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From "Andrew Stitcher (JIRA)" <qpid-...@incubator.apache.org>
Subject [jira] Commented: (QPID-1868) Implicit TLS variables in AsyncIO can cause access violation when dynamically loading qpidcommon and qpidclient DLLs
Date Thu, 21 May 2009 18:23:45 GMT

    [ https://issues.apache.org/jira/browse/QPID-1868?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12711741#action_12711741

Andrew Stitcher commented on QPID-1868:

As noted on the devlist (dev@qpid.apache.org) these thread locals vars are not used overall
in the functioning of the windows port IO. So they could be removed entirely, which would
make a much simpler fix.

However qpid definitely contains some other thread local usage (though possibly not in the
windows port) and this issue would occur there. Of course we could just say anything prior
to Vista/Win 2008 is not supported!

> Implicit TLS variables in AsyncIO can cause access violation when dynamically loading
qpidcommon and qpidclient DLLs
> --------------------------------------------------------------------------------------------------------------------
>                 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
>         Attachments: AsyncIO_TLS.patch
> 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
> ...[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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