I'm using the boost interprocess library to create server and client programs for passing opencv mat objects around in shared memory. Each server and client process has two boost threads which are members of a boost::thread_group. One handles command line IO while the other manages data processing. Shared memory access is synchronized using boost::interprocess condition_variables.
Since this program involves shared memory, I need to do some manual cleaning before exiting. My problem is that if the server terminates prematurely, then the processing thread on the client blocks at the wait() call since the server is responsible for sending notifications. I need to somehow interrupt the thread stuck at wait() to initiate shared memory destruction. I understand that calling interrupt() (in my case, thread_group.interrupt_all()) on the thread will cause theboost::thread_interrupted exception to be thrown upon reaching a interruption point (such as wait()), which if left unhandled, would allow the shared memory destruction to proceed. However, when I try to interrupt the thread while it is in wait(), nothing seems to happen. For instance, this prints nothing to the command line:
try {
shared_mat_header->new_data_condition.wait(lock);
} catch (...) {
std::cout << "Thread interrupt occurred\n";
}
I am not at all sure, but it seems like the interrupt() call needs to occur before the thread enters wait() for the exception to be thrown. Is this true? If not, then what is the proper way to interrupt a boost thread that is blocked by a condition_variable.wait() call?
Thanks for any insight.
Edit
I accepted Chris Desjardins' answer, which does not answer the question directly, but has the intended effect. Here I'm translating his code snippet for use with boost::interprocess condition variables, which have slightly different syntax than boost::thread condition variables:
while (_running) {
boost::system_time timeout = boost::get_system_time() + boost::posix_time::milliseconds(1);
if (shared_mat_header->new_data_condition.timed_wait(lock, timeout))
{
//process data
}
}
I prefer to wait with timeouts, then check the return code of the wait call to see if it timed out or not. In fact I have a thread pattern I like to use that resolves this situation (and other common problems with threads in c++).
http://blog.chrisd.info/how-to-run-threads/
The main point for you is to not block infinitely in a thread, so your thread would look like this:
while (_running == true)
{
if (shared_mat_header->new_data_condition.wait_for(lock, boost::chrono::milliseconds(1)) == boost::cv_status::no_timeout)
{
// process data
}
}
Then in your destructor you set _running = false; and join the thread(s).
Try using the "notify function". Keep a pointer to your condition variable and call that instead of interrupting the threads. Interrupting is much more costly than a notify call.
So instead of doing
thread_group.interrupt_all()
call this instead
new_data_condition_pointer->notify_one()
Related
I am creating multiple threads in my program. On pressing Ctrl-C, a signal handler is called. Inside a signal handler, I have put exit(0) at last. The thing is that sometimes the program terminates safely but the other times, I get runtime error stating
abort() has been called
So what would be the possible solution to avoid the error?
The usual way is to set an atomic flag (like std::atomic<bool>) which is checked by all threads (including the main thread). If set, then the sub-threads exit, and the main thread starts to join the sub-threads. Then you can exit cleanly.
If you use std::thread for the threads, that's a possible reason for the crashes you have. You must join the thread before the std::thread object is destructed.
Others have mentioned having the signal-handler set a std::atomic<bool> and having all the other threads periodically check that value to know when to exit.
That approach works well as long as all of your other threads are periodically waking up anyway, at a reasonable frequency.
It's not entirely satisfactory if one or more of your threads is purely event-driven, however -- in an event-driven program, threads are only supposed to wake up when there is some work for them to do, which means that they might well be asleep for days or weeks at a time. If they are forced to wake up every (so many) milliseconds simply to poll an atomic-boolean-flag, that makes an otherwise extremely CPU-efficient program much less CPU-efficient, since now every thread is waking up at short regular intervals, 24/7/365. This can be particularly problematic if you are trying to conserve battery life, as it can prevent the CPU from going into power-saving mode.
An alternative approach that avoids polling would be this one:
On startup, have your main thread create an fd-pipe or socket-pair (by calling pipe() or socketpair())
Have your main thread (or possibly some other responsible thread) include the receiving-socket in its read-ready select() fd_set (or take a similar action for poll() or whatever wait-for-IO function that thread blocks in)
When the signal-handler is executed, have it write a byte (any byte, doesn't matter what) into the sending-socket.
That will cause the main thread's select() call to immediately return, with FD_ISSET(receivingSocket) indicating true because of the received byte
At that point, your main thread knows it is time for the process to exit, so it can start directing all of its child threads to start shutting down (via whatever mechanism is convenient; atomic booleans or pipes or something else)
After telling all the child threads to start shutting down, the main thread should then call join() on each child thread, so that it can be guaranteed that all of the child threads are actually gone before main() returns. (This is necessary because otherwise there is a risk of a race condition -- e.g. the post-main() cleanup code might occasionally free a resource while a still-executing child thread was still using it, leading to a crash)
The first thing you must accept is that threading is hard.
A "program using threading" is about as generic as a "program using memory", and your question is similar to "how do I not corrupt memory in a program using memory?"
The way you handle threading problem is to restrict how you use threads and the behavior of the threads.
If your threading system is a bunch of small operations composed into a data flow network, with an implicit guarantee that if an operation is too big it is broken down into smaller operations and/or does checkpoints with the system, then shutting down looks very different than if you have a thread that loads an external DLL that then runs it for somewhere from 1 second to 10 hours to infinite length.
Like most things in C++, solving your problem is going to be about ownership, control and (at a last resort) hacks.
Like data in C++, every thread should be owned. The owner of a thread should have significant control over that thread, and be able to tell it that the application is shutting down. The shut down mechanism should be robust and tested, and ideally connected to other mechanisms (like early-abort of speculative tasks).
The fact you are calling exit(0) is a bad sign. It implies your main thread of execution doesn't have a clean shutdown path. Start there; the interrupt handler should signal the main thread that shutdown should begin, and then your main thread should shut down gracefully. All stack frames should unwind, data should be cleaned up, etc.
Then the same kind of logic that permits that clean and fast shutdown should also be applied to your threaded off code.
Anyone telling you it is as simple as a condition variable/atomic boolean and polling is selling you a bill of goods. That will only work in simple cases if you are lucky, and determining if it works reliably is going to be quite hard.
Additional to Some programmer dude answer and related to discussion in the comment section, you need to make the flag that controls termination of your threads as atomic type.
Consider following case :
bool done = false;
void pending_thread()
{
while(!done)
{
std::this_thread::sleep(std::milliseconds(1));
}
// do something that depends on working thread results
}
void worker_thread()
{
//do something for pending thread
done = true;
}
Here worker thread can be your main thread also and done is terminating flag of your thread, but pending thread need to do something with given data by working thread, before exiting.
this example has race condition and undefined behaviour along with it, and it's really hard to find what is the actual problem int the real world.
Now the corrected version using std::automic :
std::atomic<bool> done(false);
void pending_thread()
{
while(!done.load())
{
std::this_thread::sleep(std::milliseconds(1));
}
// do something that depends on working thread results
}
void worker_thread()
{
//do something for pending thread
done = true;
}
You can exit thread without being concern of race condition or UB.
I am recently working with threads in C++11. now I am thinking about how to force stop a thread. I couldn't find it on stackoverflow, and also tried these.
One variable each thread : not so reliable
return in the main thread : I have to force quit only one not all
and I have no more ideas. I have heard about WinAPI, but I want a portable solution. (that also means I wont use fork())
Can you please give me a solution of this? I really want to do it.
One of the biggest problems with force closing a thread in C++ is the RAII violation.
When a function (and subsequently, a thread), gracefully finishes, everything it held is gracefully cleaned up by the destructors of the objects the functions/threads created.
Memory gets freed,
OS resources (handles, file descriptors etc.) are closed and returned to the OS
Locks are getting unlocked so other threads can use the shared resources they protect.
other important tasks are preformed (such as updating counters, logging, etc.).
If you brutally kill a thread (aka by TerminateThread on Windows, for example), non of these actually happen, and the program is left in a very dangerous state.
A (not-so) common pattern that can be used is to register a "cancellation token" on which you can monitor and gracefully shut the thread if other thread asks so (a la TPL/PPL). something like
auto cancellationToken = std::make_shared<std::atomic_bool>();
cancellationToken->store(false);
class ThreadTerminator : public std::exception{/*...*/};
std::thread thread([cancellationToken]{
try{
//... do things
if (cancellationToken->load()){
//somone asked the thred to close
throw ThreadTerminator ();
}
//do other things...
if (cancellationToken->load()){
//somone asked the thred to close
throw ThreadTerminator ();
}
//...
}catch(ThreadTerminator){
return;
}
});
Usually, one doesn't even open a new thread for a small task, it's better to think of a multi threaded application as a collection of concurrent tasks and parallel algorithms. one opens a new thread for some long ongoing background task which is usually performed in some sort of a loop (such as, accepting incoming connections).
So, anyway, the cases for asking a small task to be cancelled are rare anyway.
tldr:
Is there a reliable way to force a thread to stop in C++?
No.
Here is my approach for most of my designs:
Think of 2 kinds of Threads:
1) primary - I call main.
2) subsequent - any thread launched by main or any subsequent thread
When I launch std::thread's in C++ (or posix threads in C++):
a) I provide all subsequent threads access to a boolean "done", initialized to false. This bool can be directly passed from main (or indirectly through other mechanisms).
b) All my threads have a regular 'heartbeat', typically with a posix semaphore or std::mutex, sometimes with just a timer, and sometimes simply during normal thread operation.
Note that a 'heartbeat' is not polling.
Also note that checking a boolean is really cheap.
Thus, whenever main wants to shut down, it merely sets done to true and 'join's with the subsequent threads.
On occasion main will also signal any semaphore (prior to join) that a subsequent thread might be waiting on.
And sometimes, a subsequent thread has to let its own subsequent thread know it is time to end.
Here is an example -
main launching a subsequent thread:
std::thread* thrd =
new std::thread(&MyClass_t::threadStart, this, id);
assert(nullptr != thrd);
Note that I pass the this pointer to this launch ... within this class instance is a boolean m_done.
Main Commanding shutdown:
In main thread, of course, all I do is
m_done = true;
In a subsequent thread (and in this design, all are using the same critical section):
void threadStart(uint id) {
std::cout << id << " " << std::flush; // thread announce
do {
doOnce(id); // the critical section is in this method
}while(!m_done); // exit when done
}
And finally, at an outer scope, main invokes the join.
Perhaps the take away is - when designing a threaded system, you should also design the system shut down, not just add it on.
I am using ZThreads to illustrate the question but my question applies to PThreads, Boost Threads and other such threading libraries in C++.
class MyClass: public Runnable
{
public:
void run()
{
while(1)
{
}
}
}
I now launch this as follows:
MyClass *myClass = new MyClass();
Thread t1(myClass);
Is it now possible to kill (violently if necessary) this thread? I can do this for sure instead of the infinite loop I had a Thread::Sleep(100000) that is, if it is blocking. But can I kill a spinning thread (doing computation). If yes, how? If not, why not?
As far as Windows goes (from MSDN):
TerminateThread is a dangerous function that should only be used in
the most extreme cases. You should call TerminateThread only if you
know exactly what the target thread is doing, and you control all of
the code that the target thread could possibly be running at the time
of the termination. For example, TerminateThread can result in the
following problems:
If the target thread owns a critical section, the critical section will not be released.
If the target thread is allocating memory from the heap, the heap lock will not be released.
If the target thread is executing certain kernel32 calls when it is terminated, the kernel32 state for the thread's process could be inconsistent.
If the target thread is manipulating the global state of a shared DLL, the state of the DLL could be destroyed, affecting other users of the DLL.
Boost certainly doesn't have a thread-killing function.
A general solution to the kind of question posted can be found in Herb Sutter article:
Prefer Using Active Objects Instead of Naked Threads
This permits you to have something like this (excerpt from article):
class Active {
public:
typedef function<void()> Message;
private:
Active( const Active& ); // no copying
void operator=( const Active& ); // no copying
bool done; // le flag
message_queue<Message> mq; // le queue
unique_ptr<thread> thd; // le thread
void Run() {
while( !done ) {
Message msg = mq.receive();
msg(); // execute message
} // note: last message sets done to true
}
In the active object destructor you can have then:
~Active() {
Send( [&]{ done = true; } ); ;
thd->join();
}
This solution promotes a clean thread function exist, and avoids all other issues related to an unclean thread termination.
It is possible to terminate a thread forcefully, but the call to do it is going to be platform specific. For example, under Windows you could do it with the TerminateThread function.
Keep in mind that if you use TerminateThread, the thread will not get a chance to release any resources it is using until the program terminates.
If you need to kill a thread, consider using a process instead.
Especially if you tell us that your "thread" is a while (true) loop that may sleep for a long period of time performing operations that are necessarily blocking. To me, that indicate a process-like behavior.
Processes can be terminated in a various number of ways at almost any time and always in a clean way. They may also offer more reliability in case of a crash.
Modern operating systems offer an array of interprocess communications facilities: sockets, pipes, shared memory, memory mapped files ... They may even exchange file descriptors.
Good OSes have copy-on-write mechanism, so processes are cheap to fork.
Note that if your operations can be made in a non-blocking way, then you should use a poll-like mechanism instead. Boost::asio may help there.
You can with TerminateThread() API, but it is not recommended.
More details at:
http://msdn.microsoft.com/en-us/library/windows/desktop/ms686717(v=vs.85).aspx
As people already said, there is no portable way to kill a thread, and in some cases not possible at all. If you have control over the code (i.e. can modify it) one of the simplest ways is to have a boolean variable that the thread checks in regular intervals, and if set then terminate the thread as soon as possible.
Can't you do add something like below
do {
//stuff here
} while (!abort)
And check the flag once in a while between computations if they are small and not too long (as in the loop above) or in the middle and abort the computation if it is long?
Not sure of the other libraries but in pthread library pthread_kill function is available pthread_kill
Yes,
Define keepAlive variable as an int .
Initially set the value of keepAlive=1 .
class MyClass: public Runnable
{
public:
void run()
{
while(keepAlive)
{
}
}
}
Now, when every you want to kill thread just set the value of keepAlive=0 .
Q. How this works ?
A. Thread will be live until the execution of the function continuous . So it's pretty simple to Terminate a function . set the value of variable to 0 & it breaks which results in killing of thread . [This is the safest way I found till date] .
How can I wait for a detached thread to finish in C++?
I don't care about an exit status, I just want to know whether or not the thread has finished.
I'm trying to provide a synchronous wrapper around an asynchronous thirdarty tool. The problem is a weird race condition crash involving a callback. The progression is:
I call the thirdparty, and register a callback
when the thirdparty finishes, it notifies me using the callback -- in a detached thread I have no real control over.
I want the thread from (1) to wait until (2) is called.
I want to wrap this in a mechanism that provides a blocking call. So far, I have:
class Wait {
public:
void callback() {
pthread_mutex_lock(&m_mutex);
m_done = true;
pthread_cond_broadcast(&m_cond);
pthread_mutex_unlock(&m_mutex);
}
void wait() {
pthread_mutex_lock(&m_mutex);
while (!m_done) {
pthread_cond_wait(&m_cond, &m_mutex);
}
pthread_mutex_unlock(&m_mutex);
}
private:
pthread_mutex_t m_mutex;
pthread_cond_t m_cond;
bool m_done;
};
// elsewhere...
Wait waiter;
thirdparty_utility(&waiter);
waiter.wait();
As far as I can tell, this should work, and it usually does, but sometimes it crashes. As far as I can determine from the corefile, my guess as to the problem is this:
When the callback broadcasts the end of m_done, the wait thread wakes up
The wait thread is now done here, and Wait is destroyed. All of Wait's members are destroyed, including the mutex and cond.
The callback thread tries to continue from the broadcast point, but is now using memory that's been released, which results in memory corruption.
When the callback thread tries to return (above the level of my poor callback method), the program crashes (usually with a SIGSEGV, but I've seen SIGILL a couple of times).
I've tried a lot of different mechanisms to try to fix this, but none of them solve the problem. I still see occasional crashes.
EDIT: More details:
This is part of a massively multithreaded application, so creating a static Wait isn't practical.
I ran a test, creating Wait on the heap, and deliberately leaking the memory (i.e. the Wait objects are never deallocated), and that resulted in no crashes. So I'm sure it's a problem of Wait being deallocated too soon.
I've also tried a test with a sleep(5) after the unlock in wait, and that also produced no crashes. I hate to rely on a kludge like that though.
EDIT: ThirdParty details:
I didn't think this was relevant at first, but the more I think about it, the more I think it's the real problem:
The thirdparty stuff I mentioned, and why I have no control over the thread: this is using CORBA.
So, it's possible that CORBA is holding onto a reference to my object longer than intended.
Yes, I believe that what you're describing is happening (race condition on deallocate). One quick way to fix this is to create a static instance of Wait, one that won't get destroyed. This will work as long as you don't need to have more than one waiter at the same time.
You will also permanently use that memory, it will not deallocate. But it doesn't look like that's too bad.
The main issue is that it's hard to coordinate lifetimes of your thread communication constructs between threads: you will always need at least one leftover communication construct to communicate when it is safe to destroy (at least in languages without garbage collection, like C++).
EDIT:
See comments for some ideas about refcounting with a global mutex.
To the best of my knowledge there's no portable way to directly ask a thread if its done running (i.e. no pthread_ function). What you are doing is the right way to do it, at least as far as having a condition that you signal. If you are seeing crashes that you are sure are due to the Wait object is being deallocated when the thread that creates it quits (and not some other subtle locking issue -- all too common), the issue is that you need to make sure the Wait isn't being deallocated, by managing from a thread other than the one that does the notification. Put it in global memory or dynamically allocate it and share it with that thread. Most simply don't have the thread being waited on own the memory for the Wait, have the thread doing the waiting own it.
Are you initializing and destroying the mutex and condition var properly?
Wait::Wait()
{
pthread_mutex_init(&m_mutex, NULL);
pthread_cond_init(&m_cond, NULL);
m_done = false;
}
Wait::~Wait()
{
assert(m_done);
pthread_mutex_destroy(&m_mutex);
pthread_cond_destroy(&m_cond);
}
Make sure that you aren't prematurely destroying the Wait object -- if it gets destroyed in one thread while the other thread still needs it, you'll get a race condition that will likely result in a segfault. I'd recommend making it a global static variable that gets constructed on program initialization (before main()) and gets destroyed on program exit.
If your assumption is correct then third party module appears to be buggy and you need to come up with some kind of hack to make your application work.
Static Wait is not feasible. How about Wait pool (it even may grow on demand)? Is you application using thread pool to run?
Although there will still be a chance that same Wait will be reused while third party module is still using it. But you can minimize such chance by properly queing vacant Waits in your pool.
Disclaimer: I am in no way an expert in thread safety, so consider this post as a suggestion from a layman.
Is the following safe?
I am new to threading and I want to delegate a time consuming process to a separate thread in my C++ program.
Using the boost libraries I have written code something like this:
thrd = new boost::thread(boost::bind(&myclass::mymethod, this, &finished_flag);
Where finished_flag is a boolean member of my class. When the thread is finished it sets the value and the main loop of my program checks for a change in that value.
I assume that this is okay because I only ever start one thread, and that thread is the only thing that changes the value (except for when it is initialised before I start the thread)
So is this okay, or am I missing something, and need to use locks and mutexes, etc
You never mentioned the type of finished_flag...
If it's a straight bool, then it might work, but it's certainly bad practice, for several reasons. First, some compilers will cache the reads of the finished_flag variable, since the compiler doesn't always pick up the fact that it's being written to by another thread. You can get around this by declaring the bool volatile, but that's taking us in the wrong direction. Even if reads and writes are happening as you'd expect, there's nothing to stop the OS scheduler from interleaving the two threads half way through a read / write. That might not be such a problem here where you have one read and one write op in separate threads, but it's a good idea to start as you mean to carry on.
If, on the other hand it's a thread-safe type, like a CEvent in MFC (or equivilent in boost) then you should be fine. This is the best approach: use thread-safe synchronization objects for inter-thread communication, even for simple flags.
Instead of using a member variable to signal that the thread is done, why not use a condition? You are already are using the boost libraries, and condition is part of the thread library.
Check it out. It allows the worker thread to 'signal' that is has finished, and the main thread can check during execution if the condition has been signaled and then do whatever it needs to do with the completed work. There are examples in the link.
As a general case I would neve make the assumption that a resource will only be modified by the thread. You might know what it is for, however someone else might not - causing no ends of grief as the main thread thinks that the work is done and tries to access data that is not correct! It might even delete it while the worker thread is still using it, and causing the app to crash. Using a condition will help this.
Looking at the thread documentation, you could also call thread.timed_join in the main thread. timed_join will wait for a specified amount for the thread to 'join' (join means that the thread has finsihed)
I don't mean to be presumptive, but it seems like the purpose of your finished_flag variable is to pause the main thread (at some point) until the thread thrd has completed.
The easiest way to do this is to use boost::thread::join
// launch the thread...
thrd = new boost::thread(boost::bind(&myclass::mymethod, this, &finished_flag);
// ... do other things maybe ...
// wait for the thread to complete
thrd.join();
If you really want to get into the details of communication between threads via shared memory, even declaring a variable volatile won't be enough, even if the compiler does use appropriate access semantics to ensure that it won't get a stale version of data after checking the flag. The CPU can issue reads and writes out of order as long (x86 usually doesn't, but PPC definitely does) and there is nothing in C++9x that allows the compiler to generate code to order memory accesses appropriately.
Herb Sutter's Effective Concurrency series has an extremely in depth look at how the C++ world intersects the multicore/multiprocessor world.
Having the thread set a flag (or signal an event) before it exits is a race condition. The thread has not necessarily returned to the OS yet, and may still be executing.
For example, consider a program that loads a dynamic library (pseudocode):
lib = loadLibrary("someLibrary");
fun = getFunction("someFunction");
fun();
unloadLibrary(lib);
And let's suppose that this library uses your thread:
void someFunction() {
volatile bool finished_flag = false;
thrd = new boost::thread(boost::bind(&myclass::mymethod, this, &finished_flag);
while(!finished_flag) { // ignore the polling loop, it's besides the point
sleep();
}
delete thrd;
}
void myclass::mymethod() {
// do stuff
finished_flag = true;
}
When myclass::mymethod() sets finished_flag to true, myclass::mymethod() hasn't returned yet. At the very least, it still has to execute a "return" instruction of some sort (if not much more: destructors, exception handler management, etc.). If the thread executing myclass::mymethod() gets pre-empted before that point, someFunction() will return to the calling program, and the calling program will unload the library. When the thread executing myclass::mymethod() gets scheduled to run again, the address containing the "return" instruction is no longer valid, and the program crashes.
The solution would be for someFunction() to call thrd->join() before returning. This would ensure that the thread has returned to the OS and is no longer executing.