Question about QThread and how thread work in general.

[J.Hilk]

@Kent-Dorfman well yes, I wrote objects/data/pointers and meant to write objects/data - pointers ;-)

[Factao]

@J.Hilk Interesting, but after looking QMutex, I have one quesition: If I'm locking a variable within a thread, may other thread "read" it? Within the docs, the following sentence does not specifiy that: "Then only one thread can modify number...". (after the example within the QMutex class). If I don't need to modify this value, then thread should not have any difficulties to read a variable that only one thread is modifying.

@Kent-Dorfman Yes, I need a near real-time synchronisation, so I can't use too many signals or slots.

[Kent-Dorfman]

@Factao said in Question about QThread and how thread work in general.:

If I'm locking a variable within a thread, may other thread "read" it?

Multithreaded reads are only consistent on atomic data items. IOW, ordinal variables that can be read or written with a single processor native instruction. You have no way of guaranteeing when a context switch will occur, so values of more complex variables are undefined unless you guard their access with a critical section lock (ie semaphore/mutex)

[Factao]

@Kent-Dorfman I'm not really sure to understand your answer, so here is an incomplete example of what I'm trying to do:

class A: public QObject
{
public:
//constructor an destructor
void methoda(int a);
int geta() const; //return m_a
B *m_B;//see below
 private:
int m_a;
}
class B: public QObject
{
public:
//constructor an destructor
void methodb(int b);
int getb() const; //return m_b
A *m_A;
 private:
int m_b;
}
 int main();
{
//everything is initialized
B *BB= new *B();
A  *AA= new *A();
//somehow set BB->m_A=AA and AA->m_B=BB
QThread thread(BB);
thread.start();
//two action that read the other class
AA->methoda(AA->m_B->getb());
BB->methodb(BB->m_A->geta());
//do stuff...

At no moment something is modified. Does that need mutex or not?

[kshegunov]

If you don't have experience with low-level threading primitives, then I kindly suggest you stick to signal-slot connections. The former can lead to difficult to diagnose errors that may not be trivial to fix. At least with the signal-slot mechanism you get serialization out of the box, which by the way is suitable for most practical cases.

[Factao]

@kshegunov Please, don't think that it is the first time that I encounter something that I'm not familiar with. If I would have stick to what I know in order to accomplish something, I wouldn't accomplished anything. If you are trying to tell me that what I'm asking can't be answered here because its too complicated for a simple post and you might need to wrote a book in order to do so, then tell it to me.

[jsulm]

@Factao I think what @kshegunov tried to tell you is that in most cases signals/slots are enough and the right way in Qt to communicate between threads. Low level multi-threading is indeed a complex topic, but nobody here forbids you to learn it. But it is not really possible to explain it in a post in a forum. You should look for tutorials/books.
Regarding locking for read access see https://doc.qt.io/qt-5/qreadwritelock.html

[CP71]

@Factao
Hi @Factao,
well, as always I hope I’m being understood correctly due to my English ;)
I joined at forum about a year ago and I’ve been active only for some months, I think nobody knows somebody, at least, is true for most of us, so it is not easy knowing coding level of someone only by his/her post.
I have always found someone helps me, directly to me or indirectly about post of somebody else.
Sometimes the answers can seem obvious, only because who wrote don’t know your or my level of coding or Qt.
Please don't take it personally, at least I do this.

Having said that, let’s come back about synchronization of thread, if you don’t want to use signals and slots you must use some mechanisms to avoid a simultaneous access to a member of thread, normally I use QMutex, so what I say it is certainly true for QMutex, but I think it is same for the other mechanisms.

In both thead you must declare a Qmutex and you must call lock and unlock before access a member of class.

In your H:
QMutex myMutex.

In your cpp
void myClass::setMyIntValue(int newValue)
{
myMutex,lock();
mMyMemberInt = newValue;
myMutex,unlock();
}

Please pay attention, if 2 or more threads call setMyIntValue only the first will be executed, the others will waiting for unlock to be executed.
In Short lock of mutex blocks execution of next caller until unlock.

Regards
CP71

[kshegunov]

@jsulm said in Question about QThread and how thread work in general.:

I think what @kshegunov tried to tell you is that in most cases signals/slots are enough and the right way in Qt to communicate between threads

That's exactly right. This is what I meant. There's nothing wrong with low-level threading, but most of the time with Qt it's not necessary.

[Factao]

@kshegunov Then I didn't understand the meaning of your first intervention. I apoligize

@kshegunov and @jsulm . I'm using slots and signals in order to command the worker thread, but this last one, when not doing a specific task, is within a loop, constantly reading value from a servo and saving them. Since there is a lot of variable that change frequently, emitting a signal for every one of them might slow the application down, due to how signal/slot work.

@CP71 Thanks, this is the example that I was looking for. To summarize, I'm locking when a thread is using a function that use a specific value and nothing more. So making a specific and short function that only return or modify the value is a good way to avoid traffic while multiple thread are accessing the same thing.
edit: After trying it, now I see that there is an issue with the function other than void, I can't do:

int m_something;// private element of class
int class::returnSomething() const
{
mutex.lock()
return m_something;
mutex.unlock()
};

If I'm doing that:

int class::returnSomething() const
{
mutex.lock();
int something(m_something);
mutex.unlock();
return something;
}

Is this what we are trying to do here? From what I understand, our goal Is to prevent two thread to access the same data simultaneously, not the same function.

So, to be sure of getting it, here is a summarize of what I get out of this topic:
We need to lock data when a thread is accessing it to be sure that the data going to stay where it should be during the operation. If we don't do it, a thread from another core could possibly "steal" the data that we need and create a little problems in our program.

Is this approximately true? Or do I miss something?

[Kent-Dorfman]

@Factao said in Question about QThread and how thread work in general.:

We need to lock data when a thread is accessing it to be sure that the data going to stay where it should be during the operation. If we don't do it, a thread from another core could possibly "steal" the data that we need and create a little problems in our program.

Steal the data isn't the correct terminology, but you are essentially correct. You shouldn't care about multiple siumulatneous reads. It's only when something is changing the data that locking becomes a concern.

And for what it's worth, your application is beginning to smell very much like something that is not really appropriate for high level Qt on a PC or real CPU. real-time functions should usually be delegated to micro-controllers running RTOS programs specifically designed to handle real-time IO and then send agregate info to a higher level computer program on a real CPU.

Of course you don't elaborate on whether this is a hobbyist level, academic/learning, or for work, project.

[Factao]

@Kent-Dorfman Your right, I did not precise what is my project, so here is for you:

I am trying to make a bio-mimetic (in apearance, at least) robotical arm, wich is requiring around 20 smart servo to operate the hand. I'm using a raspberry pi 3B+ to operate everything. Since every servo cost $50 usd, I'm planning to make a loop that verify every servo and emit an emergency signal if something is going wrong. I'm planning to also use this verification as a feedback that would update the virtual representation of the force and position of the member. Since I'm more a guy that just get admitted in mechanical engineering that someone with a great knowledge of electronics, I'm planning to stick on the raspberry pi until I got some result out of this, but perhaps I will used a dedicated micro-controller for the communication and this loop. However, I'm planning to completed my personal exploration of c/c++ before doing so.

[CP71]

Hi,
@Factao wow! very beautiful project.
I think I could learn from you ;)

Technically @Kent-Dorfman is right, You shouldn't care about multiple simultaneous reads because changes can become a problem.

Anyway, when a function, that uses variables member, starts I prefer, if is possible, copying members (Note: only variables that can be changed by external class, for example via setXXXX functions) and I do this under lock and unlock.
That's because I normally want to have a constant instant photo for all during of function.
To avoid deadlock, if is possible, I prefer using QMutex with moderation and, always if is possible, only for changing and reading variables. Always to avoid deadlock I use QMutex to protect all body function very rarely. This is how I use QMutex, but doesn’t mean is right way, is right for me.

Qt offers other classes for thread synchronization, for these I suggest you to read the manual.

Regards
Paolo

[kshegunov]

@Factao said in Question about QThread and how thread work in general.:

I'm using slots and signals in order to command the worker thread, but this last one, when not doing a specific task, is within a loop, constantly reading value from a servo and saving them. Since there is a lot of variable that change frequently, emitting a signal for every one of them might slow the application down, due to how signal/slot work.

The signal-slot connections work very similarly to how you'd serialize access with a mutex, as a matter of fact they do exactly that - serialize access to the event loop through a mutex. So unless you can prove that emission and handling of signals is a bottleneck, that concern is moot. The big advantage, however, is that thread access is defined strictly and clearly for each QObject involved, so it makes for convenient and fast coding.

[CP71]

Hi @kshegunov,
I'm Qt developer only for few years, so I’m quite newbie ;)
Is there difference using signals&slots or direct method between threads with different priority?
I think yes but I’m not sure.
Thanks

[J.Hilk]

@CP71 the main difference is, that your signal argument, if it is a struct or class, needs to have a copy constructor.

Where as with direct mutex lock, you simply lock the data block, access it and unlock it.

[CP71]

@J.Hilk
Thank you,
yes this I know, my question was incomplete, sorry.
but I’m thinking about response time.

[Factao]

Well, thank you everyone, thank you. I think that we can conclude this topic. Once again, thank you.

@CP71 said in Question about QThread and how thread work in general.:

I think I could learn from you ;)

If you need help on a subject that is more mechanical or mathematical, don't hesitate, I will be more than happy to help you back.

[CP71]

@Factao
Sure, count on it.
Thanks

[KroMignon]

@Factao said in Question about QThread and how thread work in general.:

int m_something;// private element of class
int class::returnSomething() const
{
mutex.lock()
return m_something;
mutex.unlock()
};

Hi Factao, just give you a little hint abour QMutex usage, the easiest way to deal with QMutex, is to use QMutexLocker, to avoid having mutex lock when leaving function:

int class::returnSomething() const
{
    QMutexLocker lock(&mutex);
    Q_UNUSED(lock) // just to remove warnings
    return m_something;
};

At function begin, mutex will be locked, and when lock is distroyed (by living function) mutex will be released.
This will avoid having deadlocks!

But as writen in other comments, using Signals/Slots mechanisme to exhange information between object in different thread is the most easiest way. You don't have to deal with mutex. The disadvantage is that will have shadow copies of your data, so if there are big amonth of data, this could be a problem.

Hope this will help you