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Date: Tue, 04 Sep 2012 10:39:31 -0400
From: Ryan Johnson <ryan DOT johnson AT cs DOT utoronto DOT ca>
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Subject: Re: limitations of TLS using GCC's __thread keyword and Cygwin
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On 04/09/2012 8:58 AM, Václav Zeman wrote:
> Hi.
>
> I am am porting a library that can use the __thread keyword in its
> internals to provide thread local storage. Now, with MSVC there is a
> limitation on pre-Vista Windows (see [1]) that DLLs using
> __declspec(thread) (MSVC equivalent of GCC's __thread) cannot be
> loaded using LoadLibrary() because pre-Vista Windows allocate the TLS
> declared that way only on process startup. Vista and later Windows do
> not seem to have the limitation. Since Cygwin officially still
> supports at least Windows XP, I want to provide a library that works
> there as well.
>
> Does Cygwin's GCC and its TLS emulation work around this problem? IOW,
> are Cygwin DLLs using TLS declared using __thread keyword safe to be
> loaded using LoadLibrary()/dlopen() or are they not safe to be loaded
> that way?
>
> [1] http://msdn.microsoft.com/en-us/library/2s9wt68x.aspx
>
I suspect it's not a problem, but if I were you I'd write a simple test 
program to see. Unfortunately, TLS in general seems broken on my machine 
when I tried it, but that might be due to my home-brew gcc being 
configured wrong or something.

Ryan


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