QProcess not working in QT Creator (using qt version 6.3.0)

BigBen

Hi.

I am trying to run a simple QProcess task in a simple application written on QT Creator.

The code is as follows:

void Structure::on_button1_clicked()
{
    p = new QProcess(this);
    p->start("dir C:/Users");
    p->waitForFinished();
    QString output = p->readAllStandardOutput();
    qDebug<<output;
}

However, the output I get is an empty string, even though the Users directory has a number of files and folders.
I don't know what could be going wrong in such a simple process.

I have already included the QProcess header.

JonB

@BigBen

QProcess::start(const QString &program, const QStringList &arguments = {}, QIODeviceBase::OpenMode mode = ReadWrite) does not accept a complete command line, only a single executable as its first argument. Change over to the correct new overload.
QProcess::readAllStandardOutput() is not going to read an error message. You must call QProcess::readAllStandardError() too. You also don't attach to signal QProcess::errorOccurred(). I think what you have will be generating one of these, possibly the latter.
dir is not an executable program under Windows. It is built into cmd.exe, which you must run. (Unless Qt handles this for you, I don't have Qt under Windows.)
dir C:/Users is wrong/lucky if it works under Windows. Use proper native \s for paths in QProcess commands, it does not know an argument is a path and will not translate /s to \s for you, unlike e.g. QFile.

p->start("cmd", { "/c", "dir C:\\Users" });
p->waitForFinished();
QString output = p->readAllStandardOutput();
QString error = p->readAllStandardError();

UPDATE
At Qt 6.0 it seems they introduced void QProcess::startCommand(const QString &command, QIODeviceBase::OpenMode mode = ReadWrite). So this would probably also work:

p->startCommand("dir C:\\Users")

BigBen

@JonB
Hi. Thank you for the explanation. The command p->start("cmd", { "/c", "dir C:\\Users" }); gives me the expected output.

However, p->startCommand("dir C:\\Users") gives me no error and no output.

I want to ask that in p->start("cmd", { "/c", "dir C:\\Users" });, what does the /c mean in the second argument?

JonB

@BigBen said in QProcess not working in QT Creator (using qt version 6.3.0):

However, p->startCommand("dir C:\\Users") gives me no error and no output.

Well that is interesting. As I said I only just discovered looking at the docs. It's new in Qt 6.0. I do not use Qt 6, nor Windows, so I cannot comment. If you look at he examples at https://doc.qt.io/qt-6/qprocess.html#startCommand you can see them claiming to issue dir commands that way, that's all I can say.

I want to ask that in p->start("cmd", { "/c", "dir C:\\Users" });, what does the /c mean in the second argument?

The Windows/DOS cmd.exe takes various arguments, as you can see from running cmd /?. Including:

/C      Carries out the command specified by string and then terminates

Basically without the /c it runs an interactive Command Prompt, the command-line console. With the /c it interprets and executes the command line string following it, i.e. your dir .... It does not open a Command Prompt window and it returns to your code immediately after executing the command. dir is built into cmd.exe --- along with many other "simple" commands like echo or mkdir --- there is no external dir.exe file to execute, so we need cmd /c dir to perform it.