Is this an efficient use of QProcess?

graniteDev

What I have works, but I'm just curious if it's the best way to go about this. I'm re-using a process to execute various external .exe files and pass them arguments.

process = new QProcess(this);
process->execute(file, args);
process->waitForFinish();
process->close();

process = new QProcess(this);
process->execute(file2, args2);
process->waitForFinish();
process->close();

etc.....

Like I said this is working, but I'm just wondering if I'm being efficient or not. Maybe there is a better way to do this? I do need these processes to happen in sequence as well, so the forced wait and close and restart ensures the sequence is always correct.

aha_1980

@graniteDev

if your code is in a thread, this is Ok. otherwise it would be better to connect the finished signal to a slot and start the next process from the to avoid blocking your event loop.

SGaist

Hi,

You are leaking Process objects. While you are giving them a parent, they won't be deleted until the parent object is so currently you might just be filling your memory.

Why not just create one QProcess on the stack and then execute each command in sequence with that object ? You could even have a vector of QPair<QString, QStringList> that would be your commands with their respective parameters and use a for loop to execute them all.

graniteDev

@aha_1980

No, it's not in a different thread. I've not worked with multi threading before so was unsure how to use it for this. It does lock up the system until it's done.

graniteDev

@SGaist can i execute different scripts and exe files off of one qprocess? I'm not sure how to do that...

aha_1980

@graniteDev yeah, should work.

just leave out the subsequent new lines.

@Lifetime-Qt-Champion well spotted , didn't see the leak...

SGaist

I didn't realise... You are using execute which is a static method that runs the commands and waits for it to finish. Thus you are currently creating QProcess objects for nothing.

graniteDev

@SGaist please explain?

SGaist

Static methods are not attached to an object, you should even have a compile warning telling you that you are calling a static method on an instance. See here for more details.

JonB

@SGaist said in Is this an efficient use of QProcess?:

I didn't realise... You are using execute which is a static method that runs the commands and waits for it to finish. Thus you are currently creating QProcess objects for nothing.

In which case, how come his process->waitForFinish(); even returns/doesn't error, because as you say the instance has never been start()ed?

SGaist

waitForFinished has a timeout with a default value but in this case, since the process is not even running it will return early.

graniteDev

I wonder if the confusion is that I left out the beginning code, the first process starts out:

QProcess *process = new QProcess(this);
process->execute(file1,args1);
process->waitforFinish();
process->close();

process = new QProcess(this);
process->execute(file2,args2);
process->waitforFinish();
process->close();

process = new QProcess(this);
process->execute(file3,args3);
process->waitforFinish();
process->close();

I get no compile errors, and it's working. Per the Qt documentation close() deletes the object, so I don't believe I'm creating a memory leak with what I'm doing.

However, if I can eliminate creating a new object and then deleting it every time, I'm for improving the code.

JonB

@SGaist
Ah, OK, I thought it might error "process not yet started".

SGaist

@graniteDev Warning doesn't mean error unless you enabled "all warning be errors" for your compiler. As for close, it doesn't delete anything. It will kill the process that you started and close the communication channels. It's unrelated to memory management.

@JonB waitForFinished will return false because of that.

graniteDev

@SGaist Ok, how should I go about this properly then? Could you provide some example code?

SGaist

QProcess::execute(file1, args1);

graniteDev

@SGaist I could run these from a static function? How then do I know when they are finished if there is no object to emit the signal?

SGaist

Did you read the documentation of the function ? execute runs the command and waits for it to finish. The returned value indicates what happened.

graniteDev

@SGaist yes but if i just run QProcess::execute(file1, args1); there is no object there for me to deal with.

SGaist

What do you want to deal with ? In the code you shown until now, what you are doing with your QProcess objects is already done "internally" by execute.