Reload a DLL and _getptd returns zero

Michael Ost most at
Fri Jan 11 13:24:55 CST 2013

Hi list,

Does anyone know why _getptd() calls would return zero on a DLL that has 
been reloaded? Does something happen to the TLS used by _getptd?

At Muse Research we have been struggling with a deep bug for years where 
there is a crash _sometimes_ if you load a DLL a second time.

Finally (with a big tip of the hat to Julien Pommier at PianoTeq!) we 
have a test case that demonstrates a (if not _the_) crash.

The sequence is:
1. [main] start a thread
2. [main] load a library
3. [thread] call std::cout<< from the library
4. [main] free the library
5. [main] reload the library
6. [thread] call std::cout<< from the library
-> crash!

It looks like the crash happens because the MSVCRT function _getptd() is 
returning zero in step 6. This is dereferenced and crashes. In windows, 
there is no crash.

The library in question is using static vc runtime linkage, so _getptd() 
is linked in and I don't know exactly what it is doing. But Wine's 
MSVCRT implementation of _getptd() gets its data from TlsGetValue. And I 
can see that just before the crash there is a TlsGetValue call that 
returns zero.

Looking at TlsGetValue(), there must be something wrong with the values 
in TlsSlots. Maybe they persist for DLLs in Windows in a way they don't 
for Wine...? Or maybe Wine doesn't reinitialize them the same way when 
the DLL is reloaded?

Any hints, thoughts? clues? Thanks!

-- Michael Ost

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