Read output of multiple write in QProcess one by one - qt

Question

Can anyone help me read the output of qprocess after write and loop until all task is done?
I have this code
wifi->write("scan\n");
wifi->closeWriteChannel();
wifi->waitForBytesWritten(100);
wifi->waitForReadyRead(100);
wifi->waitForFinished(100);
qDebug() << "read output" << wifi->readAllStandardOutput();
wifi->write("scan\n");
wifi->closeWriteChannel();
wifi->waitForBytesWritten(100);
wifi->waitForReadyRead(100);
wifi->waitForFinished(100);
qDebug() << "read output" << wifi->readAllStandardOutput();
the expected output must be
"OK"
"scan results"
but the ouput is
"OK"
""
thanks.

Answer 1

Your multiple waits are not useful for anything. All you care about is when the process finishes, so have a single waitForFinished call with a much longer timeout (those scans don't happen in ~100ms, a few seconds is a good minimum).
You should not be using the blocking waitForXxx methods. They trip up everyone and are a source of unending grief. Forget that they exist. Use process's signals to react to events as they happen.
Qt 5 + C++11
This is the way forward. This is why you should insist on using a modern development environment, if you can. It's less typing and easier to understand.
void MyObject::startWifi() {
auto process = new QProcess(this);
process->start("program", QStringList() << "argument");
connect(process, &QProcess::started, [process]{
process->write("scan\n");
process->closeWriteChannel();
});
connect(process, &QProcess::finished, [process]{
qDebug() << process->readAllStandardOutput();
process->deleteLater();
});
}
Qt 4
class MyObject : public QObject {
Q_OBJECT
QProcess m_wifi;
Q_SLOT void onStarted() {
m_wifi.write("scan\n");
m_wifi.closeWriteChannel();
}
Q_SLOT void onFinished() {
qDebug() << m_wifi.readAllStandardOutput();
}
public:
MyObject(QObject * parent = 0) : QObject(parent) {
connect(&m_wifi, SIGNAL(started()), SLOT(onStarted()));
connect(&m_wifi, SIGNAL(finished(int,QProcess::ExitStatus)),
SLOT(onFinished()));
}
Q_SLOT void start() {
m_wifi.start("program", QStringList() << "argument");
}
};
Then invoke the start method/slot on an instance of this object. That's all.