Date: Tue, 15 Mar 2011 16:28:17 -0400
From: Christopher Faylor <cgf-use-the-mailinglist-please AT cygwin DOT com>
To: cygwin AT cygwin DOT com
Subject: Re: Calling sem_wait() in DllMain( DLL_THREAD_DETACH) crashes
Message-ID: <20110315202816.GA8747@ednor.casa.cgf.cx>
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On Tue, Mar 15, 2011 at 04:01:35PM -0400, Charles Wilson wrote:
>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?

The python Twisted stuff works on UNIX systems so there should be no
reason to use Windows-isms in Cygwin code - especially when the
Windows-ism *calls* UNIX code.  The ability to get into trouble when you
mix Windows stuff like DllMain with Cygwin stuff is very high.

>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.

i.e., "you aren't supposed to be able to call anything you like here"

i.e., "you can't do that"

cgf

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