I'm new with multi-threading and I need to get the whole idea about the "join" and do I need to join every thread in my application ?, and how does that work with multi-threading ?
no, you can detach one thread if you want it to leave it alone.
If you start a thread, either you detach it or you join it before the program ends, otherwise this is undefined behaviour.
To know that a thread needs to be detached you need to ask yourself this question: "do I want the the thread to run after the program main function is finished?". Here are some examples:
When you do File/New you create a new thread and you detach it: the thread will be closed when the user closes the document Here you don't need to join the threads
When you do a Monte Carlo simulation, some distributed computing, or any Divide And Conquer type algorithms, you launch all the threads and you need to wait for all the results so that you can combine them. Here you explicitly need to join the thread before combining the results
Not joining a thread is like not deleteing all memory you new. It can be harmless, or it could be a bad habit.
A thread you have not synchronized with is in an unknown state of execution. If it is a file writing thread, it could be half way through writing a file and then the app finishes. If it is a network communications thread, it could be half way through a handshake.
The downside to joining every thread is if one of them has gotten into a bad state and has blocked, your app can hang.
In general you should try to send a message to your outstanding threads to tell them to exit and clean up. Then you should wait a modest amount of time for them to finish or otherwise respond that they are good to die, and then shut down the app. Now prior to this you should signify your program is no longer open for business -- shit down GUI windows, respond to requests from other processes that you are shutting down, etc -- so if this takes longer than anticipated the user is not bothered. Finally if things go imperfectly -- if threads refuse to respond to your request that they shut down and you give up on them -- then you should log errors as well, so you can fix what may be a symptom of a bigger problem.
The last time a worker thread unexpectedly hung I initially thought was a problem with a network outage and a bug in the timeout code. Upon deeper inspection it was because one of the objects in use was deleted prior to the shutdown synchronization: the undefined behaviour that resulted just looked like a hang in my reproduction cases. Had we not carefully joined, that bug would have been harder to track down (now, the right thing to do would have been to use a shared resource that we could not delete: but mistakes happen).
The pthread_join() function suspends execution of the calling thread
until the target thread terminates, unless the target thread has
already terminated. On return from a successful pthread_join() call
with a non-NULL value_ptr argument, the value passed to pthread_exit()
by the terminating thread is made available in the location referenced
by value_ptr. When a pthread_join() returns successfully, the target
thread has been terminated. The results of multiple simultaneous calls
to pthread_join() specifying the same target thread are undefined. If
the thread calling pthread_join() is canceled, then the target thread
will not be detached.
So pthread_join does two things:
Wait for the thread to finish.
Clean up any resources associated
with the thread.
This means that if you exit the process without call to pthread_join, then (2) will be done for you by the OS (although it won't do thread cancellation cleanup), and (1) will not be done.
So whether you need to call pthread_join depends whether you need (1) to happen.
Detached thread
If you don't need the thread to run, then you may as well pthread_detach it. A detached thread cannot be joined (so you can't wait on its completion), but its resources are freed automatically if it does complete.
do I need to join every thread in my application ?
Not necessarily - depends on your design and OS. Join() is actively hazardous in GUI apps - tend If you don't need to know, or don't care, about knowing if one thread has terminated from another thread, you don't need to join it.
I try very hard to not join/WaitFor any threads at all. Pool threads, app-lifetime threads and the like often do not require any explicit termination - depends on OS and whether the thread/s own, or are explicitly bound to, any resources that need explicit termination/close/whatever.
Threads can be either joinable or detached. Detached threads should not be joined. On the other hand, if you didn't join the joinable thread, you app would leak some memory and some thread structures. c++11 std::thread would call std::terminate, if it wasn't marked detached and thread object went out of scope without .join() called. See pthread_detach and pthread_create. This is much alike with processes. When the child exits, it will stay as zombee while it's creater willn't call waitpid. The reson for such behavior is that thread's and process's creater might want to know there exit code.
Update: if pthread_create is called with attribute argument equal to NULL (default attributes are used), joinable thread will be created. To create a detached thread, you can use attributes:
pthread_attr_t attrs;
pthread_attr_init(&attrs);
pthread_attr_setdetachstate(&attrs, PTHREAD_CREATE_DETACHED);
pthread_create(thread, attrs, callback, arg);
Also, you can make a thread to be detached by calling pthread_detach on a created one. If you will try to join with a detached thread, pthread_join will return EINVAL error code. glibc has a non portable extension pthread_getattr_np that allows to get attributes of a running thread. So you can check if thread is detached with pthread_attr_getdetachstate.
Related
If I terminate a thread on Windows using the TerminateThread function, is that thread actually terminated once the function returns or is termination asychnronous?
Define "actually terminated". The documentation says the thread can not execute any more user-mode code, so effectively: yes, it is terminated, nothing of your code is going to be executed by that thread any more.
If you "WaitForSingleObject" on it right after terminating, I guess there could still be some slight delay because of cleanup that Windows is doing.
By the way: TerminateThread is the worst way of ending a thread. Try using some other means of synchronization, like a global variable that tells the thread to stop, or an event for example.
Terminating a thread is akin to killing a process, only on a per-thread level. It may in fact be implemented by raising an (uncatchable) signal in the targeted thread.
The result is essentially the same: Your program is not in any particular, predictable state. There's not much you can do with the dead thread. The control flow of your program becomes generally indeterminate, and thus it is extremely hard to reason about your program's behaviour in the presence of thread termination.
Basically, unless your thread is doing something extremely narrow, specific and restricted (e.g. increment an atomic counter once every second), there's no good model for the need to terminate a thread, and for the state of the program after the thread termination.
Don't do it. Design your threads so that you can communicate with them and so that their entry functions can return. Design your program so that you can always join all threads eventually and account for everything.
It is a synchronous call. That does not mean that it necessarily returns quickly - there may be some blocking involved if the OS has to resort to using its inter-core driver to stop the thread, (ie. it's actually running on a different core than the thread requesting the termination).
There are issues with calling TerminateThread from user code during an app run, (as distinct from the kernel using it during app/process termination), as clearly posted by others.
I try very hard to never terminate threads at all during an app run, with TerminateThread or by any other means. App-lifetime threads and thread pools often do not require any explicit termination before the OS destroys them on app close.
I'm a little confused about clean-up order when you're using PThreads with regard to cancellation. Normally, if your thread is detached, it automatically cleans up when it terminates. If it's not detached, you need to join it to reclaim the system resources.
The textbook I'm reading states the following which strangely sounds like joining is optional with regard to cancellation:
"If you need to know when the thread has actually terminated, you must
join with it by calling pthread_join after cancelling it."
So, do I need to join a cancelled thread to free its resources - and if not, then why?
TLPI says this:
Upon receiving a cancellation request, a thread whose cancelability is
enabled and deferred terminates when it next reaches a cancellation
point. If the thread was not detached, then some other thread in the
process must join with it, in order to prevent it from becoming a
zombie thread.
Also, since canceling a thread isn't usually done immediately (read more about "cancellation points") without joining you can't be sure the thread was actually canceled.
From man pthread_join:
After a canceled thread has terminated, a join with that thread using
pthread_join(3) obtains PTHREAD_CANCELED as the thread's exit status.
(Joining with a thread is the only way to know that cancellation has
completed.)
It seems that joining is not necessary for execution it is necessary if you want know what you did actually succeed.
From Doccumentation of pthread_cancel():
After a canceled thread has terminated, a join with that thread using pthread_join(3) obtains PTHREAD_CANCELED as the thread's exit status. (Joining with a thread is the only way to know that cancellation has completed.)
A thread using pthread can have following cancelling statuses:
PTHREAD_CANCEL_ENABLE
PTHREAD_CANCEL_DISABLE
If you try to cancel a thread you do not 100% know if the thread will really get cancelled. Using a join delivers the information to you if the thread was really cancelled or not. There are also cancel types to be considered and respective pthread functions for setting the cancel type and state:
int pthread_setcancelstate (int state, int *oldstate);
int pthread_setcanceltype (int type, int *oldtype);
Here is a sample code borrowed from http://www.ijon.de/comp/tutorials/threads/cancel.html
EDIT: Either I am too stupid to post a few lines of code or the formatter is really going on my nerves today. Just look up the code in the link above, please.
If something goes wrong in a thread or it is stopped from with in somehow it will always be tidied up by the OS. So it's all nice and safe.
You only need to join the thread if you have to be sure it has actually stopped executing, like merging two parallel tasks. (E.g. if you have various threads working on various parts a split structure you need to join them all, as in wait until they are all finished, when you want to combine the structure again)
I'm designing a thread library. So far I have a method that initializes the library, one that creates threads, and one that yields the current thread to the next one on a queue of ready threads.
Before I move on to implementing semaphores for the threads, I figured I should probably kill the threads as soon as they are done and free up their allocated memory, but I'm having trouble figuring out how to do that. How do I tell when a thread has "finished"?
You don't just kill threads safely or reliably -- let them exit naturally (when their entry returns).
Although the system provides a means to kill the thread, nearly any C++ program out there could expect undefined behavior if it were to continue. You could dream up cases where killing could be accomplished without side effects (to the rest of the program), but that program does not at all resemble idiomatic C++. Such a program would be very exotic, with many unusual and severe restrictions.
When you want to known when a thread has exited or not, you can add some cleanup before it exits in order to track its status.
When you want the ability to request a thread exit (naturally), consider run loops and messages.
You don't explicitly kill the threads when they are finished running their forked procedures as the code which would be doing that would still be in the context of the thread to be killed.
You have a scheduler/interrupt handler which handles the context switching of the threads and maintains a few queues for managing this. You can have it save a reference to to the threads to be killed, something like scheduler->SetThreadToKill( currentThread ); inside probably your finish() method (or similar), which sets a flag for the corresponding threads.
When a context switch occurs, and you have swapped out all data structures of the current thread with that of the next thread, you scheduler can call the destructor for all the threads which have the toBeKilled flag set.
The best policy, by far, for killing threads is to not explicitly do it, (unless you are an OS, ie. on app shutdown). Queue messages and tasks to threads that loop around some queue to perform more work. If you don't write any code to continually new, create, start, terminate, delete, test, check, enlist, delist, enqueue, dequeue and otherwise micro-manage threads, then that code cannot contain bugs.
The deal is:
I want to create a thread that works similarly to executing a new .exe in Windows, so if that program (new thread) crashes or goes into infinite loop: it will be killed gracefully (after the time limit exceeded or when it crashed) and all resources freed properly.
And when that thread has succeeded, i would like to be able to modify some global variable which could have some data in it, such as a list of files for example. That is why i cant just execute external executable from Windows, since i cant access the variables inside the function that got executed into the new thread.
Edit: Clarified the problem a lot more.
The thread will already run after calling CreateThread.
WaitForSingleObject is not necessary (unless you really want to wait for the thread to finish); but it will not "force-quit" the thread; in fact, force-quitting - even if it might be possible - is never such a good idea; you might e.g. leave resources opened or otherwise leave your application in a state which is no good.
A thread is not some sort of magical object that can be made to do things. It is a separate path of execution through your code. Your code cannot be made to jump arbitrarily around its codebase unless you specifically program it to do so. And even then, it can only be done within the rules of C++ (ie: calling functions).
You cannot kill a thread because killing a thread would utterly wreck some of the most fundamental assumptions a programmer makes. You would now have to take into account the possibility that the next line doesn't execute for reasons that you can neither predict nor prevent.
This isn't like exception handling, where C++ specifically requires destructors to be called, and you have the ability to catch exceptions and do special cleanup. You're talking about executing one piece of code, then suddenly ending the execution of that entire call-stack. That's not going to work.
The reason that web browsers moved from a "thread-per-tab" to "process-per-tab" model is exactly this: because processes can be terminated without leaving the other processes in an unknown state. What you need is to use processes instead of threads.
When the process finishes and sets it's data, you need to use some inter-process communication system to read that data (I like Boost.Interprocess myself). It won't look like a regular C++ global variable, but you shouldn't have a problem with reading it. This way, you can effectively kill the process if it's taking too long, and your program will remain in a reasonable state.
Well, that's what WaitForSingleObject does. It blocks until the object does something (in case of a thread it waits until the thread exits or the timeout elapses). What you need is
HANDLE thread = CreateThread(0, 0, do_stuff, NULL, 0, 0);
//rest of code that will run paralelly with your new thread.
WaitForSingleObject(thread, 4000); // wait 4 seconds or for the other thread to exit
If you want your worker thread to shut down after a period of time has elapsed, the best way to do that is to have the thread itself monitor the elapsed time in some way and then exit when the time is up.
Another way to do this is to monitor the elapsed time in the main thread or even a third, monitor type thread. When the time has elapsed, set an event. Your worker thread could wait for this event in it's main loop, and then exit when it has been raised. These kinds of events, which are used to signal the thread to kill itself, are sometimes called "death events." (Or at least, I call them that.)
Yet another way to do this is to queue a user job to the worker thread, which needs to be in an alterable wait state. The APC can then set some internal state variable which will trigger the death sequence in the thread when it resumes.
There is another method which I hesitate even mentioning, because it should only be used in extremely dire circumstances. You can kill the thread. This is a very dangerous method akin to turning off your sink by detonating an atomic bomb. You get the sink turned off, but there could be other unintended consequences as well. Please don't do this unless you know exactly what you're doing and why.
Remove the call to WaitForSingleObject. That causes your parent thread to wait.
Remove the WaitForSingleObject call?
This question seems like it's probably a duplicate, but I was unable to find one. If I missed a previous question, apologies.
In Java, where I have most of my experience, if your main() forks a thread and immediately returns the process continues to run until all (non-daemon) threads in the process have stopped.
In C++, this appears not to be the case - as soon as the main thread returns the process is terminating with other threads still running. For my current app this is easily solved with the application of pthread_join() but I'm wondering what causes this behavior. Is this compiler (gcc) specific, pthreads specific, or is kind of behavior shared across most/all platforms for which C++ has been implemented? Is this behavior configurable within pthreads (I've looked through the pthread api at the pthread_attr_*() functions and didn't see anything that looked relevant.)?
Completely separate question, but while you're here ... what would one use pthread_detatch() for?
Yes. In modern linux (more importantly newer versions of GNU libc) exit_group is the system call used when main returns, not plain exit. exit_group is described as follows:
This system call is equivalent to
exit(2) except that it terminates not
only the calling thread, but all
threads in the calling process's
thread group.
It is worth noting that current the c++ standard makes no mention of threads, so this behavior is not c++ specific, but instead is specific to your particular implementation. That said, every implementation I've personally seen kills all threads when the main thread terminates.
EDIT: It is also worth noting Jonathan Leffler's answer which points out that the POSIX standard does indeed specify this behavior, so it is certainly normal for an application using pthreads for its threading.
EDIT: To answer the follow up about pthread_detach. Basically it is considered a resource leak if you do not join a non-detached thread. If you have a long running task which you have no need to "wait for", and it just "ends when it ends" then you should detach it which will not have a resource leak when it terminates with no join. The man page says the following:
The pthread_detach() function marks
the thread identified by thread as
detached. When a detached thread
terminates, its resources are
automatically released back to the
system without the need for another
thread to join with the terminated
thread.
So a quick and dirty answer is: "when you don't care when it ends, detach it. If another thread cares when it ends and must wait for it to terminate, then don't."
Yes
The POSIX standard says:
ยง3.297 Process Termination
There are two kinds of process termination:
Normal termination occurs by a return from main(), when requested with the exit(), _exit(), or _Exit() functions; or when the last thread in the process terminates by returning from its start function, by calling the pthread_exit() function, or through cancellation.
Abnormal termination occurs when requested by the abort() function or when some signals are received.
The first normal termination condition applies. (Note that the C++ (1998, 2003) standard says nothing about threads.)
Regarding pthread_detach()
The POSIX standard (again) says:
The pthread_detach() function shall indicate to the implementation that storage for the thread thread can be reclaimed when that thread terminates. If thread has not terminated, pthread_detach() shall not cause it to terminate.
And the rationale says:
The pthread_join() or pthread_detach() functions should eventually be called for every thread that is created so that storage associated with the thread may be reclaimed.
It has been suggested that a "detach" function is not necessary; the detachstate thread creation attribute is sufficient, since a thread need never be dynamically detached. However, need arises in at least two cases:
In a cancellation handler for a pthread_join() it is nearly essential to have a pthread_detach() function in order to detach the thread on which pthread_join() was waiting. Without it, it would be necessary to have the handler do another pthread_join() to attempt to detach the thread, which would both delay the cancellation processing for an unbounded period and introduce a new call to pthread_join(), which might itself need a cancellation handler. A dynamic detach is nearly essential in this case.
In order to detach the "initial thread" (as may be desirable in processes that set up server threads).
This is not compiler specific and is standard behavior; the application terminates when main() exits, so if you want to prevent the application from terminating, you need main() to block until all threads have terminated, which you do by joining those threads. When you invoke pthread_create, it allocates resources for that thread. The resources are not deallocated unless you do a pthread_join (which blocks until the thread terminates) or pthread_detach (which causes the thread to automatically release resources when that thread exits). You should use pthread_detach whenever you launch a background thread that will terminate when its task is completed and for which you do not need to wait.
To make this a little bit more concrete, suppose you have several threads that perform a piece of a computation, and then you aggregate the result in some way. That would be a case where you would use join, because you need the results of the threads to proceed. Now, consider a case where a thread listens on a socket and processes incoming requests, until a flag indicates that the thread should quit. In this case, you would use pthread_detach, since nothing needs the thread to terminate in order to proceed, and so the resources associated with that thread should go away automatically.