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