I want to know if there is a way to unpause a sleeping thread in Qt
I pause the thread using QThread::msleep(ms) function
And I want to unpause the thread before time runs out
Is there any way to do it ?
First of all, If you are using QSerialPort, you really don't have to mess up with threads. You should deal with it asynchronously. So, you read() in slots connected to readyRead() signal, and if you want to have some delay between writes use a QTimer and write() in slots connected to its timeout() signal as suggested by #deW1's comment.
If you really want to go with multiple threads, #sploid's code approach has many mistakes:
A mutex has to be unlocked only by the thread that locked it. Here is what the documentation for QMutex::unlock() says:
Unlocks the mutex. Attempting to unlock a mutex in a different thread to the one that locked it results in an error. Unlocking a mutex that is not locked results in undefined behavior.
A QMutex is used to protect an object, data structure or section of code so that only one thread can access it at a time. It is not used for a thread to signal some other thread.
A QWaitCondition allows a thread to tell other threads that some sort of condition has been met. This is what you need to use to tell the other thread to "unpause".
Here is how it should have been done:
class Thread : public QThread{
public:
Thread(QObject* parent=nullptr):QThread(parent){
}
~Thread(){}
void InterruptWaitState(){
QMutexLocker locker(&mutex);
cond.wakeAll();
}
protected:
void run(){
//waiting thread
QMutexLocker locker(&mutex);
if(cond.wait(&mutex, 5000)){
// unlock from another place
} else {
// exit by timeout
}
}
private:
QMutex mutex;
QWaitCondition cond;
};
QMutex mut;
void InterruptMethod() {
mut.unlock();
}
void SleepMethod() {
mut.lock();
const int kTimeout = 5000;
bool over_event_come = mut.tryLock(kTimeout); // here was sleep
if (over_event_come) {
// unlock from another place
} else {
// exit by timeout
}
mut.unlock();
}
Related
I am trying to use an std::condition_variable from C++11 for a data transaction between between UI thread & worker thread.
Situation:
m_calculated_value is a value which calculated after a complex logic. This is required on a trigger of a event from the UI thread. UI thread calls MyClass::GetCalculatedValue to fetch the value of m_calculated_value which needs to be calculated by the worker thread function that is MyClass::ThreadFunctionToCalculateValue.
Code:
std::mutex m_mutex;
std::condition_variable m_my_condition_variable;
bool m_value_ready;
unsigned int m_calculated_value;
// Gets called from UI thread
unsigned int MyClass::GetCalculatedValue() {
std::unique_lock<std::mutex> lock(m_mutex);
m_value_ready = false;
m_my_condition_variable.wait(lock, std::bind(&MyClass::IsValueReady, this));
return m_calculated_value;
}
bool MyClass::IsValueReady() {
return m_value_ready;
}
// Gets called from an std::thread or worker thread
void MyClass::ThreadFunctionToCalculateValue() {
std::unique_lock<std::mutex> lock(m_mutex);
m_calculated_value = ComplexLogicToCalculateValue();
m_value_ready = true;
m_my_condition_variable.notify_one();
}
Problem:
But the problem is that m_my_condition_variable.wait never returns.
Question:
What am I doing wrong here?
Is it a correct approach to make UI thread wait on a condition variable signal from worker thread? How do I get out of a situation where the condition_variable never triggers due to an error in the worker thread function? Is there a way I can somehow use a timeout here?
Trying to understand how it works:
I see in many examples they use a while loop checking the state of a boolean variable around a condition_var.wait. Whats the point of loop around on a variable? Cant I expect m_my_condition_variable to return out of wait when notify_one is called from other thread ?
What is most likely to happen:
Your worker thread owns and holds the mutex until it's done with the calculation. The main thread has to wait until it can acquire the lock. The worker will signal the CV before it releases the lock (in the destructor), by which time no other thread that would want to wait on the condition variable could have been acquired the lock that it still occupied by the notifying thread. Therefore the other thread never got a chance to wait on the condition variable at the time it gets notified as it just managed to acquire the lock after the notification event took place, causing it to wait infinitely.
The solution would be to remove the lock-acquisition in MyClass::ThreadFunctionToCalculateValue(), it is not required there at all, or at least, shouldn't be.
But anyways, why do you want to re-invent the wheel? For such problems, std::future has been created:
auto future = std::async(std::launch::async, ComplexLogicToCalculateValue);
bool is_ready = future.wait_for(std::chrono::seconds(0)) == std::future_status::ready;
auto result = future.get();
Here, you can easily define timeouts, you don't have to worry about condition_variables and alike.
Cant I expect m_my_condition_variable to return out of wait when notify_one is called from other thread ?
No, not exclusively. Spurious wakeups still may occur.
Take a look at this example here:
http://en.cppreference.com/w/cpp/thread/condition_variable
Changes to the code in question noted in comments in the example code below. You might want to consider using the same "handshake" as used in the cppreference.com example to synchronize when it's safe to calculate a new value (the UI thread has a wait / notify, the worker thread has a notify / wait).
Before condition variable wait, the lock needs to be locked. The wait will unlock, wait for a notify, then lock and with the predicate function, check for ready and if not ready (spurious wake up), repeat the cycle.
Before notify_one, the lock should be unlocked, else the wait gets woke up, but fails to get a lock (since it's still locked).
std::mutex m_mutex;
std::condition_variable m_my_condition_variable;
bool m_value_ready = false; // init to false
unsigned int m_calculated_value;
// Gets called from UI thread
unsigned int MyClass::GetCalculatedValue() {
std::unique_lock<std::mutex> lock(m_mutex);
m_my_condition_variable.wait(lock, std::bind(&MyClass::IsValueReady, this));
m_value_ready = false; // don't change until after wait
return m_calculated_value;
} // auto unlock after leaving function scope
bool MyClass::IsValueReady() {
return m_value_ready;
}
// Gets called from an std::thread or worker thread
void MyClass::ThreadFunctionToCalculateValue() {
std::unique_lock<std::mutex> lock(m_mutex);
m_calculated_value = ComplexLogicToCalculateValue();
m_value_ready = true;
lock.unlock(); // unlock before notify
m_my_condition_variable.notify_one();
}
or alternative:
// Gets called from an std::thread or worker thread
void MyClass::ThreadFunctionToCalculateValue() {
{ // auto unlock after leaving block scope
std::lock_guard<std::mutex> lock(m_mutex);
m_calculated_value = ComplexLogicToCalculateValue();
m_value_ready = true;
} // unlock occurs here
m_my_condition_variable.notify_one();
}
Please see the following code:
std::mutex mutex;
std::condition_variable cv;
std::atomic<bool> terminate;
// Worker thread routine
void work() {
while( !terminate ) {
{
std::unique_lock<std::mutex> lg{ mutex };
cv.wait(lg);
// Do something
}
// Do something
}
}
// This function is called from the main thread
void terminate_worker() {
terminate = true;
cv.notify_all();
worker_thread.join();
}
Is the following scenario can happen?
Worker thread is waiting for signals.
The main thread called terminate_worker();
The main thread set the atomic variable terminate to true, and then signaled to the worker thread.
Worker thread now wakes up, do its job and load from terminate. At this step, the change to terminate made by the main thread is not yet seen, so the worker thread decides to wait for another signal.
Now deadlock occurs...
I wonder this is ever possible. As I understood, std::atomic only guarantees no race condition, but memory order is a different thing. Questions:
Is this possible?
If this is not possible, is this possible if terminate is not an atomic variable but is simply bool? Or atomicity has nothing to do with this?
If this is possible, what should I do?
Thank you.
I don't believe, what you describe is possible, as cv.notify_all() afaik (please correct me if I'm wrong) synchronizes with wait(), so when the worker thread awakes, it will see the change to terminate.
However:
A deadlock can happen the following way:
Worker thread (WT) determines that the terminate flag is still false.
The main thread (MT) sets the terminate flag and calls cv.notify_all().
As no one is curently waiting for the condition variable that notification gets "lost/ignored".
MT calls join and blocks.
WT goes to sleep ( cv.wait()) and blocks too.
Solution:
While you don't have to hold a lock while you call cv.notify, you
have to hold a lock, while you are modifying terminate (even if it is an atomic)
have to make sure, that the check for the condition and the actual call to wait happen while you are holding the same lock.
This is why there is a form of wait that performs this check just before it sends the thread to sleep.
A corrected code (with minimal changes) could look like this:
// Worker thread routine
void work() {
while( !terminate ) {
{
std::unique_lock<std::mutex> lg{ mutex };
if (!terminate) {
cv.wait(lg);
}
// Do something
}
// Do something
}
}
// This function is called from the main thread
void terminate_worker() {
{
std::lock_guard<std::mutex> lg(mutex);
terminate = true;
}
cv.notify_all();
worker_thread.join();
}
I have following scenario:
condition_variable cv;
mutex mut;
// Thread 1:
void run() {
while (true) {
mut.lock();
// create_some_data();
mut.unlock();
cv.notify_all();
}
}
// Thread 2
void thread2() {
mutex lockMutex;
unique_lock<mutex> lock(lockMutex);
while (running) {
cv.wait(lock);
mut.lock();
// copy data
mut.unlock();
// process data
}
}
// Thread 3, 4... - same as Thread 2
I run thread 1 all the time to get new data. Other threads wait with condition_variable until new data is available, then copy it and do some work on it. Work perfomed by threads differs in time needed to finish, the idea is that threads will get new data only when they finished with the old one. Data got in meantime is allowed to be "missed". I don't use shared mutex (only to access data) because I don't want threads to depend on each other.
Above code works fine on Windows, but now I run it on Ubuntu and I noticed that only one thread is being notified when notify_all() is called and the other ones just hangs on wait().
Why is that? Does Linux require different approach for using condition_variable?
Your code exhibits UB immediately as it relocks the unique lock that the cv has relocked when it exits wait.
There are other problems, like not detecting spurious wakeups.
Finally cv notify all onky notified currently waiting threads. If a thread shows up later, no dice.
It's working by luck.
The mutex and the condition variable are two parts of the same construct. You can't mix and match mutexes and cvs.
try this:
void thread2() {
unique_lock<mutex> lock(mut); // use the global mutex
while (running) {
cv.wait(lock);
// mutex is already locked here
// test condition. wakeups can be spurious
// copy data
lock.unlock();
// process data
lock.lock();
}
}
Per this documentation:
Any thread that intends to wait on std::condition_variable has to
acquire a std::unique_lock, on the same mutex as used to
protect the shared variable
execute wait, wait_for, or wait_until. The wait operations atomically release the mutex and suspend the execution of the
thread.
When the condition variable is notified, a timeout expires, or a spurious wakeup occurs, the thread is awakened, and the mutex is
atomically reacquired. The thread should then check the condition
and resume waiting if the wake up was spurious.
This code
void thread2() {
mutex lockMutex;
unique_lock<mutex> lock(lockMutex);
while (running) {
doesn't do that.
class MyClass
{
public:
void PushMessage(MyMessage m) // Thread 1 calls this
{
boost::mutex::scoped_lock lock(mMutex);
mQueue.push_back(m);
mCondition.notify_one();
}
MyMessage PopMessage()
{
boost::mutex::scoped_lock lock(mMutex);
while(mQueue.empty())
mCondition.wait(lock);
MyMessage message = mQueue.front();
mQueue.pop_front();
return message;
}
void foo() // thread 2 is running this loop, and supposed to get messages
{
for(;;)
{
MyMessage message = PopMessage();
do_something(message);
}
}
private:
std::deque<MyMessage> mQueue;
boost::mutex mMutex;
boost::condition mCondition;
};
When I run the code, PushMessage is called, and foo() is waiting on PopMessage(), but PopMessage never returns.
What does do_something here is not irrelevant I think.
What am I doing wrong here?
Strangely, the above code worked fine under mac, but I'm having trouble on linux.
boost version is 1.44.0
Thank you
Rather than letting the scope of the lock object expire before it unlocks, you could try to manually unlock the mutex in PushMessage() before you unblock the waiting thread, i.e.,
void PushMessage(MyMessage m) // Thread 1 calls this
{
boost::mutex::scoped_lock lock(mMutex);
mQueue.push_back(m);
lock.unlock(); // <== manually unlock
mCondition.notify_one();
}
That way when thread 2 unblocks, there will be no "cross-over" time where thread 1 contains the lock, and thread 2 is trying to obtain a lock on your mutex. I don't see why that would create problems, but again, at least you won't have thread 2 trying to call lock.lock() while thread 1 still contains the lock.
I think you need 2 mutex objects, one is for synchronizing method call in different threads, one is for condition wait. You mixed them.
Will this code ever wait on the mutex inside the producer's void push(data)?
If so how do I get around that?
boost::mutex access;
boost::condition cond;
// consumer
data read()
{
boost::mutex::scoped_lock lock(access);
// this blocks until the data is ready
cond.wait(lock);
// queue is ready
return data_from_queue();
}
// producer
void push(data)
{
//<--- will a block ever happen here?
boost::mutex::scoped_lock lock(access);
// add data to queue
cond.notify_one();
}
Let's say I have a thread pool for(;;) loop and I have read() being called from a thread in this pool. Then I process data on it. And I call push() with some external thread. My question is, can that external thread ever block on its call to push(data)?
wait can return without notify ever being called. This is called a spurious wakeup. To handle this, code using a condition should always have a loop around the wait that checks that the expected condition really is in effect. For example:
queue data_queue;
boost::mutex access;
boost::condition cond;
// consumer
data read()
{
boost::mutex::scoped_lock lock(access);
while (queue.is_empty()) {
// this blocks until the data is ready
cond.wait(lock);
}
// queue is ready
return data_from_queue();
}
// producer
void push(data)
{
boost::mutex::scoped_lock lock(access);
// add data to queue
queue.push_back(data);
cond.notify_one();
}
Conceptually, "condition" is kind of misleading. Instead you can think of it as a signal. You are signalling another thread or threads to wake up, but you are not promising anything. Just, "Hey, maybe there's some data ready, why don't you go check eh?"
When .wait() is called it will block the calling thread in your thread pool and release the mutex. It will return when someone calls notify_one() or notify_all(). Before the thread that was blocked returns though, it will re-acquire the mutex and unblock the thread in your thread pool.
So the call to void push(data) by your external thread will only block temporarily up until .wait() is called.
See the boost documentation on the condition's wait function.