Child process stdin doesn't get data, sent by parent process

Question

Parent process write string "Message\n" to child process stdin. But child process don't receive it. Where is the problem in the code?

Qt 4.7.3

Parent process code:

// class TestParent : public QMainWindow
void TestParent::createChildProcess()
{
    childProcess = new QProcess( this );
    connect( childProcess, SIGNAL( started() ),
        this, SLOT( childProcessStarted() ) );
    connect( childProcess, SIGNAL( bytesWritten( qint64 ) ),
        this, SLOT( bytesWritten( qint64 ) ) );
    childProcess->start( "TestChild.exe", QProcess::ReadWrite );
}

void TestParent::writeToChildProcessOutput()
{
    qint64 bytesWritten = childProcess->write( "Message\n" );
    qDebug() << "ret: " << bytesWritten << " bytes written";
}

void TestParent::bytesWritten()
{
    qDebug() << "slot: " << bytesWritten << " bytes written";
}

Child process code:

// class TestChild : public QMainWindow
void TestChild::TestChild()
    // QFile TestChild::input;
    connect( &input, SIGNAL( readyRead() ),
        this, SLOT( readInput() ) );
    input.open( 0, QIODevice::ReadOnly ); // stdin
}

void TestChild::readInput()
{
    QString line;
    line.append( '(' );
    line.append( QString::number( input.bytesAvailable() ) )
    line.append( ')' );
    line.append( input.readAll() );

    list.append( line ); // add line to QListView
}

Answer 1

There is no way to portably hookup in the Qt event loop for stdin/stdout events. The following works on non-Windows platforms:

QSocketNotifier *n1 = new QSocketNotifier(0, QSocketNotifier::Read, this);
connect(n1, SIGNAL(activated(int)), this, SLOT(readInputChannel()));

QSocketNotifier *n2 = new QSocketNotifier(0, QSocketNotifier::Exception, this);
connect(n2, SIGNAL(activated(int)), this, SLOT(brokenInputChannel()));

"0" is the file descriptor (stdin).

I would use the above and then simulate something similar on Windows through a blocking thread that reads from stdin and generates a signal:

class StdinThread : public QThread
{
    Q_OBJECT
signals:
    void incomingData(QByteArray data);

public:
    void run(void)
    {
         char buf[1024];
         while (1)
         {
             int sz = fread(buf, 1, 1024, stdin);
             if (sz == 0)
                return;
             emit incomingData(QByteArray(buf, sz));
         }
     }
};

then, in the child process:

StdinThread *t = new StdinThread(this);
connect(t, SIGNAL(incomingData(QByteArray)), this, SLOT(processInputData(QByteArray)));
connect(t, SIGNAL(finished()), this, SLOT(brokenInputChannel()));
t->run();

Answer 2

The documentation says that QFile doesn't ever emit the signal readyRead().

But there is a private class: QWinEventNotifier in src/corelib/kernel/qwineventnotifier_p.h(link) that might work with GetStdHandle(STD_INPUT_HANDLE).

Another alternative is to wait for input with a blocking loop inside a dedicated thread:

QTextStream cin(stdin, QIODevice::ReadOnly);
while(!cin.atEnd())
{
   QString line = cin.readLine();
   emit newLine(line);
}

You could also look at other RPC methods (e.g. QLocalSocket, QSharedMemory).