Forking with QT - qt

Question

using QtCreator to make a loftier interface to a sofware.
There is basically a set of buttons to tune and inputs, a start and stop job
buttons, my problem comes from an infinite loop that freezes the display so I came up with using fork() so that the loop have to compete with the main program instead of eating up the whole resources (no multithreading), but the program crashes spiting:
[xcb] Unknown sequence number while processing queue
[xcb] Most likely this is a multi-threaded client and XInitThreads has not
been called
[xcb] Aborting, sorry about that.
a.out: ../../src/xcb_io.c:274: poll_for_event: Assertion
`!xcb_xlib_threads_sequence_lost' failed.
the fonction calling the loop is called 'ON', 'OFF' is supposed to exit the forked process.
//in button 'ON' func
ps = fork();
if(getpid() == ps)
{
while(1)
{
strcpy(word, charset(minlength, maxlength, N));
ui->pass->setText(word);//operation on the display
....SNIP
}
}
//In button 'OFF' func
if(getpid() == ps)
exit(0);
I'm really asking what is the correct way of starting a while(1) and be able to break, exit, or return from it while not freezing the window using QT, thanks.

Answer 1

You crash probably here:
ui->pass->setText(word);//operation on the display
as in Qt, you can not change UI from non UI threads directly. Only from signals and slots mechanism.
The proper way to not freeze UI is obviously to compute lengthy operations in another thread.
You can achieve this in several ways:
One is by sub-classing QObject class to create 'worker object' which would perform all heavy operations. You create new QThread object that should live as long as you need your object. And use QObject::moveToThread method to move created object to new thread. To control your worker object you should send signals from object and call it's slots also via signal-slot mechanism. If you call them directly - they will be executed in caller thread (so do not perform stuff like worker->startHeavyJob(); in UI thread). Instead emit signal in UI (emit sigStartHeavyStuff();) and connect it to slot of your worker object (slotDoHeavyStuff();)
if you do not want to bother with this (if operation is pretty small)
- you can use QApplication::processEvents() to process events in UI event loop while going in your infinite while loop.
Another way is to use QtConcurrentRun framework to run function in separate thread which manages itself. Threads are taken from thread pool and are managed by Qt. This approach looks like something you want to do. Though you still will be able to access UI objects only through signals and slots.

Answer 2

I see one big issue in the presented code that is causing your freeze: You never let Qt process anything while you are in the loop. You need to allow Qt to run it's event loop. The easiest way is to use QApplication::processEvents() inside the loop.
I'm also not a fan of a while(1) loop for a couple of reasons. The first of which is that it can eat machine cycles waiting for things to happen. I doubt you really need to run the code as fast as possible, you could probably get away with some sleeping in your loop.
The other issue is that it is hard to break out. A cleaner approach would be something like this
void MyClass::on_pushButton_ON_clicked()
{
MyClass::done = false; // this is a class attribute
while (!MyClass::done) {
QApplication::processEvents();
//...
}
}
void MyClass::on_pushButton_OFF_clicked()
{
MyClass::done = true;
}