limitations of TLS using GCC's __thread keyword and Cygwin

[Christopher Faylor]

On Tue, Sep 04, 2012 at 10:50:09PM +0200, V??clav Zeman wrote:
>On 09/04/2012 04:39 PM, Ryan Johnson wrote:
>> On 04/09/2012 8:58 AM, V??clav 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.
>I would have done that already but I do not have any Windows XP machine
>to try this on.

I don't believe that __thread is implemented on Cygwin.

cgf

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