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Re: multi threads programming

From: Jiang Xu <jxu158_at_yahoo.com>
Date: Fri, 4 Jan 2008 17:53:26 -0800 (PST)

Well, I thought all the functions in libcurl are
thread safe. Doesn't it means that the data, handler,
should be protected for thread safety?

Thanks

Jiang

--- Richard Atterer <richard_at_2008.atterer.net> wrote:

> On Fri, Jan 04, 2008 at 03:47:33PM -0800, Jiang Xu
> wrote:
> > My question is: if the work thread is in the
> middle of the callback that
> > is called by curl_multi_perform(), what will
> happen if the control thread
> > calls curl_multi_remove_handle() at the same time
> and they are all
> > dealing with the same handle?
>
> All sorts of interesting things will happen! But you
> probably won't like
> the result. ;-)
>
> Isn't the whole idea of the control thread *not* to
> make calls to libcurl
> directly? Only the work thread should do this (and
> thus encapsulate the
> fact that curl is used for the download), and the
> two should communicate
> via message passing, monitors or similar.
>
> If you have e.g. a case where the worker thread is
> busy, but the user has
> decided to stop the download by clicking on a
> button, then the only option
> is to delay destroying the curl_easy download a bit:
> The control thread
> must send a message to the worker via a
> semaphore-protected message queue.
> Sooner or later, libcurl will return from
> curl_multi_perform(), and at that
> point the _worker_ should notice the new message and
> stop/destroy the
> curl_easy download.
>
> BTW, the worker can cause curl_multi_perform() to
> return earlier by
> returning the value 0 from its CURLOPT_WRITEFUNCTION
> which indicates to
> libcurl that something is wrong. In practice, you
> probably won't need to do
> that, it usually doesn't matter if the download
> continues to run for
> another 0.01 seconds...
>
> HTH,
>
> Richard
>
> --
> __ _
> |_) /| Richard Atterer | GnuPG key: 888354F7
> | \/| http://atterer.net | 08A9 7B7D 3D13 3EF2
> 3D25 D157 79E6 F6DC 8883 54F7
> '`
>

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Received on 2008-01-05