Re: [pgsql-hackers-win32] [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