Implicit TLS variables in AsyncIO can cause access violation when dynamically
loading qpidcommon and qpidclient DLLs
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Key: QPID-1868
URL: https://issues.apache.org/jira/browse/QPID-1868
Project: Qpid
Issue Type: Bug
Components: C++ Client
Affects Versions: 0.5
Environment: Windows XP w/SP3
Visual Studio 2005
Reporter: David Rennalls
src/qpid/sys/windows/AsynchIO.cpp is using some implicit thread local storage
variables. If the qpid DLLs are dynamically loaded (or another DLL that
statically links in qpid) no space will be allocated for the TLS data. So if
client code tries to access one of the TLS variables defined in AsyncIO.cpp it
will get an access violation. Taken from [1]...
"..If a DLL declares any nonlocal data or object as __declspec( thread ), it
can cause a protection fault if dynamically loaded. After the DLL is loaded
with LoadLibrary, it causes system failure whenever the code references the
nonlocal __declspec( thread ) data. Because the global variable space for a
thread is allocated at run time, the size of this space is based on a
calculation of the requirements of the application plus the requirements of all
of the DLLs that are statically linked. When you use LoadLibrary, there is no
way to extend this space to allow for the thread local variables declared with
__declspec( thread ). Use the TLS APIs, such as TlsAlloc, in your DLL to
allocate TLS if the DLL might be loaded with LoadLibrary..."
...[2] also warns against this..
"Windows Server 2003 and Windows XP: The Visual C++ compiler supports a syntax
that enables you to declare thread-local variables: _declspec(thread). If you
use this syntax in a DLL, you will not be able to load the DLL explicitly using
LoadLibrary on versions of Windows prior to Windows Vista. If your DLL will be
loaded explicitly, you must use the thread local storage functions instead of
_declspec(thread)."
I ran into this when my DLL that links in qpid client and common libs
statically is loaded dynamically by a python wrapper, the first access to the
one of the TLS variables cause a crash. For reference , [3] (part 1 of 8) has a
good overview of TLS on Windows, and implicit vs explicit TLS
[1] http://msdn.microsoft.com/en-us/library/2s9wt68x(vs.71).aspx - Rules and
Limitations for TLS
[2] http://msdn.microsoft.com/en-us/library/ms684175.aspx - LoadLibrary function
[3] http://www.nynaeve.net/?p=180 - Thread Local Storage, part 1: Overview
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