I am having some timeout issue using libcurl in C/C++.

The device I am working on talks with a server, through a GSM modem, using
POST messages over https (or http).
This is always the device which initiate the communication with the server,
by sending a POST message.
Once the server receive the message, it will reply back with another
message to the device.
It might happen that the server has to delay replying to the device, but
the delay will never exceed three minutes.

What I am seeing:
- once the GSM link is established (pppd), the server *always* receive the
POST message sent by the device.
- 'most of the time', the device receive the reply sent by the server, even
if there is no delay in replying... But not always: in about 20 to 25% of
the cases, the device do not receive the message back from the server, and
times out.

I have control on both sides (device and server).
On the server side, I am using libmicrohttpd.
When the device did not receive the reply back from the server, sending
back the reply does not trigger any error or warning on the libmicrohttpd
side...

Here is a snapshot of the code I am using on the device side, using the
latest libcurl (I simplified it);

    struct curl_httppost *formpost = NULL;
    struct curl_httppost *lastptr = NULL;
    struct curl_slist *headerlist = NULL;

       // Fill in the file upload field
    curl_formadd( &formpost,
        &lastptr,
        CURLFORM_COPYNAME, "request",
        CURLFORM_FILE, SERVER_REQUEST_XML,
        CURLFORM_END );

    // Fill in the filename field
    curl_formadd( &formpost,
           &lastptr,
           CURLFORM_COPYNAME, "filename",
           CURLFORM_COPYCONTENTS, "request.xml",
           CURLFORM_END );

    // Fill in the submit field too, even if this is rarely needed
    curl_formadd( &formpost,
        &lastptr,
        CURLFORM_COPYNAME, "submit",
        CURLFORM_COPYCONTENTS, "send",
        CURLFORM_END );

    CURLcode curl_code;

    headerlist = curl_slist_append( headerlist, "Expect:" );
    headerlist = curl_slist_append( headerlist, "Keep-Alive: 300" );
    headerlist = curl_slist_append( headerlist, "Cache-Control: max-age=0"
);
    CURL_EASY_SETOP( curl, CURLOPT_HTTPHEADER, headerlist );

    CURL_EASY_SETOP( curl, CURLOPT_VERBOSE, 1L );
    CURL_EASY_SETOP( curl, CURLOPT_DEBUGFUNCTION, debug_function );
    CURL_EASY_SETOP( curl, CURLOPT_DEBUGDATA, curl_debug_fp );
    CURL_EASY_SETOP( curl, CURLOPT_READFUNCTION, read_callback );

    CURL_EASY_SETOP( curl, CURLOPT_POST, 1L );
    CURL_EASY_SETOP( curl, CURLOPT_HTTPPOST, formpost );

    CURL_EASY_SETOP( curl, CURLOPT_FOLLOWLOCATION, 1 ) ;
    CURL_EASY_SETOP( curl, CURLOPT_SSL_VERIFYPEER, 0L );
    CURL_EASY_SETOP( curl, CURLOPT_SSL_VERIFYHOST, 0L );
    CURL_EASY_SETOP( curl, CURLOPT_SSL_SESSIONID_CACHE, 0L );

    CURL_EASY_SETOP( curl, CURLOPT_WRITEDATA, (void *)fp_result );
    CURL_EASY_SETOP( curl, CURLOPT_WRITEFUNCTION, write_callback );

    CURL_EASY_SETOP( curl, CURLOPT_POSTFIELDSIZE, request_len );

    CURL_EASY_SETOP( curl, CURLOPT_DNS_CACHE_TIMEOUT, 30L );
    CURL_EASY_SETOP( curl, CURLOPT_TIMEOUT, timeout );
    CURL_EASY_SETOP( curl, CURLOPT_CONNECTTIMEOUT, 30L );

    CURL_EASY_SETOP( curl, CURLOPT_TCP_KEEPALIVE, 5L );
    CURL_EASY_SETOP( curl, CURLOPT_TCP_KEEPIDLE, 5L );
    CURL_EASY_SETOP( curl, CURLOPT_TCP_KEEPINTVL, 30L );

    CURL_EASY_SETOP( curl, CURLOPT_URL, url );

    CURLcode res = curl_easy_perform( curl );

When the device misses the reply back from the server, it times out
(CURLE_OPERATION_TIMEDOUT), and never receives the message.
What I do not understand is that netstat shows the TCP connection
established on both sides...

If I do not setup a timeout, the device never receives the message and
hangs forever, at least until the GSM link is cut off...

Any idea what I am missing, or doing wrong ?

Many TIA if somebody could point me in the right direction!
Olivier

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Received on 2013-10-07