CODEWITHSUNDEEP
c++qtqprocess
chteng2002
chteng2002

Reputation: 13

Trouble with live communication with QProcess and VS C++

I'm trying to establish a live communication between a QT program and a VS C++ program. However, I can read anything unless I close the writechannel in which I can't write anything anymore. Furthermore, the code I have now reads a continuous stream of output when I write one line to the VS C++ program when it should be waiting for the next input. Is there a way to establish synchronous communication with the two? What is wrong with my current program?

I've read documentation and can't seem to get a clear answer.

My Qt code:

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);
    process = new QProcess(this);
    connect(process, SIGNAL(readyReadStandardOutput()), this, SLOT(readOutput()));
    //connect(process, SIGNAL(readyReadStandardError()),this,SLOT(readOutput()));
    process->setArguments(args);
    process->setProgram("C:\\Users\\chten\\OneDrive\\Desktop\\QProcess\\test\\Debug\\test.exe");
    process->start();
    process->waitForStarted();
}

MainWindow::~MainWindow()
{
    delete ui;
}




void MainWindow::readOutput() {
    ui->input->append(process->readAllStandardOutput());
}

void MainWindow::on_pushButton_2_pressed()
{
    process->write("left");
    process->waitForBytesWritten();
    process->closeWriteChannel();

}

My C++ code:

    cout << "waiting for response..." << endl;
    string input_line;
    //getline(cin, input_line);
    //cout << input_line << endl;
    while (true) {
        getline(cin, input_line);
        cout << input_line << endl;

        for(int i = 0; i<9999999; i++){}
    }
    return 0;

Upvotes: 1

Views: 205

Answers (1)

G.M.
G.M.

Reputation: 12879

I think the problem is that you never write a line to the child process, you just have...

process->write("left");

No newline. In the meantime the child is executing...

getline(cin, input_line);

waiting for the newline delimiter.

The reason closing the write channel appears to work is that it will cause the getline call in the child to receive an end-of-file condition and return. However, it will also set the eofbit in the input stream's state causing further calls to getline to return immediately: hence the "continuous stream of output" you refer to.

Try changing the implementation of MainWindow::on_pushButton_2_pressed to...

void MainWindow::on_pushButton_2_pressed ()
{
    process->write("left\n");
    process->waitForBytesWritten();
}

and change the child's source code to...

std::cout << "waiting for response..." << endl;
std::string input_line;
while (std::getline(std::cin, input_line)) {
    std::cout << input_line << std::endl;

    for(int i = 0; i<9999999; i++) {
    }
}
return 0;

(All untested.)

As an aside, using Qt functions such as waitFoStarted, waitForBytesWritten etc. can be convenient but should be avoided. Far better to connect to and handle the various signals available.

Upvotes: 3

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