How to properly quit blocking thread

JeKK666

I have a Qthread that blocks, waiting to read hardware, like this:

void MyThread::run() {
    while (1) {
        poll(...);
    }
}

Suppose i need to stop said thread to change some configuration and restart it, how would i go about it?

I've read the docs for QThread, found about methods quit(), exit() and terminate(), but there are two aspects i don't get:

is the main thread supposed to call t.quit() where t is an object of type MyThread, just as i have invoked t.start()? Or does t have to call quit itself, inside its event loop?
this being a blocking thread with an infinite loop, if the main thread calls t.quit(), is it ever gonna execute and terminate the event loop, so it can be restarted later? Or do i have to put a flag inside the while(1){...} to check if i need to stop?

JonB

@JeKK666 said in How to properly quit blocking thread:

@JonB i need to block indefinetely, unless a configuration changes, which will require changing how the sysfs endpoint are polled, and therefore run a slightly modified version of the thread code; i think my best option is going to be trying to throw a fake interrupt, so as to have the code react to isInterruptionRequested().

Indeed you can do it that way, raising a (Linux) signal() from the main thread to interrupt the poll() in the other thread. And if that is what you want or need to do, fine.

However it is not unusual to write code which calls poll() with a timeout and then restart the poll if it timed out. Code like:

void MyThread::run() {
    while(!isInterruptionRequested()) {
        // do stuff here, including...
        if (poll(..., timeout) != 0)
        {
             // genuine arrival of data, *not* timeout
             // if timeout occurs, code goes through the `while` again
             // which first checks for `isInterruptionRequested()` and then re starts a new `poll(..., timeout)`
        }
    }    
}

per the doc's

A return value of zero indicates that the system call timed out

before any file descriptors became read.

jsulm

@JeKK666 See https://doc.qt.io/qt-5/qthread.html#requestInterruption

JeKK666

So i have to put an if inside the loop, like this:

void MyThread::run() {
    while (1) {
        poll(...);
        if (isInterruptionRequested()) {
            quit();
        }
    }
}

?

But since the call to poll() is blocking, the only way it could get to the if statement would be in consequence of an event, which may or may not occur...
Is there a way to ... uhm... cleanly force terminate the thread? XD

SGaist

Hi,

Then you should use a proper timeout and handle that case. There's no clean thread termination if you have to kill it.

JonB

@JeKK666 said in How to properly quit blocking thread:

But since the call to poll() is blocking

Really? The point of a poll is to test and return immediately with a "yes" or "no", not to block!

jsulm

@JonB said in How to properly quit blocking thread:

The point of a poll is to test and return immediately with a "yes" or "no", not to block!

Not really (man poll):
"If none of the events requested (and no error) has occurred for any of the file descriptors, then poll() blocks until one of the events occurs."

But there is timeout parameter to prevent poll from blocking for longer time.

JeKK666

@JonB Then i guess those who invented sysfs got it wrong, somehow 😝

@SGaist uhm...fair enough, unfortunately it's a system which waits for user input, which may or may not come in, say, a day, and there's the possibility i need to change the software configuration before said interaction. I'll have to come up with something, perhaps i can trick the system into registering a fake interrupt of sorts and have poll() return as needed, on my own terms 😉

KroMignon

@JeKK666 said in How to properly quit blocking thread:

void MyThread::run() {
    while (1) {
        poll(...);
        if (isInterruptionRequested()) {
            quit();
        }
    }
}

Not really, I would do it like this:

void MyThread::run() {
    while(!isInterruptionRequested()) {
        // do stuff here
    }    
}

J.Hilk

@JeKK666 said in How to properly quit blocking thread:

quit

since you're not using the QEventLoop, that QThread provides, calling quit() will have no effect. The threat should keep on running. You'll have to call break or return

JeKK666

@KroMignon
...speechless... thank you for putting me in front of my own dumbness, this is exaclty what i need and apparentely could not figure out by myself :O

JonB

@jsulm , @JeKK666
Fine, let me try again. If you are using the Linux poll() system call, that has a "timeout" parameter which allows you to test and return, right? If you are using some other poll() function does it offer a similar option?

JonB

@JeKK666 said in How to properly quit blocking thread:

@KroMignon
...speechless... thank you for putting me in front of my own dumbness, this is exaclty what i need and apparentely could not figure out by myself :O

I do not see the difference of behaviour between

void MyThread::run() {
    while (1) {
        poll(...);
        if (isInterruptionRequested()) {
            quit();
        }
    }
}

and

void MyThread::run() {
    while(!isInterruptionRequested()) {
        // do stuff here, including...
        poll(...);
    }    
}

if your poll() is blocking as you said. [Yes the second avoids the quit() if no event loop, but not poll() blocking and failing to see isInterruptionRequested().]

JeKK666

@JonB yikes... that's right

JeKK666

@JonB i'm indeed using the Linux poll() detailed in man section 2, excerpt:

poll() performs a similar task to select(2): it waits for one of a set of file descriptors to become ready to perform I/O.

I need it to block indefinetely until a HW interrupt occurs.

KroMignon

@JonB said in How to properly quit blocking thread:

if your poll() is blocking as you said. [Yes the second avoids the quit() if no event loop, but not poll() blocking and failing to see isInterruptionRequested().]

Using quit() don't make sense here. This thread do not have an event loop, so not signals/slots can be used here.

JonB

@JeKK666
man 2 poll:

int poll(struct pollfd *fds, nfds_t nfds, int timeout);

   The timeout argument specifies the number of milliseconds that

   poll() should block waiting for a file descriptor to become

   ready.  The call will block until either:



   • a file descriptor becomes ready;



   • the call is interrupted by a signal handler; or



   • the timeout expires.

You need to pass a timeout.

I need it to block indefinetely until a HW interrupt occurs.

If that were 100% true you would not be asking about how to interrupt the thread with isInterruptionRequested().

JonB

@KroMignon said in How to properly quit blocking thread:

@JonB said in How to properly quit blocking thread:

if your poll() is blocking as you said. [Yes the second avoids the quit() if no event loop, but not poll() blocking and failing to see isInterruptionRequested().]

Using quit() don't make sense here. This thread do not have an event loop, so not signals/slots can be used here.

Absolutely which is why I wrote what I did: your code is definitely better in that it does not attempt to use a useless quit(), but since the OP wants (thought he wanted) a blocking poll() call into the body it won't address that problem.

JeKK666

@JonB i need to block indefinetely, unless a configuration changes, which will require changing how the sysfs endpoint are polled, and therefore run a slightly modified version of the thread code; i think my best option is going to be trying to throw a fake interrupt, so as to have the code react to isInterruptionRequested().

Thanks everyone for the insights and suggestions :)

KroMignon

@JonB said in How to properly quit blocking thread:

Absolutely which is why I wrote what I did: your code is definitely better in that it does not attempt to use a useless quit(), but since the OP wants (thought he wanted) a blocking poll() call into the body it won't address that problem.

I apologies, I read too quickly your post and misunderstood it, sorry for the noise!
And yes, you are right, the poll() call definitively requires a timeout to ensure thread will exit loop without have to wait until a file event occurs.

JonB

@JeKK666 said in How to properly quit blocking thread:

@JonB i need to block indefinetely, unless a configuration changes, which will require changing how the sysfs endpoint are polled, and therefore run a slightly modified version of the thread code; i think my best option is going to be trying to throw a fake interrupt, so as to have the code react to isInterruptionRequested().

Indeed you can do it that way, raising a (Linux) signal() from the main thread to interrupt the poll() in the other thread. And if that is what you want or need to do, fine.

However it is not unusual to write code which calls poll() with a timeout and then restart the poll if it timed out. Code like:

void MyThread::run() {
    while(!isInterruptionRequested()) {
        // do stuff here, including...
        if (poll(..., timeout) != 0)
        {
             // genuine arrival of data, *not* timeout
             // if timeout occurs, code goes through the `while` again
             // which first checks for `isInterruptionRequested()` and then re starts a new `poll(..., timeout)`
        }
    }    
}

per the doc's

A return value of zero indicates that the system call timed out

before any file descriptors became read.