X-Recipient: archive-cygwin AT delorie DOT com
X-SWARE-Spam-Status: No, hits=-5.3 required=5.0	tests=AWL,BAYES_00,DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,FREEMAIL_FROM,KHOP_RCVD_TRUST,KHOP_THREADED,RCVD_IN_DNSWL_LOW,RCVD_IN_HOSTKARMA_YE
X-Spam-Check-By: sourceware.org
MIME-Version: 1.0
In-Reply-To: <20120904215129.GA30731@ednor.casa.cgf.cx>
References: <CAKw7uVgy4qgeqVMk1dRPb=Lr-so5NP3e6XjER=ZGqd+B+MT3ZQ AT mail DOT gmail DOT com>	<504612A3 DOT 6070605 AT cs DOT utoronto DOT ca>	<50466981 DOT 70005 AT gmail DOT com>	<20120904215129 DOT GA30731 AT ednor DOT casa DOT cgf DOT cx>
Date: Wed, 5 Sep 2012 10:05:33 +0200
Message-ID: <CAKw7uVgXkEktUOE_c6uhZXkZOng0DB6ja2mMHwPik+3iJemo3w@mail.gmail.com>
Subject: Re: limitations of TLS using GCC's __thread keyword and Cygwin
From: =?UTF-8?Q?V=C3=A1clav_Zeman?= <vhaisman AT gmail DOT com>
To: cygwin AT cygwin DOT com
Content-Type: text/plain; charset=UTF-8
X-IsSubscribed: yes
Mailing-List: contact cygwin-help AT cygwin DOT com; run by ezmlm
Precedence: bulk
List-Id: <cygwin.cygwin.com>
List-Unsubscribe: <mailto:cygwin-unsubscribe-archive-cygwin=delorie DOT com AT cygwin DOT com>
List-Subscribe: <mailto:cygwin-subscribe AT cygwin DOT com>
List-Archive: <http://sourceware.org/ml/cygwin/>
List-Post: <mailto:cygwin AT cygwin DOT com>
List-Help: <mailto:cygwin-help AT cygwin DOT com>, <http://sourceware.org/ml/#faqs>
Sender: cygwin-owner AT cygwin DOT com
Mail-Followup-To: cygwin AT cygwin DOT com
Delivered-To: mailing list cygwin AT cygwin DOT com

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

--
Problem reports:       http://cygwin.com/problems.html
FAQ:                   http://cygwin.com/faq/
Documentation:         http://cygwin.com/docs.html
Unsubscribe info:      http://cygwin.com/ml/#unsubscribe-simple