I don't understand why when an std::thread is destructed it must be in join() or detach() state.
Join waits for the thread to finish, and detach doesn't.
It seems that there is some middle state which I'm not understanding.
Because my understanding is that join and detach are complementary: if I don't call join() than detach() is the default.
Put it this way, let's say you're writing a program that creates a thread and only later in the life of this thread you call join(), so up until you call join the thread was basically running as if it was detached, no?
Logically detach() should be the default behavior for threads because that is the definition of what threads are, they are parallelly executed irrespective of other threads.
So when the thread object gets destructed why is terminate() called? Why can't the standard simply treat the thread as being detached?
I'm not understanding the rationale behind terminating a program when either join() or detached() wasn't called before the thread was destructed. What is the purpose of this?
UPDATE:
I recently came across this. Anthony Williams states in his book, Concurrency In Action, "One of the proposals for C++17 was for a joining_thread class that would be similar to std::thread, except that it would automatically join in the destructor much like scoped_thread does. This didn’t get consensus in the committee, so it wasn’t accepted into the standard (though it’s still on track for C++20 as std::jthread)..."
Technically the answer is "because the spec says so" but that is an obtuse answer. We can't read the designers' minds, but here are some issues that may have contributed:
With POSIX pthreads, child threads must be joined after they have exited, or else they continue to occupy system resources (like a process table entry in the kernel). This is done via pthread_join().
Windows has a somewhat analogous issue if the process holds a HANDLE to the child thread; although Windows doesn't require a full join, the process must still call CloseHandle() to release its refcount on the thread.
Since std::thread is a cross-platform abstraction, it's constrained by the POSIX requirement which requires the join.
In theory the std::thread destructor could have called pthread_join() instead of throwing an exception, but that (subjectively) that may increase the risk of deadlock. Whereas a properly written program would know when to insert the join at a safe time.
See also:
https://en.wikipedia.org/wiki/Zombie_process
https://learn.microsoft.com/en-us/windows/win32/api/processthreadsapi/nf-processthreadsapi-createprocessa
https://learn.microsoft.com/en-us/windows/win32/procthread/terminating-a-process
You're getting confused because you're conflating the std::thread object with the thread of execution it refers to. A std::thread object is a C++ object (a bunch of bytes in memory) that acts as a reference to a thread of execution. When you call std::thread::detach what happens is that the std::thread object is "detached" from the thread of execution -- it no longer refers to (any) thread of execution, and the thread of execution continues running independently. But the std::thread object still exists, until it is destroyed.
When a thread of execution completes, it stores its exit info into the std::thread object that refers to it, if there is one (If it was detached, then there isn't one, so the exit info is just thrown away.) It has no other effect on the std::thread object -- in particular the std::thread object is not destroyed and continues to exist until someone else destroys it.
You might want a thread to completely clean up after itself when it's done leaving no traces. This would mean that you could start a thread and then forget about it.
But you might also want to be able to manage a thread while it was running and get any return value it had provided when it was done. In this case, if a thread cleaned up after itself when it was done, your attempt to manage it could cause a crash because you would be accessing a handle that might be invalid. And to check for the return value when the thread finishes, the return value has to be stored somewhere, which means the thread can't be fully cleaned up because the place where the return value is stored has to be left around.
In most frameworks, by default, you get the second option. You can manage the thread (by interrupting it, sending signals to it, joining it, or whatever) but it can't clean up after itself. If you prefer the first option, there's a function to get that behavior (detach) but that means that you may not be able to access the thread because it may or may not continue to exist.
When a thread handle for an active thread goes out of scope you have a couple of options:
join
detach
kill thread
kill program
Each one of these options is terrible. No matter which one you pick it will be surprising, confusing and not what you wanted in most situations.
Arguably the joining thread you mentioned already exists in the form of std::async which gives you a std::future that blocks until the created thread is done, so doing an implicit join. But the many questions about why
std::async(std::launch::async, f);
g();
does not run f and g concurrently indicate how confusing that is. The best approach I'm aware of is to define it to be a programming error and have the programmer fix it, so an assert would be most appropriate. Unfortunately the standard went with std::terminate instead.
If you really want a detaching thread just write a little wrapper around std::thread that does if (thread.joinable()) thread.detach(); in its destructor or whichever handler you want.
Question: "So when the thread object gets destructed why is terminate() called? Why can't the standard simply treat the thread as being detached?"
Answer: Yes, I agree that it terminates the program badly but such design has its reasons. Without the std::terminate() mechanism in the destructor std::thread::~thread, if the users really wanted to do join(), but for some reason "join" didn't execute (for e.g. exception was thrown) then the new_thread will run in the background just like the detach() behaviors. This might cause undefined behaviors because that was not the original intention of the user to have a detached thread.
Related
Let's say I have a small operation which I want to perform in a separate thread. I do not need to know when it completes, nor do I need to wait for its completion, but I do not want the operation blocking my current thread. When I write the following code, I will get a crash:
void myFunction() {
// do other stuff
std::thread([]()
{
// do thread stuff
});
}
This crash is solved by assigning the thread to a variable, and detaching it:
void myFunction() {
// do other stuff
std::thread t([]()
{
// do thread stuff
});
t.detach();
}
Why is this step necessary? Or is there a better way to create a small single-use thread?
Because the std::thread::~thread() specification says so:
A thread object does not have an associated thread (and is safe to destroy) after
it was default-constructed
it was moved from
join() has been called
detach() has been called
It looks like detach() is the only one of these that makes sense in your case, unless you want to return the thread object (by moving) to the caller.
Why is this step necessary?
Consider that the thread object represents a long-running "thread" of execution (a lightweight process or kernel schedulable entity or similar).
Allowing you to destroy the object while the thread is still executing, leaves you no way to subsequently join (and find the result of) that thread. This may be a logical error, but it can also make it hard even to correctly exit your program.
Or is there a better way to create a small single-use thread?
Not obviously, but it's frequently better to use a thread pool for running tasks in the background, instead of starting and stopping lots of short-lived threads.
You might be able to use std::async() instead, but the future it returns may block in the destructor in some circumstances, if you try to discard it.
See the documentation of the destructor of std:thread:
If *this has an associated thread (joinable() == true), std::terminate() is called.
You should explicitly say that you don't care what's going to happen with the thread, and that you're OK with loosing any control over it. And that is what detach is for.
In general, this looks like a design problem so crashing makes sense: it's hard to propose a general and not surprising rule about what should happen in such a case (e.g. your program might as well normally end its execution - what should happen with the thread?).
Basically, your use case requires a call to detach() because your use case is pretty weird, and not what C++ is trying to make easy.
While Java and .Net blithely let you toss away a Thread object whose associated thread is still running, in the C++ model the Thread is closer to being the thread, in the sense that the existence of the Thread object coincides with the lifetime, or at least joinability, of the execution it refers to. Note how it's not possible to create a Thread without starting it (except in the case of the default constructor, which is really just there in the service of move semantics), or to copy it or to make one from a thread id. C++ wants Thread to outlive the thread.
Maintaining that condition has various benefits. Final cleanup of a thread's control data doesn't have to be done automagically by the OS, because once a Thread goes away, nothing can ever try to join it. It's easier to ensure that variables with thread storage get destroyed in time, since the main thread is the last to exit (barring some move shenanigans). And a missing join -- which is an extremely common type of bug -- gets properly flagged at runtime.
Letting some thread wander off into the distance, in contrast, is allowed, but it's an unusual thing to do. Unless it's interacting with your other threads through sync objects, there's no way to ensure it's done whatever it was meant to do. A detached thread is on the level of reinterpret_cast: You're allowed to tell the compiler that you know something it doesn't, but that has to be explicit, not just the consequence of the function you didn't call.
Consider this: thread A creates thread B and thread A leaves its scope of execution. The handle for thread B is about to be lost. What should happen now? There are several possibilities, with most obvious as follows:
Thread B is detached and continues its execution indempedently
Thread A waits (joins) thread B before quiting its own scope
Now you can argue which is better: 1 or 2? How should we (the compiler) decide on which one of these is better?
So what the designers did was something different: crash terminate the code so that the developer picks one of these solutions explicitely. In order to avoid implicit (perhaps unwanted) behaviuor. It's a signal for you: "hey, pay attention now, this piece of code is important and I (the compiler) don't want to decide for you".
I have an object that uses pthreads. Its constructor creates several threads.
The class's destructor calls pthread_join on all these threads.
What would happen during a delete, if it didn't? I.e., what happens to a non-terminated thread, if the delete operator suddenly tries to deallocate the object?
Thanks in advance for all replies. :)
With the join, your destructor will block until all of the joined threads have exited.
pthread_join
The pthread_join() function waits for the thread specified by thread
to terminate. If that thread has already terminated, then
pthread_join() returns immediately. The thread specified by thread
must be joinable.
If you don't join with the threads, they will continue to run. This can have various consequences depending on if you invoke undefined behavior (eg, try to reference the deleted object after the destructor exits from one of the still running threads).
Deleting the pthread_t objects could have consequences depending on your system (I don't know every implementation of pthreads), but in general they're only references to the underlying system object representing the thread. So you will lose your reference to that thread, and potentially be unable to join with it later, but the thread would continue to run.
I would like to have boost thread object being deleted together with exiting from thread entry function. Is it something wrong if I start the thread function and pass a shared pointer to object, which owns thread object instance and then, when thread function exits, it destroys the this object together with thread object at the same time?
EDIT:
Maybe I will describe why I want to do that. I have to use low level dbus API. What I want to do is to create the adapter class, which will start its own thread and wait for incoming messages until the DISCONNECT message arrives. If it arrives I want to close the thread and kill the Adapter itself. The adapter is an Active Object, which runs the method sent to its scheduler. These methods put themselves on the scheduler queue once again after reading message from dbus. But if it is DISCONNECT message, they should not sent the method but just exit scheduler thread, destroying the Adapter object. hmmm looks like it is too complicated...
From the Boost.Thread documentation you can see that a thread object that is joinable should not be deleted, otherwise std::terminate will be called.
So you should assure that if the thread is joinable, either join() or detach() should be called in the destructor of the object owning the thread. Note: if the thread itself is destroying the object, join() is not an option. The thread would attempt to join itself, resulting in a deadlock.
However, if you keep these restrictions in mind, you can destroy a thread from within its own thread of execution.
You can do this, but you probably should not.
The main purpose of the boost::thread object is that you can monitor the associated thread. Having a thread monitor itself does not make much sense in most scenarios.
As was suggested by the other answers, you could just detach the thread and throw the boost::thread object away. Doing this is usually considered bad style, unless the monitoring responsibility has been transferred to another object first. For example, many simple worker threads set a future upon completion. The future already provides all the monitoring we need, so we can detach the thread.
You should never detach a thread completely such that you lose all means of monitoring it. You must at least be able to guarantee a clean shutdown, which becomes impossible for all but the most trivial threads if you detach them completely.
I am not sure if that addresses your use case but it sounds to me like you don't have to do this.
The lifetime of the boost::thread object does not necessarily coincide with the thread itself. Meaning that if you don't care you can just as well start the thread, call detach() on it and have the object run out of scope. Then it is deleted but the thread will still run until it's function is finished. The only thing is, you won't be able to join it. So if your program finishes while the thread still runs it will crash.
In case you do care about this stuff, the question might be wrong because in this case you would store the objects and call join() on them before deleting.
I am working on a multithreaded program using C++ and Boost. I am using a helper thread to eagerly initialize a resource asynchronously. If I detach the thread and all references to the thread go out of scope, have I leaked any resources? Or does the thread clean-up after itself (i.e. it's stack and any other system resources needed for the itself)?
From what I can see in the docs (and what I recall from pthreads 8 years ago), there's not explicit "destory thread" call that needs to be made.
I would like the thread to execute asynchronously and when it comes time to use the resource, I will check if an error has occured. The rough bit of code would look something like:
//Assume this won't get called frequently enough that next_resource won't get promoted
//before the thread finishes.
PromoteResource() {
current_resource_ptr = next_resource_ptr;
next_resource_ptr.reset(new Resource());
callable = bind(Resource::Initialize, next_resource); //not correct syntax, but I hope it's clear
boost::thread t(callable);
t.start();
}
Of course--I understand that normal memory-handling problems still exist (forget to delete, bad exception handling, etc)... I just need confirmation that the thread itself isn't a "leak".
Edit: A point of clarification, I want to make sure this isn't technically a leak:
void Run() {
sleep(10 seconds);
}
void DoSomething(...) {
thread t(Run);
t.run();
} //thread detaches, will clean itself up--the thread itself isn't a 'leak'?
I'm fairly certain everything is cleaned up after 10 seconds-ish, but I want to be absolutely certain.
The thread's stack gets cleaned up when it exits, but not anything else. This means that anything it allocated on the heap or anywhere else (in pre-existing data structures, for example) will get left when it quits.
Additionally any OS-level objects (file handle, socket etc) will be left lying around (unless you're using a wrapper object which closes them in its destructor).
But programs which frequently create / destroy threads should probably mostly free everything that they allocate in the same thread as it's the only way of keeping the programmer sane.
If I'm not mistaken, on Windows Xp all resources used by a process will be released when the process terminates, but that isn't true for threads.
Yes, the resources are automatically released upon thread termination. This is a perfectly normal and acceptable thing to do to have a background thread.
To clean up after a thread you must either join it, or detach it (in which case you can no longer join it).
Here's a quote from the boost thread docs that somewhat explains that (but not exactly).
When the boost::thread object that
represents a thread of execution is
destroyed the thread becomes detached.
Once a thread is detached, it will
continue executing until the
invocation of the function or callable
object supplied on construction has
completed, or the program is
terminated. A thread can also be
detached by explicitly invoking the
detach() member function on the
boost::thread object. In this case,
the boost::thread object ceases to
represent the now-detached thread, and
instead represents Not-a-Thread.
In order to wait for a thread of
execution to finish, the join() or
timed_join() member functions of the
boost::thread object must be used.
join() will block the calling thread
until the thread represented by the
boost::thread object has completed. If
the thread of execution represented by
the boost::thread object has already
completed, or the boost::thread object
represents Not-a-Thread, then join()
returns immediately. timed_join() is
similar, except that a call to
timed_join() will also return if the
thread being waited for does not
complete when the specified time has
elapsed.
In Win32, as soon as the thread's main function, called ThreadProc in the documentation, finishes, the thread is cleaned up. Any resources allocated by you inside the ThreadProc you'll need to clean up explicitly, of course.
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.