QProcess and QLocalSocket - IPC messages aren't received

Question

Following the Fortune Client example.

Have a subclass of QLocalServer which starts the another application (client) using the Qprocess and this client will send some messages using the QLocalSocket. Only the first message from the client appears in the main (server) application that too only when the client process exits.

server app:

localserver.h

#ifndef LOCALSERVER_H
#define LOCALSERVER_H

#include <QLocalServer>

class LocalServer : public QLocalServer
{
    Q_OBJECT

public:

    explicit LocalServer(QObject* prnt = nullptr);
};

#endif // LOCALSERVER_H

localserver.cpp

#include "localserver.h"

#include <QLocalSocket>
#include <QDebug>

LocalServer::LocalServer(QObject* prnt)
    : QLocalServer(prnt)
{
    connect(this, &QLocalServer::newConnection, this, [&]() {
        qDebug() << "Socket connected";
        QLocalSocket* socket = this->nextPendingConnection();
        connect(socket, &QLocalSocket::disconnected, socket, &QLocalSocket::deleteLater);
        connect(socket, &QLocalSocket::readyRead, [&, socket]() {
            qDebug() << socket->readAll();
        });
    });
}

main.cpp (server)

#include <QCoreApplication>
#include <QProcess>

int main(int argc, char* argv[])
{
    QCoreApplication a(argc, argv);

    LocalServer::removeServer("testServer");
    LocalServer server;

    if (server.listen("testServer")) {
        QProcess process;
        process.setProgram("/home/ram/work/build/QtExamples/build-LocalSocketClient-Qt_5_12_1_Desktop-Debug/LocalSocketClient");
        process.start();

        if (process.waitForStarted() && process.waitForFinished()) {
            a.exit(0);
        }
    }

    return a.exec();
}

client app:

main.cpp (client)

#include <QCoreApplication>

#include <QLocalSocket>
#include <QTimer>
#include <QDebug>

int main(int argc, char* argv[])
{
    QCoreApplication a(argc, argv);

    QLocalSocket localSocket;

    QObject::connect(&localSocket, &QLocalSocket::connected, &localSocket, []() {
        qDebug() << "Socket connected";
    });

    localSocket.setServerName("testServer");
    localSocket.connectToServer();

    if (localSocket.waitForConnected()) {
        qDebug() << "Connected!";
        auto timer = new QTimer(qApp);
        QObject::connect(timer, &QTimer::timeout, &localSocket, [&localSocket]() {
            static int msgCount(0);
            QString msg(QString("Message from client %1").arg(++msgCount));
            qDebug() << localSocket.write(msg.toLatin1()) << msg;

            if (msgCount > 5) {
                qApp->exit(0);
            }
        });
        timer->start(1000);
    } else {
        return 1;
    }

    return a.exec();
}

When I run two applications separately i.e., without Qprocess part in main.cpp in server app, I see the following output.

Socket connected
"Message from client 1"
"Message from client 2"
"Message from client 3"
"Message from client 4"
"Message from client 5"

With QProcess all the five messages appear all at once.

Socket connected
"Message from client 1Message from client 2Message from client 3Message from client 4Message from client 5"

Am I missing anything here? Do QProcess need anymore info?

Also, i have tried using system() function as well - it behaved the same.

This really needs a [mcve] but I suspect at least part of the problem is caused by having two `QTimer` instances named `timer` one of which shadows the other. The one you are using will go out of scope and be destructed immediately after calling `timer.start`. — G.M., Apr 01 '19 at 11:10
@G.M. sorry, that was a typo. fixed it now. The missing part in this code is main functions. I'm not at my desk and just put out the code as much as I could. — ramtheconqueror, Apr 01 '19 at 11:12
Ok, but if the newly edited code is correct then the variable name shadowing has gone but the scoping problem remains -- you start the timer and it's then immediately destructed. — G.M., Apr 01 '19 at 11:19
change `if (cient.waitForStarted() &&` to `if (client.waitForStarted() &&` — eyllanesc, Apr 01 '19 at 11:25
Your code does not help because it has many inconsistencies, you must provide a [mcve] — eyllanesc, Apr 01 '19 at 11:27
@G.M. Sorry, I've now updated the question with full working code. — ramtheconqueror, Apr 01 '19 at 13:46

score 2 · Accepted Answer · answered Apr 01 '19 at 14:58

Never use the waitXXX methods because they block the event loop inpidiendo that the signals are transmitted. In your case the data is obtained one by one but because no one consumes the information since the signal is not transmitted it accumulates in the buffer and only shows when the eventloop is not blocked showing all the text. So the solution in your case is not to use waitXXX but to use the signals:

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);

    LocalServer::removeServer("testServer");
    LocalServer server;

    if (server.listen("testServer")) {
        QProcess process;
        process.setProgram("/home/ram/work/build/QtExamples/build-LocalSocketClient-Qt_5_12_1_Desktop-Debug/LocalSocketClient");
        process.start();
        QObject::connect(&process, QOverload<int>::of(&QProcess::finished), &QCoreApplication::quit);
        return a.exec();
    }
    return 0;
}

Thanks, this worked. I'm aware of the fact that `waitForFinished` is a blocking call but didn't realise that it will block network socket related activity as well. Oh well, lesson learned. :-) — ramtheconqueror, Apr 02 '19 at 14:51

QProcess and QLocalSocket - IPC messages aren't received

1 Answers1