I'm watching a video about mutex and RAII.
In the video the author explains that a good application of RAII (using an object to manage a resource) is a mutex. He has the following class:
class Lock {
private:
Mutext_t* m_pm;
public:
explicit Lock(Mutex_t* pm) { Mutex_lock(pm); m_pm = pm; };
~Lock() { Mutex_unlock(m_pm); };
};
I'm not sure how he implemented Mutex_unlock and Mutex_lock.
My question is: Is the following implementation comparable, and will it achieve the desired result of unlocking a mutex should the function that class object is instantiated in throw an exception?
mutex mu;
class Lock { // Lock will be destroyed if stack is unwound
private:
mutex* p_Mutex;
public:
explicit Lock(mutex* Mutex) {
lock_guard<mutex> u_Lock(*Mutex);
p_Mutex = Mutex;
};
};
void functionB() {
Lock myLock(&mu);
// .. Do stuff that throws an error.
// Unwind stack and destroy myLock, unlocking the mutex?
}
This is very much a simple implementation for learning purposes. Thanks for your help.
Related
I have two libs, one is thread safe called class A, The other lib called class B, which used class A to realize functions.
class A {
public:
void Get() {
std::lock_guard<std::mutex> lock(mutex_);
do_something
}
void Put() {
std::lock_guard<std::mutex> lock(mutex_);
do_something
}
private:
std::mutex mutex_;
};
class B {
public:
void Get() {
a.Get();
}
void Put() {
a.Put();
}
private:
A a;
};
So is class B thread safe?
I know that judging whether the thread is safe depends on whether the operation is atomic. If the put operate is not atomic then it's not thread safe. According to the above requirements, I think class B is not an atomic operation, so it is not thread-safe?
When the operation is not atomic, it may not be thread safe. for example we add some operate like below, Is it right?
class B {
public:
void Get() { // But Get is not atomic!!!
do_some_thing(); // atomic
a.Get(); // atomic
do_some_thing(); // atomic
}
void Put() {
do_some_thing();
a.Put();
do_some_thing();
}
private:
A a;
};
Thread safety concerns about the race conditions and data races.
Now, Since the methods of class B don't use any data directly but via delegating other methods in class A that as you said are thread-safe, the methods in B are thread-safe.
I need to work with the same mutex and unique_lock across the main function and class instances. However, I am having trouble assigning the mutex/unique_lock address to a class member variable (that is a mutex&).
This is what I have:
Worker.h
class Worker
{
private:
std::mutex &m_mu;
std::unique_lock<std::mutex> &locker;
public:
void start(std::mutex &mu, std::unique_lock<std::mutex> &locker);
};
Worker.cpp
void Worker::start(std::mutex &mu, std::unique_lock<std::mutex> &locker)
{
this->mu = mu; // error
this->locker = locker; // error
}
I tried doing this->mu(mu); but that doesn't work either. Is there anything I can do to make this work?
Thanks.
You need to pass the mutex reference when you construct your class.
Worker::Worker(std::mutex &mu, std::unique_lock<std::mutex> &locker)
:m_mu(mu), locker(locker)
{}
That's the only place you can initialize a reference. Once it's constructed, you cannot change what it references.
Why do you need the locker? The mutex makes the synchronization, the lock is just a RAII object to ease acquiring the mutex.
You don't need to pass the lock object to the function. As long as the class is referring to the correct mutex you can lock the mutex inside the function like this:
class Worker
{
private:
std::mutex& m_mu;
public:
Worker(std::mutex& mu): m_mu(mu) {} // bind reference during initialization
void start();
};
// Worker.cpp
void Worker::start()
{
std::unique_lock<std::mutex> locker(m_mu); // lock the shared resource
// Do something with it here
}
int main()
{
std::mutex mu;
std::vector<Worker> workers(4, Worker(std::ref(mu)));
// etc...
}
I'm new to the boost threads library. I have a situation where I acquire a scoped_lock in one function and need to wait on it in a callee.
The code is on the lines of:
class HavingMutex
{
public:
...
private:
static boost::mutex m;
static boost::condition_variable *c;
static void a();
static void b();
static void d();
}
void HavingMutex::a()
{
boost::mutex::scoped_lock lock(m);
...
b() //Need to pass lock here. Dunno how !
}
void HavingMutex::b(lock)
{
if (some condition)
d(lock) // Need to pass lock here. How ?
}
void HavingMutex::d(//Need to get lock here)
{
c->wait(lock); //Need to pass lock here (doesn't allow direct passing of mutex m)
}
Basically, in function d(), I need to access the scoped lock I acquired in a() so that I can wait on it. How do I do that ? (Some other thread will notify).
Or can I directly wait on a mutex instead of a lock ?
Any help is appreciated. Thanks !
Pass it by reference:
void HavingMutex::d(boost::mutex::scoped_lock & lock)
{ // ^ that means "reference"
c->wait(lock);
}
I cannot use boost or the latest std::thread library. The way to go is to create a custom implementation of a scoped mutex.
In a few words when a class instance is create a mutex locks. Upon class destruction the mutex is unlocked.
Any implementation available? I don't want to re-invent the wheel.
I need to use pthreads.
resource acquisition is initialization == “RAII”
Note This is an old answer. C++11 contains better helpers that are more platform independent:
std::lock_guard
std::mutex, std::timed_mutex, std::recursive_mutex, std::recursive_timed_mutex
And other options like std::unique_lock, boost::unique_lock
Any RAII tutorial will do.
Here's the gist: (also on http://ideone.com/kkB86)
// stub mutex_t: implement this for your operating system
struct mutex_t
{
void Acquire() {}
void Release() {}
};
struct LockGuard
{
LockGuard(mutex_t& mutex) : _ref(mutex)
{
_ref.Acquire(); // TODO operating system specific
}
~LockGuard()
{
_ref.Release(); // TODO operating system specific
}
private:
LockGuard(const LockGuard&); // or use c++0x ` = delete`
mutex_t& _ref;
};
int main()
{
mutex_t mtx;
{
LockGuard lock(mtx);
// LockGuard copy(lock); // ERROR: constructor private
// lock = LockGuard(mtx);// ERROR: no default assignment operator
}
}
Of course you can make it generic towards mutex_t, you could prevent subclassing.
Copying/assignment is already prohibited because of the reference field
EDIT For pthreads:
struct mutex_t
{
public:
mutex_t(pthread_mutex_t &lock) : m_mutex(lock) {}
void Acquire() { pthread_mutex_lock(&m_mutex); }
void Release() { pthread_mutex_unlock(&m_mutex); }
private:
pthread_mutex_t& m_mutex;
};
I have a class that is shared between several projects, some uses of it are single-threaded and some are multi-threaded. The single-threaded users don't want the overhead of mutex locking, and the multi-threaded users don't want to do their own locking and want to be able to optionally run in "single-threaded mode." So I would like to be able to select between real and "dummy" mutexes at runtime.
Ideally, I would have a shared_ptr<something> and assign either a real or fake mutex object. I would then "lock" this without regard to what's in it.
unique_lock<something> guard(*mutex);
... critical section ...
Now there is a signals2::dummy_mutex but it does not share a common base class with boost::mutex.
So, what's an elegant way to select between a real mutex and a dummy mutex (either the one in signals2 or something else) without making the lock/guard code more complicated than the example above?
And, before you point out the alternatives:
I could select an implementation at compile time, but preprocessor macros are ugly and maintaining project configurations is painful for us.
Users of the class in a multi-threaded environment do not want to take on the responsibility of locking the use of the class rather than having the class do its own locking internally.
There are too many APIs and existing usages involved for a "thread-safe wrapper" to be a practical solution.
How about something like this?
Its untested but should be close to OK.
You might consider making the template class hold a value rather than a pointer
if your mutexes support the right kinds of constructions. Otherwise you could specialise the MyMutex class to get value behaviour.
Also it's not being careful about copying or destruction .. I leave that as an exercise to the reader ;) ( shared_ptr or storing a value rather than a pointer should fix this)
Oh and the code would be nicer using RAII rather than explicit lock/unlock... but that's a different question.I assume thats what the unique_lock in your code does?
struct IMutex
{
virtual ~IMutex(){}
virtual void lock()=0;
virtual bool try_lock()=0;
virtual void unlock()=0;
};
template<typename T>
class MyMutex : public IMutex
{
public:
MyMutex(T t) : t_(t) {}
void lock() { t_->lock(); }
bool try_lock() { return t_->try_lock(); }
void unlock() { t_->unlock(); }
protected:
T* t_;
};
IMutex * createMutex()
{
if( isMultithreaded() )
{
return new MyMutex<boost::mutex>( new boost::mutex );
}
else
{
return new MyMutex<signal2::dummy_mutex>( new signal2::dummy_mutex );
}
}
int main()
{
IMutex * mutex = createMutex();
...
{
unique_lock<IMutex> guard( *mutex );
...
}
}
Since the two mutex classes signals2::dummy_mutex and boost::mutex don't share a common base class you could use something like "external polymorphism" to allow to them to be treated polymorphically. You'd then use them as locking strategies for a common mutex/lock interface. This allows you to avoid using "if" statements in the lock implementation.
NOTE: This is basically what Michael's proposed solution implements. I'd suggest going with his answer.
Have you ever heard about Policy-based Design ?
You can define a Lock Policy interface, and the user may choose which policy she wishes. For ease of use, the "default" policy is precised using a compile-time variable.
#ifndef PROJECT_DEFAULT_LOCK_POLICY
#define PROJECT_DEFAULT_LOCK_POLICY TrueLock
#endif
template <class LP = PROJECT_DEFAULT_LOCK_POLICY>
class MyClass {};
This way, your users can choose their policies with a simple compile-time switch, and may override it one instance at a time ;)
This is my solution:
std::unique_lock<std::mutex> lock = dummy ?
std::unique_lock<std::mutex>(mutex, std::defer_lock) :
std::unique_lock<std::mutex>(mutex);
Is this not sufficient?
class SomeClass
{
public:
SomeClass(void);
~SomeClass(void);
void Work(bool isMultiThreaded = false)
{
if(isMultiThreaded)
{
lock // mutex lock ...
{
DoSomething
}
}
else
{
DoSomething();
}
}
};
In general, a mutex is only needed if the resource is shared between multiple processes. If an instance of the object is unique for a (possibly multi-threaded) process, then a Critical Section is often more appropriate.
In Windows, the single-threaded implementation of a Critical Section is a dummy one. Not sure what platform you are using.
Just FYI, here's the implementation I ended up with.
I did away with the abstract base class, merging it with the no-op "dummy" implementation. Also note the shared_ptr-derived class with an implicit conversion operator. A little too tricky, I think, but it lets me use shared_ptr<IMutex> objects where I previously used boost::mutex objects with zero changes.
header file:
class Foo {
...
private:
struct IMutex {
virtual ~IMutex() { }
virtual void lock() { }
virtual bool try_lock() { return true; }
virtual void unlock() { }
};
template <typename T> struct MutexProxy;
struct MutexPtr : public boost::shared_ptr<IMutex> {
operator IMutex&() { return **this; }
};
typedef boost::unique_lock<IMutex> MutexGuard;
mutable MutexPtr mutex;
};
implementation file:
template <typename T>
struct Foo::MutexProxy : public IMutex {
virtual void lock() { mutex.lock(); }
virtual bool try_lock() { return mutex.try_lock(); }
virtual void unlock() { mutex.unlock(); }
private:
T mutex;
};
Foo::Foo(...) {
mutex.reset(single_thread ? new IMutex : new MutexProxy<boost::mutex>);
}
Foo::Method() {
MutexGuard guard(mutex);
}
Policy based Option:
class SingleThreadedPolicy {
public:
class Mutex {
public:
void Lock() {}
void Unlock() {}
bool TryLock() { return true; }
};
class ScopedGuard {
public:
ScopedGuard(Mutex& mutex) {}
};
};
class MultithreadingPolicy {
public:
class ScopedGuard;
class Mutex {
friend class ScopedGuard;
private:
std::mutex mutex_;
public:
void Lock() {
mutex_.lock();
}
void Unlock() {
mutex_.unlock();
}
bool TryLock() {
return mutex_.try_lock();
}
};
class ScopedGuard {
private:
std::lock_guard<std::mutex> lock_;
public:
ScopedGuard(Mutex& mutex) : lock_(mutex.mutex_) {}
};
};
Then it can be used as follows:
template<class ThreadingPolicy = SingleThreadedPolicy>
class MyClass {
private:
typedef typename ThreadingPolicy::Mutex Mutex;
typedef typename ThreadingPolicy::ScopedGuard ScopedGuard;
Mutex mutex_;
public:
void DoSomething(){
ScopedGuard guard(mutex_);
std::cout<<"Hello World"<<std::endl;
}
};