Here is a skeleton of my thread class:
class MyThread {
public:
virutal ~MyThread();
// will start thread with svc() as thread entry point
void start() = 0;
// derive class will specialize what the thread should do
virtual void svc() = 0;
};
Somewhere in code I create an instance of MyThread and later I want to destroy it.
In this case MyThread~MyThread() is called. MyThread:svc() is still running and using the object's data members. So I need a way politely inform MyThread:svc() to stop spinning, before proceeding with the destructor.
What is the acceptable way to destroy the thread object?
Note: I'm looking for platform agnostic solution.
UPD: It's clear that the root of problem is that there's no relationship between C++ object representing thread and OS thread. So the question is: in context of object destuction, is there an acceptable way to make thread object behave like an ordinary C++ object or should it be treated as an unusual one (e.g. should we call join() before destoying it?
Considering your additional requirements posted as comment to Checkers' reply (which is the
most straightforward way to do that):
I agree that join in DTor is problematic for various reasons. But from that the lifetime of your thread object is unrelated to the lifetime of the OS thread object.
First, you need to separate the data the thread uses from the thread object itself. They are distinct entities with distinct lifetime requirements.
One approach is to make the data refcounted, and have any thread that wants to access it hold a strong reference to the data. This way, no thread will suddenly grab into the void, but the data will be destroyed as soon as noone touches it anymore.
Second, about the thread object being destroyed when the thread joins:
I am not sure if this is a good idea. The thread object is normally a way to query the state of a thread - but with a thread object that dies as soon as the thread finishes, noone can tell you wether the thread finished.
Generally, I'd completely decouple the lifetime of the thread object from the lifetime of the OS thread: Destroying your thread object should not affect the thread itself. I see two basic approaches to this:
Thread Handle Object - reference counted again, returned by thread creator, can be released as early as one likes without affecting the OS thread. It would expose methods such as Join, IsFinished, and can give access to the thread shared data.
(If the thread object holds relevant execution state, the threafFunc itself could hold a reference to it, thereby ensuring the instance won't be released before the thread ends)
Thin Wrapper - You simply create a temporary around an OS thread handle. You could not hold additional state for the thread easily, but it might be just enough to make it work: At any place, you can turn an OS thread handle into an thread object. The majority of communication - e.g. telling the thread to terminate - would be via the shared data.
For your code example, this means: separate the start() from the svc()
You'd roughly work with this API (XxxxPtr could be e.g. boost::shared_ptr):
class Thread
{
public:
bool IsFinished();
void Join();
bool TryJoin(long timeout);
WorkerPtr GetWorker();
static ThreadPtr Start(WorkerPtr worker); // creates the thread
};
class Worker
{
private:
virtual void Svc() = 0;
friend class Thread; // so thread can run Svc()
}
Worker could contain a ThreadPtr itself, giving you a guarantee that the thread object exists during execution of Svc(). If multiple threads are allowed to work on the same data, this would have to be a thread list. Otherwise, Thread::Start would have to reject Workers that are already associated with a thread.
Motivation: What to do with rogue threads that block?
Assuming a thread fails to terminate within time for one reason or another, even though you told it to. You simply have three choices:
Deadlock, your applicaiton hangs. That usually happens if you join in the destructor.
Violently terminate the thread. That's potentially a violent termination of the app.
Let the thread run to completion on it's own data - you can notify the user, who can safely save & exit. Or you simply let the rogue thread dance on it's own copy of the data (not reference by the main thread anymore) until it completes.
Usually any OS-specific threads API will allow you to "join" a thread. That is, to block indefinitely on a thread handle until the thread functions returns.
So,
Signal the thread function to return (e.g. by setting a flag in its loop to false).
Join the thread, to make sure the actual thread terminates before you try to delete the thread object.
Then you can proceed with destruction of the thread object (you may also join in the destructor, though some people object to blocking destructors.).
I've had a project before with a similar "thread worker" class and a corresponding "work item" class (a-la Java's Thread and Runnable, except thread does not terminate but waits for a new Runnable object to be executed).
In the end, there was no difference if you join in a separate "shutdown" function or in the destructor, except a separate function is a bit more clear.
If you join in a destructor and a thread blocks, you will wait indefinitely.
If you join in a separate function and a thread blocks, you will wait indefinitely.
If you detach the thread and let it finish on its own, it will usually block application from exiting, so you will wait indefinitely.
So there is no straightforward way to make a thread behave like a regular C++ object and ignore its OS thread semantics, unless you can guarantee that your thread code can terminate almost immediately when notified to do so.
You could havee somthing like this in your svc method
while (alive){ //loops}
//free resources after while.
In your destructor, you could set the alive member to false. Or, you could have a pleaseDie() method, that sets the alive member to false, and can be called from the outside requesting the Thread instance to stop processing.
void
Thread::pleaseDie()
{
this->alive = false;
}
You first need a way to communicate with the thread to tell it to shut down. The best mechanism to do this depends on what svc() is doing. If, for example, it is looping on a message queue, you could insert a "please stop" message in that queue. Otherwise, you could simply add a member bool variable (and synchronize access to it) that is periodically checked by the svc(), and set by the thread wanting to destroy the object. Your could add a pure virtual stop() function to your base class, giving the implementor a clear signal that it has to implement svc() to make its class "runnable", and to implement stop() to make it "stoppable".
After asking the thread to stop, you must wait for it to exit before destroying the object. Again, there are several ways to do this. One is to make the stop() function blocking. It could wait, for example, for a "ok, I'm really stopped now" condition variable to be set by the thread running svc(). Alternatively, the caller could "wait" on the thread running svc(). The way to "wait" is platform dependent.
Most thread systems allow you to send a signal to a thead.
Example: pthreads
pthread_kill(pthread_t thread, int sig);
This will send a signall to a thread.
You can use this to kill the thread. Though this can leave a few of the resources hanging in an undefined state.
A solution to the resource problem is to install a signall handler.
So that when the signal handler is called it throws an exception. This will cause the thread stack to unwind to the entry point where you can then get the thread to check a variable about weather it is sill alive.
NOTE: You should never allow an exception to propogate out of a thread (this is so undefined my eyes bleed thinking about it). Basically catch the exception at the thread entry point then check some state variable to see if the thread should really exit.
Meanwhile the thread that sends the signal should wait for the thread to die by doing a join.
The only issues are that when you throw out of signal handler function you need to be careful. You should not use a signal that is asynchronus (ie one that could have been generated by a signal in another thread). A good one to use is SIGSEGV. If this happens normally then you have accessed invalid memory any you thread should think about exiting anyway!
You may also need to specify an extra flag on some systems to cope.
See This article
A working example using pthreads:
#include <pthread.h>
#include <iostream>
extern "C" void* startThread(void*);
extern "C" void shouldIexit(int sig);
class Thread
{
public:
Thread();
virtual ~Thread();
private:
friend void* startThread(void*);
void start();
virtual void run() = 0;
bool running;
pthread_t thread;
};
// I have seen a lot of implementations use a static class method to do this.
// DON'T. It is not portable. This is because the C++ ABI is not defined.
//
// It currently works on several compilers but will break if these compilers
// change the ABI they use. To gurantee this to work you should use a
// function that is declared as extern "C" this guarantees that the ABI is
// correct for the callback. (Note this is true for all C callback functions)
void* startThread(void* data)
{
Thread* thread = reinterpret_cast<Thread*>(data);
thread->start();
}
void shouldIexit(int sig)
{
// You should not use std::cout in signal handler.
// This is for Demo purposes only.
std::cout << "Signal" << std::endl;
signal(sig,shouldIexit);
// The default handler would kill the thread.
// But by returning you can continue your code where you left off.
// Or by throwing you can cause the stack to unwind (if the exception is caught).
// If you do not catch the exception it is implementation defined weather the
// stack is unwound.
throw int(3); // use int for simplicity in demo
}
Thread::Thread()
:running(true)
{
// Note starting the thread in the constructor means that the thread may
// start before the derived classes constructor finishes. This may potentially
// be a problem. It is started here to make the code succinct and the derived
// class used has no constructor so it does not matter.
if (pthread_create(&thread,NULL,startThread,this) != 0)
{
throw int(5); // use int for simplicity in demo.
}
}
Thread::~Thread()
{
void* ignore;
running = false;
pthread_kill(thread,SIGSEGV); // Tell thread it may want to exit.
pthread_join(thread,&ignore); // Wait for it to finish.
// Do NOT leave before thread has exited.
std::cout << "Thread Object Destroyed" << std::endl;
}
void Thread::start()
{
while(running)
{
try
{
this->run();
}
catch(...)
{}
}
std::cout << "Thread exiting" << std::endl;
}
class MyTestThread:public Thread
{
public:
virtual void run()
{
// Unless the signal causes an exception
// this loop will never exit.
while(true)
{
sleep(5);
}
}
};
struct Info
{
Info() {std::cout << "Info" << std::endl;}
~Info() {std::cout << "Done: The thread Should have exited before this" << std::endl;}
};
int main()
{
signal(SIGSEGV,shouldIexit);
Info info;
MyTestThread test;
sleep(4);
std::cout << "Exiting About to Exit" << std::endl;
}
> ./a.exe
Info
Exiting About to Exit
Signal
Thread exiting
Thread Object Destroyed
Done: The thread Should have exited before this
>
You should add dedicated thread management class (i.e. MyThreadMngr), that handles this and other tasks, like book keeping, owning the thread handles etc. The Thread itself should somehow signal to the thread manager that its going to terminate and MyThreadMngr should i.e. have a loop like Tom proposed.
There will probably be more actions that suite into such a thread manager class.
I reckon the easiest way to do this is to wrap the thread execution code in a loop
while(isRunning())
{
... thread implementation ...
}
You can also stop your thread by doing specific calls, for instance when you're using a WIN32 thread you can call TerminateThread on the thread handle in the destructor.
i give a simple and clean design, no signal, no sync, no kill needed.
per your MyThread, i suggest renaming and adding as below:
class MyThread {
public:
virutal ~MyThread();
// will be called when starting a thread,
// could do some initial operations
virtual bool OnStart() = 0;
// will be called when stopping a thread, say calling join().
virtual bool OnStop() = 0;
// derive class will specialize what the thread should do,
// say the thread loop such as
// while (bRunning) {
// do the job.
// }
virtual int OnRun() = 0;
};
the thread interface user will control the lifetime of MyThread.
and actually the real thread object is as below:
class IThread
{
public:
virtual API ~IThread() {}
/* The real destructor. */
virtual void Destroy(void) = 0;
/* Starts this thread, it will call MyThread::OnStart()
* and then call MyThread::OnRun() just after created
* the thread. */
virtual bool Start(void) = 0;
/* Stops a thread. will call MyThread::OnStop(). */
virtual void Stop(void) = 0;
/* If Wait() called, thread won't call MyThread::OnStop().
* If could, it returns the value of MyThread::OnRun()
* returned */
virtual int Wait(void) = 0;
/* your staff */
virtual MyThread * Command(void) = 0;
};
/* The interface to create a thread */
extern IThread * ThrdCreate(MyThread *p);
See the complete interfaces
http://effoaddon.googlecode.com/svn/trunk/devel/effo/codebase/addons/thrd/include/thrd_i.h
Coding Examples
Case 1. Controlled thread loop
class ThreadLoop : public MyThread
{
private:
bool m_bRunning;
public:
virtual bool OnStart() { m_bRunning = true; }
virtual bool OnStop() { m_bRunning = false; }
virtual int OnRun()
{
while (m_bRunning) {
do your job;
}
}
};
int main(int argc, char **argv)
{
ThreadLoop oLoop;
IThread *pThread = ThrdCreate(&oLoop);
// Start the thread, it will call Loop::OnStart()
//and then call Loop::OnRun() internally.
pThread->Start();
do your things here. when it is time to stop the thread, call stop().
// Stop the thread, it will call Loop::OnStop(),
// so Loop::OnRun() will go to the end
pThread->Stop();
// done, destroy the thread
pThread->Destroy();
}
Case 2. Don't know when the thread will stop
class ThreadLoop : public MyThread
{
public:
virtual bool OnStart() { }
virtual bool OnStop() { }
virtual int OnRun()
{
do your job until finish.
}
};
int main(int argc, char **argv)
{
ThreadLoop oLoop;
IThread *pThread = ThrdCreate(&oLoop);
// Start the thread, it will call Loop::OnStart()
//and then call Loop::OnRun() internally.
pThread->Start();
do your things here. Since you don't know when the job will
finish in the thread loop. call wait().
// Wait the thread, it doesn't call Loop::OnStop()
pThread->Wait();
// done, destroy the thread
pThread->Destroy();
}
A complete IThread implementation:
see
http://effoaddon.googlecode.com/svn/trunk/devel/effo/codebase/addons/thrd/src/thrd/thrd.cpp
Related
class ThreadOne {
public:
ThreadOne();
void RealThread();
void EnqueueJob(s_info job);
std::queue<s_info> q_jobs;
private:
H5::H5File* targetFile = new H5::H5File("file.h5", H5F_ACC_TRUNC);
std::condition_variable cv_condition;
std::mutex m_job_q_;
};
ThreadOne::ThreadOne() {
}
void ThreadOne::RealThread() {
while (true) {
std::unique_lock<std::mutex> lock(m_job_q_);
cv_condition.wait(lock, [this]() { return !this->q_jobs.empty(); });
s_info info = std::move(q_jobs.front());
q_jobs.pop();
lock.unlock();
//* DO THE JOB *//
}
}
void ThreadOne::EnqueueJob(s_info job) {
{
std::lock_guard<std::mutex> lock(m_job_q_);
q_jobs.push(std::move(job));
}
cv_condition.notify_one();
}
ThreadOne *tWrite = new ThreadOne();
I want to make a thread and send it a pointer of an array and its name as a struct(s_info), and then make the thread write it into a file. I think that it's better than creating a thread whenever writing is needed.
I could make a thread pool and allocate jobs to it, but it's not allowed to write the same file concurrently in my situation, I think that just making a thread will be enough and the program will still do CPU-bound jobs when writing job is in process.
To sum up, this class (hopefully) gets array pointers and their dataset names, puts them in q_jobs and RealThread writes the arrays into a file.
I referred to a C++ thread pool program and the program initiates threads like this:
std::vector<std::thread> vec_worker_threads;
vector_worker_threads.reserve(num_threads_);
vector_worker_threads.emplace_back([this]() { this->RealThread(); });
I'm new to C++ and I understand what the code above does, but I don't know how to initiate RealThread in my class without a vector. How can I make an instance of the class that has a thread(RealThread) that's already ready inside it?
From what I can gather, and as already discussed in the comments, you simply want a std::thread member for ThreadOne:
class ThreadOne {
std::thread thread;
public:
~ThreadOne();
//...
};
//...
ThreadOne::ThreadOne() {
thread = std::thread{RealThread, this};
}
ThreadOne::~ThreadOne() {
// (potentially) notify thread to finish first
if(thread.joinable())
thread.join();
}
//...
ThreadOne tWrite;
Note that I did not start the thread in the member-initializer-list of the constructor in order to avoid the thread accessing other members that have not been initialized yet. (The default constructor of std::thread does not start any thread.)
I also wrote a destructor which will wait for the thread to finish and join it. You must always join threads before destroying the std::thread object attached to it, otherwise your program will call std::terminate and abort.
Finally, I replaced tWrite from being a pointer to being a class type directly. There is probably no reason for you to use dynamic allocation there and even if you have a need for it, you should be using
auto tWrite = std::make_unique<ThreadOne>();
or equivalent, instead, so that you are not going to rely on manually deleteing the pointer at the correct place.
Also note that your current RealThread function seems to never finish. It must return at some point, probably after receiving a notification from the main thread, otherwise thread.join() will wait forever.
I have this class Foo, such that when you create an instance of it, it starts a background thread in the constructor, and the background thread runs until the destructor is called on the object instance (which sets a flag to tell the background thread to exit) and then waits for the background thread finishes.
So, I want to write a small program that runs the background thread indefinitely
int main(int argc, char[][] argv)
{
Foo foo(argc, argv);
block_forever(); // How do I implement this portably?
return 0;
}
making something like ctrl-c/a signal to cause block_forever to return would be a nice bonus (but in general I can just kill the process)
Foo can be modeled as
class Foo
{
public:
Foo() : m_stopFlag(false)
{
m_thread = StartThread(&ThreadFn, this);
if (!m_thread) throw ...;
}
~Foo()
{
m_stopFlag = true;
JoinThread(m_thread);
}
private:
void* ThreadFn(void* threadParam)
{
Foo& foo(*static_cast<Foo*>(threadParam);
try
{
while (!foo.m_stopFlag)
{
DoSomethingInteresting();
SleepFiveSeconds();
}
return NULL;
}
catch (...)
{
abort();
}
}
volatile bool m_stopFlag;
THREAD_HANDLE m_thread;
};
So, the Foo background thread will never exit on it's own. It can only be stopped by the Foo instance destructor modifying m_stopFlag, and if anything goes wrong, any resulting exception will cause the process to abort.
I want to write a version of main() like the one above which will never (or at least not until something like a signal) reach the end of main(), and thus end my Foo background thread.
Foo is under my control, but I would rather not change it unless it was necessary.
Likely a while loop around a wait on a condition variable is what you are looking for. The condition variable is never signalled. You still need the while loop because of spurious lookups and you need a vestigial mutex. As a bonus, when you do need to break out of the wait loop, it will be obvious how to do so. Waiting on a semaphore will also work if that abstraction is available.
That said, in any real situation, there are complications. First off, The thread abstraction may or may not be within your definition of "portable." Second, in some environments a main thread has responsibilities such as running an event loop. So this will not be entirely portable. On POSIX based systems I might just use sigwait.
i'm currently writing a c/c++ dll for later use mostly in Delphi and i'm more familiar with threads in Delphi than c/c++ and especially boost. So i wonder how i can achieve the following scenario?
class CMyClass
{
private:
boost::thread* doStuffThread;
protected:
void doStuffExecute(void)
{
while(!isTerminationSignal()) // loop until termination signal
{
// do stuff
}
setTerminated(); // thread is finished
};
public:
CMyClass(void)
{
// create thread
this->doStuffThread = new boost::thread(boost::bind(&CMyClass::doStuffExecute, this));
};
~CMyClass(void)
{
// finish the thread
signalThreadTermination();
waitForThreadFinish();
delete this->doStuffThread;
// do other cleanup
};
}
I have red countless articles about boost threading, signals and mutexes but i don't get it, maybe because it's friday ;) or is it not doable how i think to do it?
Regards
Daniel
Just use an atomic boolean to tell the thread to stop:
class CMyClass
{
private:
boost::thread doStuffThread;
boost::atomic<bool> stop;
protected:
void doStuffExecute()
{
while(!stop) // loop until termination signal
{
// do stuff
}
// thread is finished
};
public:
CMyClass() : stop(false)
{
// create thread
doStuffThread = boost::thread(&CMyClass::doStuffExecute, this);
};
~CMyClass()
{
// finish the thread
stop = true;
doStuffThread.join();
// do other cleanup
};
}
To wait for the thread to finish you just join it, that will block until it is finished and can be joined. You need to join the thread anyway before you can destroy it, or it will terminate your program.
There is no need to use a pointer and create the thread with new, just use a boost::thread object directly. Creating everything on the heap is wasteful, unsafe and poor style.
There is no need to use boost::bind to pass arguments to the thread constructor. For many many years boost::thread has supported passing multiple arguments to its constructor directly and it does the binding internally.
It's important that stop has been initialized to false before the new thread is created, otherwise if the new thread is spawned very quickly it could check the value of stop before it is initialized, and might happen to read a true value from the uninitialized memory, and then it would never enter the loop.
On the subject of style, writing foo(void) is considered by many C++ programmers to be a disgusting abomination. If you want to say your function takes no arguments then just write foo().
struct Test {
bool active{true};
void threadedUpdate() {
std::this_thread::sleep_for(std::chrono::milliseconds(1));
if(!active) // crashes here after Test instance is destroyed
return;
}
Test() {
std::thread([this]{ while(true) threadedUpdate(); }).detach();
}
~Test() {
// somehow stop the detached thread?
}
};
When an instance of Test is initialized, it spawns and detaches an std::thread which runs in background. When the same instance is destroyed, the previously mentioned thread tries to access the active member, which was destroyed along with the instance, causing a crash (and an AddressSanitizer backtrace).
Is there a way to stop the detached thread on ~Test()?
The design is bad. How should a thread running in background until the caller is destroyed be spawned/handled correctly?
Make the thread a member of the class, and instead of detaching it in the constructor, join it in the destructor. To stop the thread from looping, you can have a boolean inside the class that signals whether the thread should continue running or not (std::atomic<bool> update).
The thread could be executing this: [this] { while (update) threadUpdate(); }.
In the destructor of your class, do update = false, and call thread.join()
You can't stop detached threads. That's the point of .detach() - you don't have any way to refer to the detached thread anymore, at least as far as the C++ standard specifies. If you want to keep a handle to the thread, store the std::thread and call .join() in the destructor.
I'm trying to get a class run a thread, which will call a virtual member function named Tick() in a loop. Then I tried to derive a class and override the base::Tick().
but when execute, the program just call the base class's Tick instead of override one. any solutions?
#include <iostream>
#include <atomic>
#include <thread>
#include <chrono>
using namespace std;
class Runnable {
public:
Runnable() : running_(ATOMIC_VAR_INIT(false)) {
}
~Runnable() {
if (running_)
thread_.join();
}
void Stop() {
if (std::atomic_exchange(&running_, false))
thread_.join();
}
void Start() {
if (!std::atomic_exchange(&running_, true)) {
thread_ = std::thread(&Runnable::Thread, this);
}
}
virtual void Tick() {
cout << "parent" << endl;
};
std::atomic<bool> running_;
private:
std::thread thread_;
static void Thread(Runnable *self) {
while(self->running_) {
self->Tick();
std::this_thread::sleep_for(std::chrono::milliseconds(100));
}
}
};
class Fn : public Runnable {
public:
void Tick() {
cout << "children" << endl;
}
};
int main (int argc, char const* argv[])
{
Fn fn;
fn.Start();
return 0;
}
outputs:
parent
You can't let an object run out of scope until you're finished using it! The return 0; at the end of main causes fn to go out of scope. So by the time you get around to calling tick, there's no guarantee the object even exists any more.
(The logic in ~Runnable is totally broken. Inside the destructor is way too late -- the object is already at least partially destroyed.)
The approach of using inheritance with the parent serving as control for the thread and the children implementing the functions is a bad idea in general. The common problems with this approach come from construction and destruction:
if the thread is started from the constructor in the parent (control) then it might start running before the constructor completes and the thread might call the virtual function before the complete object has been fully constructed
if the thread is stopped in the destructor of the parent, then by the time that the control joins the thread, the thread is executing a method on an object that does no longer exist.
In your particular case you are hitting the second case. The program starts executing, and in main the second thread is started. At that point there is a race between the main thread and the newly launched, if the new thread is faster (unlikely, as starting the thread is an expensive operation), it will call the member method Tick that will be dispatched to the final overrider Fn::Tick.
But if the main thread is faster it will exit the scope of main, and it will start destruction of the object, it will complete destruction of the Fn object and during construction of the Runnable it will join the thread. If the main thread is fast enough, it will make it to the join before the second thread and wait there for the second thread to call Tick on the now final overrider that is Runnable::Tick. Note that this is Undefined Behavior, and not guaranteed, since the second thread is accessing an object that is being destroyed.
Also, there are other possible orderings, like for example, the second thread could dispatch to Fn::Tick before the main thread starts destruction, but might not complete the function before the main thread destroys the Fn sub object, in which case your second thread would be calling a member function on a dead object.
You should rather follow the approach in the C++ standard: separate the control from the logic, fully construct the object that will be run and pass it to the thread during construction. Note that this is the case of Java's Runnable, which is recommended over extending the Thread class. Note that from a design point of view this separation makes sense: the thread object manages the execution, and the runnable is the code to execute.
A thread is not a ticker, but rather what controls the execution of the ticker. And in your code Runnable is not something that can be run, but rather something that runs other objects that happen to derive from it.