[QPID-1868] Implicit TLS variables in AsyncIO can cause access violation when dynamically loading qpidcommon and qpidclient DLLs

• Type: Bug
• Status: Closed
• Priority: Major
• Resolution: Auto Closed
• Affects Version/s: 0.5
• Component/s: C++ Client

Environment

Windows XP w/SP3
Visual Studio 2005

• Assignee: Unassigned
• Reporter: David Rennalls
• Created: 21/May/09 05:19
• Updated: 14/Mar/18 20:27
• Resolved: 14/Mar/18 20:27

Description

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

Issue Links

• is related to: QPID-2206 thread-local storage prevents build on Mac