How to manage single instance of a Qt app on crash? - c++

Question

I want to execute my application as single instance, currently I am using QSharedMemory and working fine. I am using Qt5.2.1 on Ubuntu 12.04.
Below is my test code:
QApplication a(argc, argv);
a.processEvents();
const char* MEM_KEY = "56";
QSharedMemory sharedMem(MEM_KEY);
if( !sharedMem.create( 512, QSharedMemory::ReadWrite) )
{
QMessageBox msgBox;
msgBox.setText( QObject::tr("Can't start more than one instance of the application.") );
msgBox.setIcon( QMessageBox::Critical );
msgBox.exec();
exit(0);
}
MainWindow w;
w.show();
int p=0;
//p=p/0; // create exception here
return a.exec();
But the if makes the application crash(as shown in above code). If I start the application again, it shows Can't start more than one instance of the application, which means the previous instance is still there even if it has crashed. It should not happen in my case.
How can I restart my application in such a situation?
Edit:
Actually my original project contains lot of source files (the above one just made for testing). I want to implement it on my original project with editing least source file, if possible only by editing main.cpp

Answer 1

You can use a QSetting:
int main(int argc, char *argv[])
{
QSettings settings;
QApplication a(argc, argv);
if(!settings.exitedNormaly()) {
// In case of crash
}
// set the flag to false
settings.setExitedNormaly(false);
MainWindow w(&settings);
w.processArg(argc, argv);
w.show();
int result = a.exec();
settings.setExitedNormaly(result == 0);
return result;
}
Combined with your shared memory, you'll be able to unlock the block in case of application crash.

Answer 2

for linux:
//----------------------------------
QProcess *m_prSystemCall;
m_prSystemCall = new QProcess();
QString Commnd = "pgrep " + qApp->applicationDisplayName();
m_prSystemCall->start(Commnd);
m_prSystemCall->waitForFinished(8000);
QString output(m_prSystemCall->readAllStandardOutput());
QStringList AppList = output.split("\n", QString::SkipEmptyParts);
qDebug() <<"pgrep out:"<<AppList;
for(int i=0;i<AppList.size()-1;i++)
{
Commnd = "kill " + AppList.at(i);
m_prSystemCall->start(Commnd);
m_prSystemCall->waitForFinished(8000);
}
//-------------------------------------------------------
and for Windows:
#include <tlhelp32.h>
#include <comdef.h>
QString pName = qApp->applicationDisplayName();
pName += ".exe";
PROCESSENTRY32 entry;
entry.dwSize = sizeof(PROCESSENTRY32);
HANDLE snapshot = CreateToolhelp32Snapshot(TH32CS_SNAPPROCESS, NULL);
if (Process32First(snapshot, &entry) == TRUE)
{
DWORD myPID = GetCurrentProcessId();
while (Process32Next(snapshot, &entry) == TRUE)
{
const WCHAR* wc = entry.szExeFile ;
_bstr_t b(wc);
const char* c = b;
if (stricmp(c, pName.toStdString().c_str()) == 0)
{
HANDLE hProcess = OpenProcess(PROCESS_ALL_ACCESS, FALSE, entry.th32ProcessID);
qDebug() <<"myPID: "<< myPID << "entry.th32ProcessID" << entry.th32ProcessID;
if(myPID != entry.th32ProcessID)
TerminateProcess(hProcess,0);
QThread::msleep(10);
CloseHandle(hProcess);
}
}
}
CloseHandle(snapshot);