QProcess and none-string arguments

[Raistlin]

I need to combine some code I have written myself.

• The main program is written in LabVIEW and has C-style DLLs calls, where the DLLs contain C++ code with Qt.
• The second part is written entirely in Qt.

I tried to integrate the second part in my LabVIEW code, but this is not trivial because of the threads, signals/slots, event handling, ... Therefore, I postponed this to later and would now like to use QProcess as a solution.

I need to pass a lot of parameters from the main program to the second application. These range from strings to ints, floats, doubles, ... I thought I could use the 'arguments' of void QProcess::start( const QString & program, const QStringList & arguments) . I am not sure how to handle this however.

• Format all parameters into QStrings and append them to the QStringList.
• Construct a QDataStream based on a QByteArray, serialize all date in there, and construct a QString based on this. I suppose however that I can run into troubles because of '0' bytes, and have to use QT_NO_CAST_FROM_ASCII to avoid strange things happening to the data.

Or is there a more elegant way? Any suggestions?

[wysota]

I suggest you format the data as XML and pass it to stdin of the other application (so not as a parameter but as a stream of data).

[Raistlin]

Thank you for the suggestion.

I do however still have some issues to get the inter process communication to work. This is how I handle it. The application is launched succesfully, but it does not seem to receive any data. I use a QFile constructed with stdin in my server to capture the input from the startserver program.

The main.cpp of my startserver program:

#include <QCoreApplication>
#include "startserver.h"
 
int main(int argc, char *argv[])
{
	QCoreApplication a(argc, argv);
	Startserver w;	
	return a.exec();
}

#include <QCoreApplication>
#include "startserver.h"

int main(int argc, char *argv[])
{
	QCoreApplication a(argc, argv);
	Startserver w;	
	return a.exec();
}

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The startserver class:

#include <QDir>
#include <QFile>
#include <QProcess>
#include <QString>
#include <QStringList>
 
class Startserver : public QObject
{
	Q_OBJECT
 
public:
	Startserver(QObject *parent = 0);
	~Startserver();
 
private slots:
	void serverFinished(int, QProcess::ExitStatus);
 
private:
	QProcess* process_;
};
 
Startserver::Startserver(QObject* parent) : QObject(parent)
{
	QString appName = QString("myapplication.exe");
 
	process_ = new QProcess(this);		
 
	process_->start(appName);
	process_->waitForStarted();	
 
	connect(process_, SIGNAL(finished(int, QProcess::ExitStatus)), this, SLOT(serverFinished(int, QProcess::ExitStatus)));	
 
	//Start a timer, which sends data to the server each second
	QTimer* timer = new QTimer(this);
	timer->setInterval(1000);
	connect(timer, SIGNAL(timeout()), this, SLOT(sendData()));
	timer->start();
}
 
Startserver::~Startserver()
{
 
}
 
void Startserver::serverFinished(int exitCode, QProcess::ExitStatus exitStatus)
{
	exit(exitCode);;
}
 
void Startserver::sendData()
{
	//Send some data to server
	//This slot is effictively called each second !!
	QByteArray block;
	QDataStream out(&block, QIODevice::WriteOnly);
	out.setVersion(QDataStream::Qt_4_3);
	out << (quint32)0;
	out << QString("Hello server");
	out.device()->seek(0);
	out << (quint32)(block.size() - sizeof(quint32));	
 
	//Data is sent to stream each time, since bytesToWrite returns same amount each time
	process_->write(block);
 
	while (process_->bytesToWrite() > 0)
	{
		if (!process_->waitForBytesWritten())
			break;
	}	
}

#include <QDir>
#include <QFile>
#include <QProcess>
#include <QString>
#include <QStringList>

class Startserver : public QObject
{
	Q_OBJECT

public:
	Startserver(QObject *parent = 0);
	~Startserver();

private slots:
	void serverFinished(int, QProcess::ExitStatus);

private:
	QProcess* process_;
};

Startserver::Startserver(QObject* parent) : QObject(parent)
{
	QString appName = QString("myapplication.exe");

	process_ = new QProcess(this);		
	
	process_->start(appName);
	process_->waitForStarted();	

	connect(process_, SIGNAL(finished(int, QProcess::ExitStatus)), this, SLOT(serverFinished(int, QProcess::ExitStatus)));	

	//Start a timer, which sends data to the server each second
	QTimer* timer = new QTimer(this);
	timer->setInterval(1000);
	connect(timer, SIGNAL(timeout()), this, SLOT(sendData()));
	timer->start();
}

Startserver::~Startserver()
{

}

void Startserver::serverFinished(int exitCode, QProcess::ExitStatus exitStatus)
{
	exit(exitCode);;
}

void Startserver::sendData()
{
	//Send some data to server
	//This slot is effictively called each second !!
	QByteArray block;
	QDataStream out(&block, QIODevice::WriteOnly);
	out.setVersion(QDataStream::Qt_4_3);
	out << (quint32)0;
	out << QString("Hello server");
	out.device()->seek(0);
	out << (quint32)(block.size() - sizeof(quint32));	

	//Data is sent to stream each time, since bytesToWrite returns same amount each time
	process_->write(block);

	while (process_->bytesToWrite() > 0)
	{
		if (!process_->waitForBytesWritten())
			break;
	}	
}

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And here is what happens in the main class of my server program. The code is a lot bigger, so I only show what I think to be the relevant parts.

#include <QByteArray>
#include <QList>
#include <QtNetwork>
#include <QtCore>
#include <QMessageBox>
 
#include "ConnectionThread.h"
 
class OctoServer : public QTcpServer
{
	Q_OBJECT
 
public:
	OctoServer(QObject *parent = 0);
	~OctoServer();
 
signals:
	//...
 
public slots:
	void readInput();
	//...
 
private:			
	QString* directory_;		
};
 
OctoServer::OctoServer(QObject* parent) : QTcpServer(parent)
{	
	QFile* stdinFile = new QFile(this);
	directory_ = new QString();	
 
	//Open a file pointing to the stdin and connect incoming data signal to custom slot
	stdinFile->open(stdin, QIODevice::ReadOnly);	
	connect(stdinFile, SIGNAL(readyRead()), this, SLOT(readInput()));
 
	//...
}
 
void OctoServer::readInput()
{
	//This slot is never called !!
	QMessageBox::information(0, QString("Data incoming"), QString("Data")) ;
}
 
OctoServer::~OctoServer()
{
	delete directory_;	
}

#include <QByteArray>
#include <QList>
#include <QtNetwork>
#include <QtCore>
#include <QMessageBox>

#include "ConnectionThread.h"

class OctoServer : public QTcpServer
{
	Q_OBJECT

public:
	OctoServer(QObject *parent = 0);
	~OctoServer();

signals:
	//...
	
public slots:
	void readInput();
	//...

private:			
	QString* directory_;		
};

OctoServer::OctoServer(QObject* parent) : QTcpServer(parent)
{	
	QFile* stdinFile = new QFile(this);
	directory_ = new QString();	

	//Open a file pointing to the stdin and connect incoming data signal to custom slot
	stdinFile->open(stdin, QIODevice::ReadOnly);	
	connect(stdinFile, SIGNAL(readyRead()), this, SLOT(readInput()));

	//...
}

void OctoServer::readInput()
{
	//This slot is never called !!
	QMessageBox::information(0, QString("Data incoming"), QString("Data")) ;
}

OctoServer::~OctoServer()
{
	delete directory_;	
}

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[wysota]

Have you tried verifying each application that they actually read/write the data to respective descriptors?

[Raistlin]

How exactly would I accomplish this?

[Raistlin]

Hmm, it seems QFile does not emit readyRead() in general. So this means I will need to poll in the idle loop for this, since QSocketNotifier is also not an option on Windows.

Good to know .

If I could only wait for the official release of Qt 4.4 and the introduction of QSharedMemory.

[wysota]

You can always use platform dependent solutions.

[Raistlin]

Well, I implemented a connection through a QTcpSocket now, and once Qt 4.4 is available (not the beta, since we have to release this code to customers) I will take a look at QLocalServer and QLocalSocket.

[wysota]

If both applications are GUI-based, you can use WinAPI PostMessage() call.