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Re: Curl_multi_perform goes into infinite loop

From: ajil koshy <>
Date: Mon, 1 Aug 2011 14:44:45 +0530

> On Thu, 28 Jul 2011, ajil koshy wrote:
> > I have an https client using libcurl (with openssl) multi APIs.
> > Occasionally, I see that if remote peer application exits, the libcurl
> > client's 'multi-loop' (the one which calls curl_multi_perform repeatedly
> > till running handles reach zero) goes into an infinite loop instead of
> > breaking out. Is this expected behavior?
> Without any kind of timeout set, I figure there's a risk that something
> stalls
> for what may be perceived as "infinitely".
> But also note that you didn't state what libcurl version or OS you're
> using.
I am using libcurl version 7.21.5 on windows.

> > Also, after each curl_multi_perform call, I also check if any errors
> > occurred on the individual transfers using curl_multi_info_read function
> in
> > which case I can exit the loop. I would like to know what is the
> definitive
> > way to find out if remote peer has disconnected (and thereby error out of
> > the multi-loop) while using libcurl?
> curl_multi_info_read() is the definitive way.

I am already calling curl_multi_info_read after calling curl_multi_perform
(as mentioned earlier). This should probably return some kind of error,
shouldn't it? The problem is that I am observing intermittent behavior here.
In some cases, I see that calling curl_multi_info_read yields a connection
closed error but on some other occasions, the loop just runs on and on.
Another observation is that the call to select () function used to wait on
inbound data returns SOCKET_ERROR if peer application has exited. However, I
cannot use this as a sufficient condition to break out of the multi-loop as
I have observed that select call fails while data transfer is in progress


> > I have come across articles which suggest we can attempt to read from the
> > socket which would but I am not sure of implications of doing this while
> > using libcurl.
> That's a really bad iadea. If you would read from a socket that is still in
> use by libcurl then you will effectively ruin that transfer. Not to mention
> that you would have to make an effort to even get to know the socket in
> tthe
> first place.
> > Is there any callback I can register for or info I can query in libcurl
> to
> > achieve this?
> You don't need any callback for that, curl_multi_info_read() is all you
> need.

(Note: Extremely sorry for re-posting. I didn't edit the subject last time.
Apologies to all for the spam and inconvenience.)


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Received on 2011-08-01