Prevent multiple TCP segments in post request

[vicsoftware]

It is Ubuntu 18, Nvidia Jetson AGX, Arm64

Requests executes on same system, why curl and chrome sends single valid post request, but QT sends splitted segments and 2 requests?

[Christian Ehrlicher]

@vicsoftware said in Prevent multiple TCP segments in post request:

but QT sends splitted segments and 2 requests?

It does not send two requests. It sends one request with two tcp packets. Maybe it's how write() is called on the socket or the moon phase or whatever.

[vicsoftware]

@Christian-Ehrlicher No. QT sends 2 request until "closed connection" error and every request has 2 TCP segments.
Curl and chrome sends single request with 1 tcp segment.

Here is 2 first QT request, third is chrome request forth is curl.

As we see length is 64 bytes for QT, and 185 for chrome and 222 for curl.

But QT post has more headers and it shoud be longer bytes. It shows 64 bytes but:

Really tcp has 235 bytes.

[Christian Ehrlicher]

No. QT sends 2 request

This is a new information.

Qt only send two requests when you programmed it to do so.

[vicsoftware]

@Christian-Ehrlicher No. I execute only single post from code. QT trying send second post before closing connection. It's look like.

If it is only attempts to connect so it is not a problem.
Problem with bad post from QT so server does not understand request.

[Christian Ehrlicher]

@vicsoftware said in Prevent multiple TCP segments in post request:

QT trying send second post before closing connection. It's look like.

It does not send a http request a second time when you don't ask for it.

[vicsoftware]

@Christian-Ehrlicher Sorry but no. It is definitely QT behavior to resend request. I think. I am doing only one call:

const auto reply{ m_networkMan->post( request, data.toUtf8() ) };

It is pushButton handler and it has single debug std::cout message only before connection error emitted.

QT GET request works and sends only one time.

[Christian Ehrlicher]

Then please provide a minimal, reproducible example for us. I don't see why a http request should be sent twice when the other side does not request it.

[vicsoftware]

@Christian-Ehrlicher If server powered off then after longer time i got Request error: "Host unreachable" and there is no requests at all in wireshark.

[vicsoftware]

Why QT makes 2 tcp segments but other methods has 1 tcp segment. Someone can explain QT post behavior?

[Christian Ehrlicher]

@vicsoftware said in Prevent multiple TCP segments in post request:

Why QT makes 2 tcp segments but other methods has 1 tcp segment. Someone can explain QT post behavior?

Again: this has not much to do with Qt but with the underlying os. Maybe Qt writes the packet in two parts to the OS and curl only one. But this is completely irrelevant.

[vicsoftware]

@Christian-Ehrlicher And chrome extension only one. And python sends normal request. But QT does not. And sends it twice. And packet length wrong. Why?

[vicsoftware]

Have tried online post service, and same QT code sends only single post request with 2 tcp segments.

[vicsoftware]

This post is deleted!

[vicsoftware]

Simple curl code works:

const std::string url{ "http://" + m_addr + "/od/2400/01" };
        CURL *curl;
        CURLcode res;

        curl_global_init(CURL_GLOBAL_ALL);

        curl = curl_easy_init();
        if(curl) {
            curl_easy_setopt(curl, CURLOPT_URL, url.c_str() );
            curl_easy_setopt(curl, CURLOPT_POSTFIELDS, "\"00000001\"");

            res = curl_easy_perform(curl);
            if(res != CURLE_OK)
                fprintf(stderr, "curl_easy_perform() failed: %s\n",
                        curl_easy_strerror(res));

            curl_easy_cleanup(curl);
        }
        curl_global_cleanup();

QNetworkManager post failed.

[J.Hilk]

@vicsoftware why do you post the code that "works" for you and not the one that doesn't!?!?!

[JonB]

@vicsoftware
Two comments:

• As @Christian-Ehrlicher has said, you need to produce (minimal, test) Qt code, not curl code, if you want someone to look at it as a Qt issue.

• You, and your program, are never supposed to care what is going on at the TCP packet level. Whether an application makes a request which gets passed as one TCP packet, or two, or other, should have no effect. We have all used TCP for years, with Qt or other toolkits, and never cared about this or noticed any behaviour such as you report with "errors" occurring. Again, for the record, it is basically down to the OS/TCP level how it chooses to packet-ise messages. Artefacts such as Nagle's algorithm will commonly affect the runtime packet behaviour.

[vicsoftware]

@J-Hilk QT code posted above.

[vicsoftware]

@JonB I have nanotec.com tcp controller C5-E-2-81. It has REST API to control driver. So i can only get/post. And i can't reproduce for you it. You do not have that controller.
I tried online service and it works. But that service do not dispatch input data, it only receives post so it is not the same as controller.

In this case, the question is why all other methods do but QT does not? It simple post but it does not work.

[mchinand]

Why does your curl data, "\"00000001\"=" and Qt data, "\"00000002\"" both have a content length of '10' (at least that's what you put in the request header for both)? Will you ever include a 'value' with your form request (i.e., don't form requests usually consist of key-value pairs)?