Qt startDetached fails under Qt 5.15.2

Question

I'm updating some old Qt 5.6 code to Qt 5.15.2

The Qt C++ code below opens a dos prompt, the runs a bat file, staying open when done.

cstring = "cmd /k " +QDir::currentPath()+"/cpath.bat";

QProcess::startDetached(cstring);

This code works fine under 5.6, does nothing under 5.15.2

How do I fix this for Qt 5.15.2?

Answer 1

You need to split the command to program and arguments. See https://doc.qt.io/qt-5/qprocess.html#startDetached-1

In your case it should be

QProcess::startDetached("cmd", {"/k", QDir::currentPath() + "/cpath.bat"});

The advantage is that now you do not need to worry for example about enclosing paths containing spaces in quotes. Consider if your current path would be "C:\my path". Then your version from Qt 5.6 would not work. While the version I presented in this answer will work out of the box. I guess it was exactly this reason that the old overload was prone to this kind of bugs that Qt decided to remove it and use only the new overload.

Answer 2

Ok, with my previous answer I though that the problem was with the way you are passing the arguments. But as it seems the problem is with showing the terminal... Since I am doing the same in one of my projects (HiFile file manager), I had a look how I am doing it. So here it is.

class DetachableProcess : public QProcess
{
public:
    explicit DetachableProcess(QObject *parent = nullptr) : QProcess(parent)
    {
    }

    bool detach()
    {
        if (!waitForStarted())
        {
            return false;
        }
        setProcessState(QProcess::NotRunning);
        return true;
    }
};

And then use it like this:

DetachableProcess process;
process.setCreateProcessArgumentsModifier( // this probably did the trick you need
    [](QProcess::CreateProcessArguments *args)
    {
        args->flags |= CREATE_NEW_CONSOLE;
        args->startupInfo->dwFlags &=~ STARTF_USESTDHANDLES;
    });

process.start("cmd", {"/k", QDir::currentPath() + "/cpath.bat"}); // here are your params...
process.detach();

I am afraid I cannot give more explanation why this complicated method works, I do not know it. And I do not know why it was changed in Qt.

I hope it will work for you. It works for me in Qt 6.2.4, which I am currently using.