When dealing with concurrency problems I often use std::unique_lock<std::mutex> and std::lock_guard<std::mutex>, no problem with both.
I also extended std::mutex to be able to use it as follow:
mutex.protect([](){
// my protected code here
}) ;
It locks the mutex and releases it around the lambda call.
Is such a similar behavior already implemented inside boost or the standard library?
Boost Thread has this: http://www.boost.org/doc/libs/1_58_0/doc/html/thread/synchronization.html#thread.synchronization.with_lock_guard
You can use it like you'd expect:
std::mutex mx;
boost::with_lock_guard(mx, []{
// protected stuff
});
It even supports the usual INVOKE semantics:
int foo(int,double) { return 42; }
// ...
int answer = boost::with_lock_guard(mx, foo, 3, 3.14);
Manual Standard Library Only implementation
You can easily add a thing like this yourself:
template <typename M, typename F, typename... Args>
auto my_with_lock_guard(M& mx, F&& f, Args&&... args) {
std::lock_guard<M> lk(mx);
return std::forward<F>(f)(std::forward<Args>(args)...);
}
If the standard ever adopts a proposal like this you can easily swap it out.
If all you want to do is protect code in a smaller scope inside a function you don't need to extend mutex by writing your own protect function. You can just do as below, use curly braces to create a local scope, and when the scope is exited the mutex will automatically be unlocked in an exception safe way.
double process_func() {
// do some stuff
{ //start a new scope block
std::lock_guard<my_mutex> g; // mutex is locked here.
[]() { } // your lambda that needs to be protected
} // Mutex is released here.
// do more stuff
}
Of course this has the disadvantage over your custom function that,
it is difficult to maintain. Someone can come later and inject more code without knowing what they are doing.
Related
Intro: For synchronization, C# offers the System.Threading.Monitorclass, offering thread synchronization routines such as Enter(), Exit(), TryEnter() and alike.
Furthermore, there is the lock statement that makes sure a lock gets destroyed when a critical code block is left, either by normal execution flow or by an exception:
private static readonly obj = new Object();
lock(obj) {
...
}
Problem: In C++, for this purpose, we got the RAII wrappers std::lock_guard and std::unique_lock that are not applied to Monitor classes but to types fulfilling the Lockable concept. However, I consider this approach syntactically weaker than the way C# implemented it for several reasons:
You pollute the local scope with a variable name that cannot be reused. This can be countered by adding new scopes like
{
std::unique_lock<std::mutex> lck{ mtx };
...
}
But I find this notation rather awkward-looking. What troubles me even more that this is valid C++:
std::unique_lock<std::mutex>{ mtx ]; // note there is no name to the lock!
...
So by forgetting to give a proper name to the lock guard, this statement will be interpreted as a variable declaration named "mtx" of type std::unique_lock<std::mutex>, without having anything locked!
I want to implement something like the lock statement from C# in C++. In C++17, this can be accomplished very easily:
#define LOCK(mutex) if(std::lock_guard<decltype(mutex)> My_Lock_{ mutex }; true)
std::mutex mtx;
LOCK(mtx) {
...
}
Q: How can I implement this in C++11/14?
Putting aside the "should you do this", here's how:
While it's not quite the same, since it requires a semi-colon, it's near enough that I feel I may present it. This pure C++14 solution basically just defines the macro to start a lambda which is immediately executed:
template<typename MTX>
struct my_lock_holder {
MTX& mtx;
my_lock_holder(MTX& m) : mtx{m} {}
};
template<typename MTX, typename F>
void operator+(my_lock_holder<MTX>&& h, F&& f) {
std::lock_guard<MTX> guard{h.mtx};
std::forward<F>(f)();
}
#define LOCK(mtx) my_lock_holder<decltype(mtx)>{mtx} + [&]
The my_lock_holder just nabs the mutex reference for later, and allows us to overload operator+. The idea is that the operator creates the guard and execute the lambda. As you can see the macro defines a default reference capture, so that lambda will be able to reference anything in the enclosing scope. Then it's pretty much straight forward:
std::mutex mtx;
LOCK(mtx) {
}; // Note the semi-colon
And you can see it build live.
Inspired by StoryTeller's great idea, I think I found a viable solution myself, despite being somewhat a "hack":
template <typename T>
struct Weird_lock final : private std::lock_guard<T> {
bool flip;
Weird_lock(T& m) : std::lock_guard<T>{ m }, flip{ true } { }
operator bool() noexcept {
bool old = flip;
flip = false;
return old;
}
};
#define LOCK(mutex) for(Weird_lock<decltype(mutex)> W__l__{ mutex }; W__l__;)
The good thing is that it doesn't need a semicolon in the end. The bad is the need for an additional bool, but from what I see in godbolt.org, the compiler optimizes this out anyways.
I suggest you do:
#define UNIQUE_NAME(name) name##__COUNTER__
#define LOCK(mutex) std::lock_guard<decltype(mutex)> UNIQUE_NAME(My_Lock){ mutex };
Using the
COUNTER preprocessor symbol will generate a unique variable name that you simply don't care about.
I try to give a std::bind to another existing thread currently waiting in a condition_variable. I really want to keep this other thread alive and not creating another one.
But I don't know how to give this std::bind to the other thread, due to the fact that everything is decided at compile-time.
I know that boost thread pool manage that, and I really wonder how and I'd like doing it without boost.
Here is some pseudo-code
class Exec
{
template<typename Func, typename... Args>
auto call(Func func, Args... args)
{
sendWork(std::bind(func, this->someMemberClass, args...)); // Async
return getResults(); // Waiting til get results
}
void waitThread()
{
//Thread waiting
// Will do the std::bind sent at sendWork
}
}
Has someone any idea?
Thank you for your time!
As mentioned in the commentaries, the only current way to pass a generic function to another thread is by using std::function<void()> which forbid any return type, but grants the ability to specify any parameters and number of parameters, in order to return results, you'll have to think about callbacks.
I want to create a thread pool for experimental purposes (and for the fun factor). It should be able to process a wide variety of tasks (so I can possibly use it in later projects).
In my thread pool class I'm going to need some sort of task queue. Since the Standard Library provides std::packaged_task since the C++11 standard, my queue will look like std::deque<std::packaged_task<?()> > task_queue, so the client can push std::packaged_tasks into the queue via some sort of public interface function (and then one of the threads in the pool will be notified with a condition variable to execute it, etc.).
My question is related to the template argument of the std::packaged_task<?()>s in the deque.
The function signature ?() should be able to deal with any type/number of parameters, because the client can do something like:
std::packaged_task<int()> t(std::bind(factorial, 342));
thread_pool.add_task(t);
So I don't have to deal with the type/number of parameters.
But what should the return value be? (hence the question mark)
If I make my whole thread pool class a template class, one instance
of it will only be able to deal with tasks with a specific signature
(like std::packaged_task<int()>).
I want one thread pool object to be able to deal with any kind of task.
If I go with std::packaged_task<void()> and the function invoked
returns an integer, or anything at all, then thats undefined behaviour.
So the hard part is that packaged_task<R()> is move-only, otherwise you could just toss it into a std::function<void()>, and run those in your threads.
There are a few ways around this.
First, ridiculously, use a packaged_task<void()> to store a packaged_task<R()>. I'd advise against this, but it does work. ;) (what is the signature of operator() on packaged_task<R()>? What is the required signature for the objects you pass to packaged_task<void()>?)
Second, wrap your packaged_task<R()> in a shared_ptr, capture that in a lambda with signature void(), store that in a std::function<void()>, and done. This has overhead costs, but probably less than the first solution.
Finally, write your own move-only function wrapper. For the signature void() it is short:
struct task {
template<class F,
class dF=std::decay_t<F>,
class=decltype( std::declval<dF&>()() )
>
task( F&& f ):
ptr(
new dF(std::forward<F>(f)),
[](void* ptr){ delete static_cast<dF*>(ptr); }
),
invoke([](void*ptr){
(*static_cast<dF*>(ptr))();
})
{}
void operator()()const{
invoke( ptr.get() );
}
task(task&&)=default;
task&operator=(task&&)=default;
task()=default;
~task()=default;
explicit operator bool()const{return static_cast<bool>(ptr);}
private:
std::unique_ptr<void, void(*)(void*)> ptr;
void(*invoke)(void*) = nullptr;
};
and simple. The above can store packaged_task<R()> for any type R, and invoke them later.
This has relatively minimal overhead -- it should be cheaper than std::function, at least the implementations I've seen -- except it does not do SBO (small buffer optimization) where it stores small function objects internally instead of on the heap.
You can improve the unique_ptr<> ptr container with a small buffer optimization if you want.
I happen to have an implementation which does exactly that. My way of doing things is to wrap the std::packaged_task objects in a struct which abstracts away the return type. The method which submits a task into the thread pool returns a future on the result.
This kind of works, but due to the memory allocations required for each task it is not suitable for tasks which are very short and very frequent (I tried to use it to parallelize chunks of a fluid simulation and the overhead was way too high, in the order of several milliseconds for 324 tasks).
The key part is this structure:
struct abstract_packaged_task
{
template <typename R>
abstract_packaged_task(std::packaged_task<R> &&task):
m_task((void*)(new std::packaged_task<R>(std::move(task)))),
m_call_exec([](abstract_packaged_task *instance)mutable{
(*(std::packaged_task<R>*)instance->m_task)();
}),
m_call_delete([](abstract_packaged_task *instance)mutable{
delete (std::packaged_task<R>*)(instance->m_task);
})
{
}
abstract_packaged_task(abstract_packaged_task &&other);
~abstract_packaged_task();
void operator()();
void *m_task;
std::function<void(abstract_packaged_task*)> m_call_exec;
std::function<void(abstract_packaged_task*)> m_call_delete;
};
As you can see, it hides away the type dependencies by using lambdas with std::function and a void*. If you know the maximum size of all possibly occuring std::packaged_task objects (I have not checked whether the size has a dependency on R at all), you could try to further optimize this by removing the memory allocation.
The submission method into the thread pool then does this:
template <typename R>
std::future<R> submit_task(std::packaged_task<R()> &&task)
{
assert(m_workers.size() > 0);
std::future<R> result = task.get_future();
{
std::unique_lock<std::mutex> lock(m_queue_mutex);
m_task_queue.emplace_back(std::move(task));
}
m_queue_wakeup.notify_one();
return result;
}
where m_task_queue is an std::deque of abstract_packaged_task structs. m_queue_wakeup is a std::condition_variable to wake a worker thread up to pick up the task. The worker threads implementation is as simple as:
void ThreadPool::worker_impl()
{
std::unique_lock<std::mutex> lock(m_queue_mutex, std::defer_lock);
while (!m_terminated) {
lock.lock();
while (m_task_queue.empty()) {
m_queue_wakeup.wait(lock);
if (m_terminated) {
return;
}
}
abstract_packaged_task task(std::move(m_task_queue.front()));
m_task_queue.pop_front();
lock.unlock();
task();
}
}
You can take a look at the full source code and the corresponding header on my github.
After receiving answers to a previous question on logging on a different thread, I am currently at the following bit of code (note: the concurrent_queue here is from ppl, but any other concurrent_queue should work):
class concurrentFuncQueue
{
private:
typedef std::function<void()> LambdaFunction;
mutable concurrency::concurrent_queue<LambdaFunction> functionQueue;
mutable std::atomic<bool> endcond;
LambdaFunction function;
std::thread thd;
public:
concurrentFuncQueue() : endcond(false), thd([=]{
while (endcond != true)
{
if (functionQueue.try_pop( function ))
{
function(); //note: I am popping a function and adding () to execute it
}
}
}){}
~concurrentFuncQueue() { functionQueue.push([=]{ endcond = true; }); thd.join(); }
void pushFunction(LambdaFunction function) const { functionQueue.push(function); }
};
Basically the functions I push are run on a different thread sequentially (ex. a logging function) as to avoid performance issues on the main thread.
Current usage is along the following:
static concurrentFuncQueue Logger;
vector<char> outstring(256);
Logger.pushFunction([=]{ OutputDebugString(debugString.c_str()) });
Great so far. I can push functions on to a concurrent queue that will run my functions sequentially on a separate thread.
One thing I also need to have, but currently don't are return values so that ex (pseudo-code):
int x = y = 3;
auto intReturn = Logger.pushFunction([=]()->int { return x * y; });
will push x * y on to the concurrent queue, and after the pop and completion of the function (on the other thread), returns the value calculated to the caller thread.
(I understand that I'll be blocking the caller thread until the pushed function is returned. That is exactly what I want)
I get the feeling that I might have to use something along the line of std::promise, but sadly my current low understanding of them prevent me from formulating something codable.
Any ideas? Thoughts on the above C++ code and any other comments are also much welcome (please just ignore the code completely if you feel another implementation is more appropriate or solves the problem).
You should be able to use something along the lines of:
template<typename Foo>
std::future<typename std::result_of<Foo()>::type> pushFunction(Foo&& f) {
using result_type = typename std::result_of<Foo()>::type; // change to typedef if using is not supported
std::packaged_task<result_type()> t(f);
auto ret_fut = t.get_future();
functionQueue.push(std::move(t));
return ret_fut;
}
For this to work you need to make your LambdaFunction a type-erased function handler.
I have to use a library that makes database calls which are not thread-safe. Also I occasionally have to load larger amounts of data in a background thread.
It is hard to say which library functions actually access the DB, so I think the safest approach for me is to protect every library call with a lock.
Let's say I have a library object:
dbLib::SomeObject someObject;
Right now I can do something like this:
dbLib::ErrorCode errorCode = 0;
std::list<dbLib::Item> items;
{
DbLock dbLock;
errorCode = someObject.someFunction(&items);
} // dbLock goes out of scope
I would like to simplify that to something like this (or even simpler):
dbLib::ErrorCode errorCode =
protectedCall(someObject, &dbLib::SomeObject::someFunction(&items));
The main advantage of this would be that I won't have to duplicate the interface of dbLib::SomeObject in order to protect each call with a lock.
I'm pretty sure that this is a common pattern/idiom but I don't know its name or what keywords to search for. (Looking at http://www.vincehuston.org/dp/gof_intents.html I think, it's more an idiom than a pattern).
Where do I have to look for more information?
You could make protectedCall a template function that takes a functor without arguments (meaning you'd bind the arguments at the call-site), and then creates a scoped lock, calls the functor, and returns its value. For example something like:
template <typename Ret>
Ret protectedCall(boost::function<Ret ()> func)
{
DbLock lock;
return func();
}
You'd then call it like this:
dbLib::ErrorCode errorCode = protectedCall(boost::bind(&dbLib::SomeObject::someFunction, &items));
EDIT. In case you're using C++0x, you can use std::function and std::bind instead of the boost equivalents.
In C++0x, you can implement some form of decorators:
template <typename F>
auto protect(F&& f) -> decltype(f())
{
DbLock lock;
return f();
}
usage:
dbLib::ErrorCode errorCode = protect([&]()
{
return someObject.someFunction(&items);
});
From your description this would seem a job for Decorator Pattern.
However, especially in the case of resources, I wouldn't recommend using it.
The reason is that in general these functions tend to scale badly, require higher level (less finegrained) locking for consistency, or return references to internal structures that require the lock to stay locked until all information is read.
Think, e.g. about a DB function that calls a stored procedure that returns a BLOB (stream) or a ref cursor: the streams should not be read outside of the lock.
What to do?
I recommend instead to use the Facade Pattern. Instead of composing your operations directly in terms of DB calls, implement a facade that uses the DB layer; This layer could then manage the locking at exactly the required level (and optimize where needed: you could have the facade be implemented as a thread-local Singleton, and use separate resources, obviating the need for locks, e.g.)
The simplest (and still straightforward) solution might be to write a function which returns a proxy for the object. The proxy does the locking and overloads -> to allow calling the object. Here is an example:
#include <cstdio>
template<class T>
class call_proxy
{
T &item;
public:
call_proxy(T &t) : item(t) { puts("LOCK"); }
T *operator -> () { return &item; }
~call_proxy() { puts("UNLOCK"); }
};
template<class T>
call_proxy<T> protect(T &t)
{
return call_proxy<T>(t);
}
Here's how to use it:
class Intf
{
public:
void function()
{
puts("foo");
}
};
int main()
{
Intf a;
protect(a)->function();
}
The output should be:
LOCK
foo
UNLOCK
If you want the lock to happen before the evaluation of the arguments, then can use this macro:
#define PCALL(X,APPL) (protect(X), (X).APPL)
PCALL(x,x.function());
This evaluates x twice though.
This article by Andrei Alexandrescu has a pretty interesting article how to create this kind of thin wrapper and combine it with dreaded volatile keyword for thread safety.
Mutex locking is a similar problem. It asked for help here: Need some feedback on how to make a class "thread-safe"
The solution I came up with was a wrapper class that prevents access to the protected object. Access can be obtained via an "accessor" class. The accessor will lock the mutex in its constructor and unlock it on destruction. See the "ThreadSafe" and "Locker" classes in Threading.h for more details.