Launch telnet from Qt GUI application

richferrara

Running the telnet command, I see this in the Application Output tab:

true
false
"\r\nC:\[path to my executable]>" ""
QProcess: Destroyed while process ("cmd.exe") is still running

Running the dir command, I see the output of dir start displaying in the tab.

I only see anything when I run my executable through Qt Creator's Run menu. If I run outside Qt Creator, I see no output.

JonB

@richferrara
I really would expect

QStringList args = {"/c", "start telnet"};
QProcess::startDetached("cmd", {args});

to work, no? I am expecting this at least to open a terminal/console where you can interact with telnet. I do not expect it to get "destroyed". I expect it to work both inside and outside Creator. But I can't test anything.

richferrara

I figured it out.

system("start telnet") works.

JonB

@richferrara said in Launch telnet from Qt GUI application:

system("start telnet") works.

As should the code I suggested sticking with QProcess.

Christian Ehrlicher

Why one needs to run a cmd.exe at all?

Simply follow the docs

QObject *parent = new QObject;
...
QString program = "telnet";
QStringList arguments;
arguments << "172.99.99.99";

QProcess *myProcess = new QProcess(parent);
myProcess->start(program, arguments);

JonB

@Christian-Ehrlicher
The OP essentially claimed to have tried this and claimed it did not work:

The first thing I tried was just launching telnet directly:
QStringList args = {"172.99.99.99"}; // using my target IP address
QProcess::startDetached("telnet.exe", {args});

That is why I tried "building up" step by step with them to see what was happening. Have you tried this under Windows (which I cannot do)? The question is whether the Windows' telnet opens any kind of "terminal/console/command" window for itself, the suggestion is it does not and hence is "invisible"?

richferrara

The windows telnet command normally runs in its own terminal window. For whatever reason, QProcess seems to only be capable of running applications in the background.

JonB

@richferrara
QProcess should be fine. You could also try having it run, say, notepad, if you don't think it can run e.g. a UI process. Did you actually try either my code or @Christian-Ehrlicher's ? If the Windows telnet opens its own window then you ought not need to run it via cmd/start? I guess I will be quiet now as I cannot test under Windows, maybe @Christian-Ehrlicher will.

Christian Ehrlicher

@richferrara said in Launch telnet from Qt GUI application:

For whatever reason, QProcess seems to only be capable of running applications in the background.

This is wrong. Please show your code and don't use QProcess::startDetached() if you want to interact with your application.

JonB

@Christian-Ehrlicher said in Launch telnet from Qt GUI application:

and don't use QProcess::startDetached() if you want to interact with your application

Oh, really??

Christian Ehrlicher

@JonB said in Launch telnet from Qt GUI application:

@Christian-Ehrlicher said in Launch telnet from Qt GUI application:

and don't use QProcess::startDetached() if you want to interact with your application

Oh, really??

That's the whole point of this function - fire and forget
https://doc.qt.io/qt-6/qprocess.html#startDetached

JonB

@Christian-Ehrlicher said in Launch telnet from Qt GUI application:

and don't use QProcess::startDetached() if you want to interact with your application

and

That's the whole point of this function - fire and forget
https://doc.qt.io/qt-6/qprocess.html#startDetached

Hi Christian. I am sorry but I do not agree with your statements. Under Linux at least startDetached() causes the child process not to get killed when the parent process exits, but it is not true to say that means you do not use if you need interaction.

Testing under Linux at least, where telnet does not open its own window (maybe it does under Windows, I don't know) while, say, gedit does, I see the following behaviour:

start("telnet"): runs telnet (in the background), no window, exiting Qt app kills the telnet.
startDetached("telnet"): same as start(), but exiting Qt app does not kill the telnet.
start("gedit"): runs gedit, that creates its own window and interacts fine, exiting Qt app kills the gedit.
startDetached("gedit"): same as start(), but exiting Qt app does not kill the gedit.

If I want a Linux spawned telnet to be visible and have a window I have to use something like e.g. xterm -e telnet as the command. And again that behaves as above: dies on parent exit with start(), continues to run afterwards with startDetached(), but same interaction in both cases.

Maybe it's different under Windows and/or with telnet there, but saying that "and don't use QProcess::startDetached() if you want to interact with your application" is not the story under Linux at least. Which is why I expressed my surprise when you wrote that, as not my experience in Linux.

Emma-2000

QProcess::startDetached() is probably the better approach here if the goal is to keep the Qt GUI responsive while the console runs independently.

I would try launching cmd.exe and using start so that Windows creates a separate console process:

QStringList args = {"/c", "start", "telnet.exe", "172.99.99.99"};
bool ok = QProcess::startDetached("cmd.exe", args);
qDebug() << "started:" << ok;

The important part is start, because simply running cmd.exe /c telnet.exe ... may not give you the separate interactive window you're expecting. startDetached() also means the child process can continue after the Qt application exits.

If telnet.exe isn't available through PATH, I'd also try using its full path.

JonB

@Emma-2000 Is that not exactly what I wrote above at https://forum.qt.io/post/839250 ?