How to check if QProcess is executing correctly? - c++

Question

QProcess process_sdcompare;
QString command_sdcompare;
QStringList args_sdcompare;
command_sdcompare = "diff";
args_sdcompare << "Filename" << "Filename";
process_sdcompare.start(command_sdcompare,args_sdcompare,QIODevice::ReadOnly);
process_sdcompare.waitForFinished();
QString StdOut_sdcompare = process_sdcompare.readAllStandardOutput(); //Reads standard output
QString StdError_sdcompare = process_sdcompare.readAllStandardError(); //Reads standard error
if(StdOut_sdcompare.isEmpty()) //the process output is checked here if empty it is a success
return 1;
I am running the above code. When I check for an error condition after comparing not similar text files, isEmpty() returns false.
How do I check if the QProcess execution occurred without errors?

Answer 1

I use QProcess::error() to query the last error (for quick debugging), however a "nicer" way to do it:
// capture any errors (do this before you run the process)
connect(&process_sdcompare, &QProcess::errorOccurred, this, &ThisClass::processError);
Then define slot:
ThisClass::processError(QProcess::ProcessError error)
{
qDebug() << "error enum val = " << error << endl;
}
update
Or with Lambda:
// No need to define a slot function...
connect(&process_sdcompare, &QProcess::errorOccurred, [=](QProcess::ProcessError error)
{
qDebug() << "error enum val = " << error << endl;
});

Answer 2

I think all the status and error functions are pretty useless to figure out if the process execution was actually successful. Example, trying to execute a program which does not exist (under windows):
error(): QProcess::ProcessError(FailedToStart)
exitStatus(): QProcess::ExitStatus(NormalExit)
exitCode(): 0
errorString(): "Process failed to start: Das System kann die angegebene Datei nicht finden."
If the process was actually executed successfully:
error(): QProcess::ProcessError(UnknownError)
exitStatus(): QProcess::ExitStatus(NormalExit)
exitCode(): 0
errorString(): "Unknown error"
So I would consider "Unknown error" not an indicator for success.
The solution I think could look like this:
QProcess *m_proc = new QProcess(this);
bool errorOccured = false;
QProcess::ProcessError procError;
QObject::connect(m_proc, &QProcess::errorOccurred, [&](QProcess::ProcessError error)
{
procError = error;
errorOccured = true;
});
m_proc->start(...);

Answer 3

When waitForFinished returns, it returns a bool that broadly indicates success. As Karsten recommended, you can then check for actual success or failure using QProcess::exitCode.

Answer 4

This is how you use QProcess
QProcess process_sdcompare;
QStringList args_sdcompare;
args_sdcompare << "Filename" << "Filename";
process_sdcompare.setProgram("diff");
process_sdcompare.setArguments(args_sdcompare);
process_sdcompare.start();
if (process_sdcompare.waitForStarted() && process_sdcompare.waitForFinished()) {
QString StdOut_sdcompare = process_sdcompare.readAllStandardOutput();
QString StdError_sdcompare = process_sdcompare.readAllStandardError();
if(StdOut_sdcompare.isEmpty())
return 1;
}
}