handling exceptions in Qt

[wysota]

But what do you mean exactly by "application may be in an undefined state"?

Applications are crashing not because people who implemented your operating system were malicious, but rather that when your application does something horribly bad, the best solution is to kill it as soon as possible to prevent serious damage to hardware, system or even your application data. When the system detects such dangerous situation, it sends a signal to the process (most often SIGSEGV, SIGFPE or SIGBUS) and aborts it right after. This may happen in any moment of execution of your program, for instance right after you tried to overwrite your own stack or even your own program code, effectively destroying data critical for the process to work. Thus you should consider any internal state of the program to be corrupt and unrecoverable.

I mean can we atleast popup a message showing the reason of the crash to the end user and write the same to a log file before quitting the application?

It should be safe to write something to a file. Doing anything with the GUI is not safe. If you are after playing some sound then you can play it using an external process (e.g. external player). This way, even if it crashes, your main program can continue safely.

[sattu]

the best solution is to kill it as soon as possible to prevent serious damage to hardware, system or even your application data.

Actually Wysota this problem is not exactly from application side because the wav files got corrupted. That's why I wanted to know how to handle such situations as they are not in our control.

If you are after playing some sound then you can play it using an external process (e.g. external player). This way, even if it crashes, your main program can continue safely.

Correct but then again not an option in our case . Sticking to QSound as of now

[wysota]

Actually Wysota this problem is not exactly from application side because the wav files got corrupted.

Of course it is the fault of the application. The library you are using for playing sounds apparently can't handle invalid files and causes a crash.

Correct but then again not an option in our case

Why not?

[sattu]

Of course it is the fault of the application. The library you are using for playing sounds apparently can't handle invalid files and causes a crash.

Well, we are using QSound. So, maybe I will see if I can work on the library part

[pradeepreddyg95]

with QProcess you can restart your appliaction when it crashed this is most useful in embedded linux .
Example:

#include "applicationcontrol.h"
 
ApplicationControl::ApplicationControl(QObject *parent) :
    QObject(parent)
{
 
 
    m_pProcess = new QProcess*[2];
 
    for(int i = 0; i < 2; i++)
    {
        m_pProcess[i] = new QProcess();
 
        m_pProcess[i]->setProcessChannelMode(QProcess::MergedChannels);
        connect(m_pProcess[i], SIGNAL(finished(int, QProcess::ExitStatus)),this, SLOT(processFinished(int, QProcess::ExitStatus)));
        connect(m_pProcess[i], SIGNAL(error(QProcess::ProcessError)),this, SLOT(processError(QProcess::ProcessError)));
    }
 
    m_pProcess[0]->setObjectName("Server");
    m_pProcess[1]->setObjectName("Client");
 
 
 
    QTimer::singleShot(5,this,SLOT(StartJob()));
 
    m_nServerStarted = false;
    m_nClientStarted = false;
}
void ApplicationControl::StartJob()
{
    m_pProcess[0]->start("/root/CCU/Server");
    m_pProcess[1]->start("/root/CCU/Client");
 
}
 
void ApplicationControl::processError(QProcess::ProcessError error)
{
 
    QProcess *pProcess = qobject_cast<QProcess*>(sender());
 
    qDebug() << "Application Name " << pProcess->objectName();
 
    if (error == QProcess::FailedToStart)
    {
       qDebug(("Unable to Start Job ..."));
       return ;
    }
    else if (error == QProcess::Crashed)
    {
 
         qDebug() << "Program Crashed error " << error << QDateTime::currentDateTime().toString();
    }
    else if (error == QProcess::Timedout)
    {
        qDebug() << "Program Timedout error " << error << QDateTime::currentDateTime().toString();
 
    }
    else if (error == QProcess::WriteError)
    {
        qDebug() << "Program WriteError error " << error << QDateTime::currentDateTime().toString();
 
    }
    else if (error == QProcess::ReadError)
    {
        qDebug() << "Program ReadError error " << error << QDateTime::currentDateTime().toString();
 
    }
    else
    {
        qDebug() << "Program Unknow error " << error << QDateTime::currentDateTime().toString();
 
    }
 
}
void ApplicationControl::processFinished(int exitCode, QProcess::ExitStatus exitStatus)
{
/* If crashed Restart Application */
     QProcess *pProcess = qobject_cast<QProcess*>(sender());
        qDebug() << "Application Name " << pProcess->objectName();
    if(exitStatus == 0)
    {
        qDebug() << "exit Success Restarting " << pProcess->objectName();
 
            return ;
 
    }
 
    if (exitStatus == QProcess::CrashExit)
    {
        qDebug() << "Program Crashed " << QDateTime::currentDateTime().toString();
       // return ;
 
    }
    else if (exitCode != 0)
    {
      qDebug() << "Failed to Start" << QDateTime::currentDateTime().toString();
    }
 
    if(pProcess->objectName() == "Server")
    {
        m_pProcess[0]->start("/root/CCU/Server");
    }
    else if(pProcess->objectName() == "Client")
    {
        m_pProcess[1]->start("/root/CCU/Client");
    }
 
 
}

#include "applicationcontrol.h"

ApplicationControl::ApplicationControl(QObject *parent) :
    QObject(parent)
{

  
    m_pProcess = new QProcess*[2];

    for(int i = 0; i < 2; i++)
    {
        m_pProcess[i] = new QProcess();

        m_pProcess[i]->setProcessChannelMode(QProcess::MergedChannels);
        connect(m_pProcess[i], SIGNAL(finished(int, QProcess::ExitStatus)),this, SLOT(processFinished(int, QProcess::ExitStatus)));
        connect(m_pProcess[i], SIGNAL(error(QProcess::ProcessError)),this, SLOT(processError(QProcess::ProcessError)));
    }
  
    m_pProcess[0]->setObjectName("Server");
    m_pProcess[1]->setObjectName("Client");


   
    QTimer::singleShot(5,this,SLOT(StartJob()));

    m_nServerStarted = false;
    m_nClientStarted = false;
}
void ApplicationControl::StartJob()
{
    m_pProcess[0]->start("/root/CCU/Server");
    m_pProcess[1]->start("/root/CCU/Client");

}

void ApplicationControl::processError(QProcess::ProcessError error)
{

    QProcess *pProcess = qobject_cast<QProcess*>(sender());

    qDebug() << "Application Name " << pProcess->objectName();

    if (error == QProcess::FailedToStart)
    {
       qDebug(("Unable to Start Job ..."));
       return ;
    }
    else if (error == QProcess::Crashed)
    {

         qDebug() << "Program Crashed error " << error << QDateTime::currentDateTime().toString();
    }
    else if (error == QProcess::Timedout)
    {
        qDebug() << "Program Timedout error " << error << QDateTime::currentDateTime().toString();

    }
    else if (error == QProcess::WriteError)
    {
        qDebug() << "Program WriteError error " << error << QDateTime::currentDateTime().toString();

    }
    else if (error == QProcess::ReadError)
    {
        qDebug() << "Program ReadError error " << error << QDateTime::currentDateTime().toString();

    }
    else
    {
        qDebug() << "Program Unknow error " << error << QDateTime::currentDateTime().toString();

    }

}
void ApplicationControl::processFinished(int exitCode, QProcess::ExitStatus exitStatus)
{
/* If crashed Restart Application */
     QProcess *pProcess = qobject_cast<QProcess*>(sender());
        qDebug() << "Application Name " << pProcess->objectName();
    if(exitStatus == 0)
    {
        qDebug() << "exit Success Restarting " << pProcess->objectName();
      
            return ;
        
    }

    if (exitStatus == QProcess::CrashExit)
    {
        qDebug() << "Program Crashed " << QDateTime::currentDateTime().toString();
       // return ;

    }
    else if (exitCode != 0)
    {
      qDebug() << "Failed to Start" << QDateTime::currentDateTime().toString();
    }

    if(pProcess->objectName() == "Server")
    {
        m_pProcess[0]->start("/root/CCU/Server");
    }
    else if(pProcess->objectName() == "Client")
    {
        m_pProcess[1]->start("/root/CCU/Client");
    }


}

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[sattu]

Wow, thanks Pradeep. This code looks really great, I will try implementing it in my context.

---

Added after 17 minutes:

you can play it using an external process (e.g. external player). This way, even if it crashes, your main program can continue safely.

Hi Wysota, I tried playing wav files in a different process but I am encountering certain problem in it. Here's the way I implemented it:-

[QUOTE].h file[/QUOTE]
class customWavePlayer:public QSound
{
public:
    customWavePlayer(const QString&, QObject*);
    ~customWavePlayer();
    bool playWaveFile(QString filename, int vol);
};
 
[QUOTE].cpp file[/QUOTE]
customWavePlayer::customWavePlayer(const QString& str, QObject* parent):QSound(str,parent)
{
}
 
customWavePlayer::~customWavePlayer()
{
}
 
bool customWavePlayer::playWaveFile(QString filename, int vol)
{
    static int child_pid=-1;
    int status;
    if(child_pid>0)
    {
        waitpid(child_pid, &status, 0);
    }
    child_pid=fork();
    if(child_pid==0) //in child
    {        
        qDebug("******************started playing");
        play(filename,vol);
        //while(!isFinished());
        qDebug("******************finished playing");
        //exit(EXIT_SUCCESS);
    }
    if(child_pid!=-1)
            return true;
    else
            return false;
}

[QUOTE].h file[/QUOTE]
class customWavePlayer:public QSound
{
public:
    customWavePlayer(const QString&, QObject*);
    ~customWavePlayer();
    bool playWaveFile(QString filename, int vol);
};

[QUOTE].cpp file[/QUOTE]
customWavePlayer::customWavePlayer(const QString& str, QObject* parent):QSound(str,parent)
{
}

customWavePlayer::~customWavePlayer()
{
}

bool customWavePlayer::playWaveFile(QString filename, int vol)
{
    static int child_pid=-1;
    int status;
    if(child_pid>0)
    {
        waitpid(child_pid, &status, 0);
    }
    child_pid=fork();
    if(child_pid==0) //in child
    {        
        qDebug("******************started playing");
        play(filename,vol);
        //while(!isFinished());
        qDebug("******************finished playing");
        //exit(EXIT_SUCCESS);
    }
    if(child_pid!=-1)
            return true;
    else
            return false;
}

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By this the file is playing but the application hangs. For more information our's is a multi-threaded GUI application. 4-5 different threads are running in the main application.
Can you suggest us some better way to play it in a different process?

[wysota]

You don't need to code your own player. I'm sure there are some available for your platform. Just install one and use it.

[sattu]

You don't need to code your own player. I'm sure there are some available for your platform. Just install one and use it.

Well, ok. Let me see to it.