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Date: Wed, 5 Sep 2012 10:05:33 +0200
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Subject: Re: limitations of TLS using GCC's __thread keyword and Cygwin
From: =?UTF-8?Q?V=C3=A1clav_Zeman?= <vhaisman@gmail.com>
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On 4 September 2012 23:51, Christopher Faylor wrote:
> 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.
Adjust your beliefs. :) It is implemented and it works correct as far
as I can tell. It implements TLS using calls to __emutls* routines.
What I am unclear about is whether the implemention works around the
limitation mentioned in the MSDN link or not.

-- 
VZ

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