On Wed, Sep 17, 2008 at 03:37:17PM +0200, Aurélien wrote:
> I'm using libcurl in a multi-threaded monitoring application that is using cURL
> to make reports via HTTP. I read the documentation about threading and libcurl,
> and deduced the following things:
>
> - cURL should be initialized and released via curl_global_*() functions, when
> there's only the main thread running.
> - A cURL handle should not be used by more than one thread at a time.
> - If using HTTPs, proper locking functions should be installed in the openssl
> library.
>
> Maybe I already missed or misunderstood something here.

Sounds like you're on the right track here. Don't forget about disabling
signals, though.

> Anyway, after libcurl is initialized as advised via curl_global_*() function -
> I'm using normal HTTP without authentication, so no need for openssl stuff, my
> application's threads are using a single function that deals with libcurl. This
> function makes a HTTP request and immediately returns the result (content +
> return code) to the caller. It can be summarized in pseudo-code as:
>
> string fetchURL(string URL, string postdata)
> {
> lock() // I'm using a pthread_mutex specific to this function.
> curl_easy_setopt(CURLOPT_WRITEFUNCTION...);
> curl_easy_setopt(CURLOPT_WRITEDATA...);
> curl_easy_setopt(CURLOPT_POSTFIELDS...);
> curl_easy_setopt(CURLOPT_HTTPHEADER...);
> curl_easy_setopt(CURLOPT_URL...);
>
> curl_easy_perform(...);
> ...
> unlock()
> }
>
> Each running thread can call this function at any time, hence the locking. If
> you need any functional details about my application, feel free to ask.

As long as you provide for mutual exclusion like this, you should be fine.
That's assuming that the other threads aren't using any of libcurl's
dependent libraries in a non-thread-safe manner, of course.

> My problem is that, if the connection to the server is lost (modem problem,
> network cable unplugged, etc), sometimes the thread currently detaining the
> lock never gets out of curl_easy_perform. I have compiled libcurl with
> debugging information and got the following backtraces:

"never" gets out? How long did you wait? TCP is designed to cope with
network outages like this by retrying, potentially for a long time. If
you don't like that, you can set a timeout option like CURLOPT_LOW_SPEED_LIMIT
to make libcurl abort early.

>>> Dan

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