I have a working thread similar to the following code. In begin_work, it will check whether the working thread is executing before creating a new working thread. However, begin_work will never create the next working thread when the current thread is exited until I call end_work.
I have tried to call detach at the end of the thread and it works fine. Is it safe to call detach at the end of the thread? Or, how can I do to safely create the next working thread without calling end_work before calling begin_work?
class thread_worker {
private:
std::thread worker;
// ... other menbers
public:
thread_worker() {};
~thread_worker() { end_work(); };
void begin_work() {
if (!worker.joinable()) {
worker = std::thread { &thread_worker::do_work, this };
}
}
void do_work() {
// ... access other members ...
if (exit not by notify) {
worker.detach(); // can I call detach?
}
}
void end_work() {
if (worker.joinable()) {
// notify worker to exit
worker.join();
}
}
};
Edit:
My purpose is to call begin_work without block. If there is one working thread on execution, then the function will return directly or returns an is_working error. Otherwise, create a new working thread seamlessly.
Since std::thread::joinable() always returns true until join or detach is called. As a result, the future call of begin_work will never create the new working thread even though the current working thread has exited.
Therefore, I need a mechanism to automatically detach at the end of the thread.
I have tried to call detach at the end of the thread and it works fine
There's data race in the access to worker - it's undefined behaviour. When begin_work tests worker.joinable(), do_work might be detaching it at the same time (the call to worker.detach()).
You can instead detach the immediately when creating it:
worker = std::thread { &thread_worker::do_work, this };
worker.detach();
However, this can leave multiple threads running at the same time, which contradicts your requirement of running one worker thread at a time (but why only one? that just makes threading pointless).
Instead you can do:
void begin_work() {
end_work();
worker = std::thread { &thread_worker::do_work, this };
}
which ensures the previous thread completed.
Based on yuor edit, you only need to check whether you can join without wait - that seems to be reason you want to detach. You can instead do that with an atomic flag. Basically, you just to take care of the data race noted above.
class thread_worker {
private:
std::thread worker;
std::atomic_bool w_done {true};
// ... other menbers
public:
thread_worker() {};
~thread_worker() { end_work(); };
void begin_work() {
if (w_done) {
end_work();
worker = std::thread { &thread_worker::do_work, this };
}
}
void do_work() {
// ... access other members ...
w_done = true;
}
void end_work() {
w_done = false;
if (worker.joinable()) {
// notify worker to exit
worker.join();
}
}
};
In a project we're creating multiple statemachines in a wrapper-class. Each wrapper runs in it's own thread. When the jobs is done, the wrapper-class destructor is being called, and in there we would like to stop the thread.
Though if we're using thread.join(), we get a deadlock (since it tries to join itself). We could somehow signal another thread, but that seems a bit messy.
Is there any way to properly terminate the thread in which a class is running in, upon object destruction?
thread.join() does not stop a thread. It waits for the thread to finish and then returns. In order to stop a thread you have to have some way of telling the thread to stop, and the thread has to check to see whether it's time to stop. One way to do that is with an atomic bool:
class my_thread {
public:
my_thread() : done(false) { }
~my_thread() { done = true; thr.join(); }
void run() { thread th(&my_thread::do_it, this); swap(th, thr); }
private:
void do_it() { while (!done) { /* ... */ } }
std::thread thr;
std::atomic<bool> done;
};
That's off the top of my head; not compiled, not tested.
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();
}
I want to implement a multithreading environment using Qt4. The idea is as follows in c++-alike pseudo-code:
class Thread : public QThread {
QList<SubThread*> threads_;
public:
void run() {
foreach(SubThread* thread : threads) {
thread.start();
}
foreach(SubThread* thread : threads) {
thread.wait();
}
}
void abort() {
foreach(SubThread* thread : threads) {
thread.cancel();
}
}
public slots:
// This method is called from the main-thread
// (sometimes via some signal-slot-connection)
void changeSomeSettings() {
abort();
// change settings
start();
}
}
class SubThread : public QThread {
bool isCancelled_;
public:
void run() {
while(!isCancelled or task completed) {
// something that takes some time...
}
}
void cancel() {
if(isRunning() {
isCancelled_ = true;
}
}
}
The purpose is that the slot changeSomeSettings() kills all running threads, commits its changes and restarts it. What I want to achieve is that once this method has been started, it calls "abort" and then waits until all threads have terminated. Using mutexes in a wrong way:
void Thread::changeSomeSettings() {
mutex1.lock();
abort();
mutex2.lock();
start();
mutex1.unlock();
}
void Thread::run() {
foreach(Thread* thread : threads) {
thread.start();
}
foreach(Thread* thread : threads) {
thread.wait();
}
mutex2.unlock();
}
This actually works in Qt under MacOSX, yet according to the documentation mutex2 must be unlocked in the same thread (and in Windows I get an error). What is the best way to achieve my goal without running into racing conditions and deadlocks? Is there a better design than the one I have proposed here?
You probably want to use a condition variable instead of a mutex for this situation. A condition variable is a way for one thread to signal another. QT's implementation appears to be the QTWaitCondition:
I might have the child thread's periodically check the state of the condition variable. This can be done with QTWaitCondition::wait() with a short/0 timeout. If it is being signaled, then lock a shared memory area containing updated data and access the data that needs to be updated. Then that thread can safely restart itself accordingly.
It's usually not a good idea to just abort a thread. You may end up leaking memory/resources/handles/locks/etc. You don't know where that thread is in it's call stack, and there may be no guarantees that the stack will be "unwound" for you and all destructors are called. This is another reason for the child threads checking a condition variable periodically for updated data and having them restart themselves safely with the new data.
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