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Date: Tue, 15 Mar 2011 16:01:35 -0400
From: Charles Wilson <cygwin AT cwilson DOT fastmail DOT fm>
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Subject: Re: Calling sem_wait() in DllMain( DLL_THREAD_DETACH) crashes
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On 3/15/2011 3:07 PM, Christopher Faylor wrote:
> On Tue, Mar 15, 2011 at 06:34:27PM +0000, Jon TURNEY wrote:
>>
>> I've come across a problem whilst trying to run the twisted test suite, where
>> some tests just cause python to segfault.  It seems to be the same issue with
>> libcrypto as reported in [2],[3]

> Actually you aren't supposed to be able to call anything you like here and
> it isn't clear why a POSIX/UNIX/Linux program would be relying on DllMain.

There are cases where cygwin-ish code does win32-ish things, like
login.exe, or the PyWin32 extensions for Python-on-Windows(and cygwin).
Maybe twisty uses PyWin32?

However, it does seem that python (or one of its extensions) is doing
something it shouldn't.

From http://msdn.microsoft.com/en-us/library/ms682583%28v=vs.85%29.aspx
> The entry-point function should perform only simple initialization 
> or termination tasks. It must not call the LoadLibrary or LoadLibraryEx
> function (or a function that calls these functions), because this may
> create dependency loops in the DLL load order. This can result in a DLL
> being used before the system has executed its initialization code.
> Similarly, the entry-point function must not call the FreeLibrary
> function (or a function that calls FreeLibrary) during process
> termination, because this can result in a DLL being used after the
> system has executed its termination code.
>
> Because Kernel32.dll is guaranteed to be loaded in the process
> space when the entry-point function is called, calling functions in
> Kernel32.dll does not result in the DLL being used before its
> initialization code has been executed. Therefore, the entry-point
> function can call functions in Kernel32.dll that do not load other DLLs.
> For example, DllMain can create synchronization objects such as critical
> sections and mutexes, and use TLS. Unfortunately, there is not a
> comprehensive list of safe functions in Kernel32.dll.
> 
> Windows 2000: Do not create a named synchronization object in
> DllMain because the system will then load an additional DLL.
> 
> Calling functions that require DLLs other than Kernel32.dll may
> result in problems that are difficult to diagnose. For example,
> calling User, Shell, and COM functions can cause access violation
> errors, because some functions load other system components.
> Conversely, calling functions such as these during termination can
> cause access violation errors because the corresponding component may
> already have been unloaded or uninitialized.
>
> Because DLL notifications are serialized, entry-point functions
> should not attempt to communicate with other threads or processes.
> Deadlocks may occur as a result.

--
Chuck


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