class Class {
public:
Class ();
private:
std::thread* updationThread;
};
Constructor:
Class::Class() {
updationThread = new std::thread(&someFunc);
}
At some point in my application, I have to pause that thread and call a function and after execution of that function I have to resume the thread. Let's say it happens here:
void Class::aFunction() {
functionToBeCalled(); //Before this, the thread should be paused
//Now, the thread should be resumed.
}
I have tried to use another thread with function functionToBeCalled() and use thread::join but was unable to do that for some reason.
How can I pause a thread or how can I use thread::join to pause a thread until the other finishes?
I don't think you can easily (in a standard way) "pause" some thread, and then resumes it. I imagine you can send SIGSTOP and SIGCONT if you are using some Unix-flavored OS, but otherwise, you should properly mark the atomic parts inside someFunc() with mutexes and locks, an wraps functionToBeCalled() with a lock on the corresponding mutex:
std::mutex m; // Global mutex, you should find a better place to put it
// (possibly in your object)
and inside the function:
void someFunc() {
// I am just making up stuff here
while(...) {
func1();
{
std::lock_guard<std::mutex> lock(m); // lock the mutex
...; // Stuff that must not run with functionToBeCalled()
} // Mutex unlocked here, by end of scope
}
}
and when calling functionToBeCalled():
void Class::aFunction() {
std::lock_guard<std::mutex> lock(m); // lock the mutex
functionToBeCalled();
} // Mutex unlocked here, by end of scope
You can use a condition variable. An example similar to your situation is given there:
http://en.cppreference.com/w/cpp/thread/condition_variable
Related
I would like to write a class that wraps around std::thread and behaves like a std::thread but without actually allocating a thread every time I need to process something async. The reason is that I need to use multi threading in a context where I'm not allow to dynamically allocate and I also don't want to have the overhead of creating a std::thread.
Instead, I want a thread to run in a loop and wait until it can start processing. The client calls invoke which wakes up the thread. The Thread locks a mutex, does it's processing and falls asleep again. A function join behaves like std::thread::join by locking until the thread frees the lock (i.e. falls asleep again).
I think I got the class to run but because of a general lack of experience in multi threading, I would like to ask if anybody can spot race conditions or if the approach I used is considered "good style". For example, I'm not sure if temporary locking the mutex is a decent way to "join" the thread.
EDIT
I found another race condition: when calling join directly after invoke, there is no reason the thread already locked the mutex and thus locks the caller of join until the thread goes to sleep. To prevent this, I had to add a check for the invoke counter.
Header
#pragma once
#include <thread>
#include <atomic>
#include <mutex>
class PersistentThread
{
public:
PersistentThread();
~PersistentThread();
// set function to invoke
// locks if thread is currently processing _func
void set(const std::function<void()> &f);
// wakes the thread up to process _func and fall asleep again
// locks if thread is currently processing _func
void invoke();
// mimics std::thread::join
// locks until the thread is finished with it's loop
void join();
private:
// intern thread loop
void loop(bool *initialized);
private:
bool _shutdownRequested{ false };
std::mutex _mutex;
std::unique_ptr<std::thread> _thread;
std::condition_variable _cond;
std::function<void()> _func{ nullptr };
};
Source File
#include "PersistentThread.h"
PersistentThread::PersistentThread()
{
auto lock = std::unique_lock<std::mutex>(_mutex);
bool initialized = false;
_thread = std::make_unique<std::thread>(&PersistentThread::loop, this, &initialized);
// wait until _thread notifies, check bool initialized to prevent spurious wakeups
_cond.wait(lock, [&] {return initialized; });
}
PersistentThread::~PersistentThread()
{
{
std::lock_guard<std::mutex> lock(_mutex);
_func = nullptr;
_shutdownRequested = true;
// wake up and let join
_cond.notify_one();
}
// join thread,
if (_thread->joinable())
{
_thread->join();
}
}
void PersistentThread::set(const std::function<void()>& f)
{
std::lock_guard<std::mutex> lock(_mutex);
this->_func = f;
}
void PersistentThread::invoke()
{
std::lock_guard<std::mutex> lock(_mutex);
_cond.notify_one();
}
void PersistentThread::join()
{
bool joined = false;
while (!joined)
{
std::lock_guard<std::mutex> lock(_mutex);
joined = (_invokeCounter == 0);
}
}
void PersistentThread::loop(bool *initialized)
{
std::unique_lock<std::mutex> lock(_mutex);
*initialized = true;
_cond.notify_one();
while (true)
{
// wait until we get the mutex again
_cond.wait(lock, [this] {return _shutdownRequested || (this->_invokeCounter > 0); });
// shut down if requested
if (_shutdownRequested) return;
// process
if (_func) _func();
_invokeCounter--;
}
}
You are asking about potential race conditions, and I see at least one race condition in the shown code.
After constructing a PersistentThread, there is no guarantee that the new thread will acquire its initial lock in its loop() before the main execution thread returns from the constructor and enters invoke(). It is possible that the main execution thread enters invoke() immediately after the constructor is complete, ends up notifying nobody, since the internal execution thread hasn't locked the mutex yet. As such, this invoke() will not result in any processing taking place.
You need to synchronize the completion of the constructor with the execution thread's initial lock acquisition.
EDIT: your revision looks right; but I also spotted another race condition.
As documented in the description of wait(), wait() may wake up "spuriously". Just because wait() returned, doesn't mean that some other thread has entered invoke().
You need a counter, in addition to everything else, with invoke() incrementing the counter, and the execution thread executing its assigned duties only when the counter is greater than zero, decrementing it. This will guard against spurious wake-ups.
I would also have the execution thread check the counter before entering wait(), and enter wait() only if it is 0. Otherwise, it decrements the counter, executes its function, and loops back.
This should plug up all the potential race conditions in this area.
P.S. The spurious wake-up also applies to the initial notification, in your correction, that the execution thread has entered the loop. You'll need to do something similar for that situation, too.
I don't understand what you're trying to ask exactly. It's a nice style you used.
It would be much safer using bools and check the single routines because void returns nothing so you could be maybe stuck caused by bugs. Check everything you can since the thread runs under the hood. Make sure the calls are running correctly, if the process had really success. Also you could read some stuff about "Thread Pooling".
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 am trying to write a class that would run a thread upon its object creation and stop the thread once the object gets deleted.
class MyThread : public boost::thread {
public:
MyThread() : bAlive(true) {
boost::thread(&MyThread::ThreadFunction,this);
}
~MyThread() {
{
boost::unique_lock<boost::mutex> lock(Mutex);
bAlive=false;
}
ConditionVariable.notify_one();
join();
}
private:
volatile bool bAlive;
boost::mutex Mutex;
boost::condition_variable ConditionVariable;
void ThreadFunction() {
boost::unique_lock<boost::mutex> lock(Mutex);
while(bAlive) {
ConditionVariable.timed_wait(lock,boost::get_system_time()+ boost::posix_time::milliseconds(MAX_IDLE));
/*******************************************
* Here goes some code executed by a thread *
*******************************************/
}
}
};
Theoretically, I want to wake the thread up instantly as soon as it needs to be finished, so I had to use timed_wait instead of Sleep.
This works fine until I try to delete an object of this class. In most cases, it deletes normally, but occasionally it causes an error either in condition_variable.hpp, thread_primitives.hpp or crtexe.c. Sometimes I am notified that "Free Heap block 3da7a8 modified at 3da804 after it was freed", and sometimes I'm not. And yes, I'm aware of the spurious wakeups of timed_wait, in this case it's not critical.
Can you please point me to the source of my problem? What am I doing wrong?
I see what you're trying to do but it doesn't work as you expect:
MyThread foo;
default constructs a boost::thread (because MyThread is derived from boost::thread).
The default constructor creates a boost::thread instance that refers to Not-a-Thread.
MyThread() {
boost::thread(&MyThread::ThreadFunction,this);
}
is actually creating a different thread and you're ignoring the returned object (the valid thread).
~MyThread() {
// ...
join();
}
is then trying to join the default constructed thread (which throws an exception inside the destructor) and you never join the thread that actually does the work.
First of all, don't derive from boost::thread. Create a member variable instead:
class MyThread {
// ...
private:
// ...
boost::thread _thread;
};
In the constructor, create and assign a thread to that member variable:
MyThread() {
_thread = boost::thread(&MyThread::ThreadFunction,this);
}
and call its join() in your destructor.
~MyThread() {
// ...
_thread.join();
}
That should fix your problem.
However, if you simply want to exit the thread when your object is destroyed (and don't have to wake it up while its running), you can use a different approach. Remove the mutex and the condition variable and use interrupt instead. This will cause sleep() to throw an exception so you have to catch it:
void ThreadFunction() {
try {
for(;;) {
boost::this_thread::sleep(boost::posix_time::milliseconds(MAX_IDLE));
// Here goes some code executed by a thread
}
} catch( const boost::thread_interrupted& e ) {
// ignore exception: thread interrupted, exit function
}
}
This will instantly exit the ThreadFunction when the thread is interrupted. If you don't need the thread to sleep every cycle, you can replace it with boost::this_thread::interruption_point(). This will just throw an exception if the thread is interrupted.
Now you can simply interrupt the thread in the destructor:
MyThread::~MyThread() {
_thread.interrupt();
_thread.join();
}
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.
I'm looking for a way to wait for multiple condition variables.
ie. something like:
boost::condition_variable cond1;
boost::condition_variable cond2;
void wait_for_data_to_process()
{
boost::unique_lock<boost::mutex> lock(mut);
wait_any(lock, cond1, cond2); //boost only provides cond1.wait(lock);
process_data();
}
Is something like this possible with condition variables. And if not are there alternative solutions?
Thanks
I don't believe you can do anything like this with boost::thread. Perhaps because POSIX condition variables don't allow this type of construct. Of course, Windows has WaitForMultipleObjects as aJ posted, which could be a solution if you're willing to restrict your code to Windows synchronization primitives.
Another option would to use fewer condition variables: just have 1 condition variable that you fire when anything "interesting" happens. Then, any time you want to wait, you run a loop that checks to see if your particular situation of interest has come up, and if not, go back to waiting on the condition variable. You should be waiting on those condition variables in such a loop anyways, as condition variable waits are subject to spurious wakeups (from boost::thread docs, emphasis mine):
void wait(boost::unique_lock<boost::mutex>& lock)
...
Effects:
Atomically call lock.unlock() and blocks the current thread. The thread will unblock when notified by a call to this->notify_one() or this->notify_all(), or spuriously. ...
As Managu already answered, you can use the same condition variable and check for multiple "events" (bool variables) in your while loop. However, concurrent access to these bool variables must be protected using the same mutex that the condvar uses.
Since I already went through the trouble of typing this code example for a related question, I'll repost it here:
boost::condition_variable condvar;
boost::mutex mutex;
bool finished1 = false;
bool finished2 = false;
void longComputation1()
{
{
boost::lock_guard<boost::mutex> lock(mutex);
finished1 = false;
}
// Perform long computation
{
boost::lock_guard<boost::mutex> lock(mutex);
finished1 = true;
}
condvar.notify_one();
}
void longComputation2()
{
{
boost::lock_guard<boost::mutex> lock(mutex);
finished2 = false;
}
// Perform long computation
{
boost::lock_guard<boost::mutex> lock(mutex);
finished2 = true;
}
condvar.notify_one();
}
void somefunction()
{
// Wait for long computations to finish without "spinning"
boost::lock_guard<boost::mutex> lock(mutex);
while(!finished1 && !finished2)
{
condvar.wait(lock);
}
// Computations are finished
}
alternative solutions?
I am not sure of Boost library but you can use WaitForMultipleObjects Function to wait for multiple kernel objects. Just check if this helps.
As Managu points out using multiple conditions might not be a good solution in the first place. What you want to do should be possible to be implemented using Semaphores.
Using the same condition variable for multiple events technically works, but it doesn't allow encapsulation. So I had an attempt at making a class that supports it. Not tested yet! Also it doesn't support notify_one() as I haven't worked out how to implement that.
#pragma once
#include <condition_variable>
#include <unordered_set>
// This is like a `condition_variable` but you can wait on multiple `multi_condition_variable`s.
// Internally it works by creating a new `condition_variable` for each `wait_any()` and registering
// it with the target `multi_condition_variable`s. When `notify_all()` is called, the main `condition_variable`
// is notified, as well as all the temporary `condition_variable`s created by `wait_any()`.
//
// There are two caveats:
//
// 1. You can't call the destructor if any threads are `wait()`ing. This is difficult to get around but
// it is the same as `std::wait_condition` anyway.
//
// 2. There is no `notify_one()`. You can *almost* implement this, but the only way I could think to do
// it was to add an `atomic_int` that indicates the number of waits(). Unfortunately there is no way
// to atomically increment it, and then wait.
class multi_condition_variable
{
public:
multi_condition_variable()
{
}
// Note that it is only safe to invoke the destructor if no thread is waiting on this condition variable.
~multi_condition_variable()
{
}
// Notify all threads calling wait(), and all wait_any()'s that contain this instance.
void notify_all()
{
_condition.notify_all();
for (auto o : _others)
o->notify_all();
}
// Wait for notify_all to be called, or a spurious wake-up.
void wait(std::unique_lock<std::mutex>& loc)
{
_condition.wait(loc);
}
// Wait for any of the notify_all()'s in `cvs` to be called, or a spurious wakeup.
static void wait_any(std::unique_lock<std::mutex>& loc, std::vector<std::reference_wrapper<multi_condition_variable>> cvs)
{
std::condition_variable c;
for (multi_condition_variable& cv : cvs)
cv.addOther(&c);
c.wait(loc);
for (multi_condition_variable& cv : cvs)
cv.removeOther(&c);
}
private:
void addOther(std::condition_variable* cv)
{
std::lock_guard<std::mutex> lock(_othersMutex);
_others.insert(cv);
}
void removeOther(std::condition_variable* cv)
{
// Note that *this may have been destroyed at this point.
std::lock_guard<std::mutex> lock(_othersMutex);
_others.erase(cv);
}
// The condition variable.
std::condition_variable _condition;
// When notified, also notify these.
std::unordered_set<std::condition_variable*> _others;
// Mutex to protect access to _others.
std::mutex _othersMutex;
};
// Example use:
//
// multi_condition_variable cond1;
// multi_condition_variable cond2;
//
// void wait_for_data_to_process()
// {
// unique_lock<boost::mutex> lock(mut);
//
// multi_condition_variable::wait_any(lock, {cond1, cond2});
//
// process_data();
// }