I am quite new to the C++11 feature std::async and I fail to grasp why the code below never prints bar.
Could someone shed some light on this for me?
class Thready {
public:
Thready() {
std::async(std::launch::async, &Thready::foo, this);
}
void foo() {
while (true) {
std::cout << "foo" << std::endl;
}
}
void bar() {
while (true) {
std::cout << "bar" << std::endl;
}
}
};
int main() {
Thready t;
t.bar();
}
See "Notes" section on this page: http://en.cppreference.com/w/cpp/thread/async
The implementation may extend the behavior of the first overload of
std::async by enabling additional (implementation-defined) bits in the
default launch policy. Examples of implementation-defined launch
policies are the sync policy (execute immediately, within the async
call) and the task policy (similar to async, but thread-locals are not
cleared) If the std::future obtained from std::async is not moved from
or bound to a reference, the destructor of the std::future will block
at the end of the full expression until the asynchronous operation
completes, essentially making code such as the following synchronous:
std::async(std::launch::async, []{ f(); }); // temporary's dtor waits for f()
std::async(std::launch::async, []{ g(); }); // does not start until f() completes
(note that the destructors of std::futures
obtained by means other than a call to std::async never block)
TL;DR:
try to save the returned value of std::async call into some variable:
auto handle = std::async(std::launch::async, &Thready::foo, this);
EDIT:
the following code should work as you expect.
#include <future>
#include <iostream>
class Thready {
public:
Thready() {
handle = std::async(std::launch::async, &Thready::foo, this);
}
void foo() {
while (true) {
std::cout << "foo" << std::endl;
}
}
void bar() {
while (true) {
std::cout << "bar" << std::endl;
}
}
std::future<void> handle;
};
int main() {
Thready t;
t.bar();
}
Related
I'm reading C++ concurrency in action.
It introduces how to implement interrupting thread using std::condition_variable_any.
I try to understand the code more than a week, but I couldn't.
Below is the code and explanation in the book.
#include <condition_variable>
#include <future>
#include <iostream>
#include <thread>
class thread_interrupted : public std::exception {};
class interrupt_flag {
std::atomic<bool> flag;
std::condition_variable* thread_cond;
std::condition_variable_any* thread_cond_any;
std::mutex set_clear_mutex;
public:
interrupt_flag() : thread_cond(0), thread_cond_any(0) {}
void set() {
flag.store(true, std::memory_order_relaxed);
std::lock_guard<std::mutex> lk(set_clear_mutex);
if (thread_cond) {
thread_cond->notify_all();
} else if (thread_cond_any) {
thread_cond_any->notify_all();
}
}
bool is_set() const { return flag.load(std::memory_order_relaxed); }
template <typename Lockable>
void wait(std::condition_variable_any& cv, Lockable& lk);
};
thread_local static interrupt_flag this_thread_interrupt_flag;
void interruption_point() {
if (this_thread_interrupt_flag.is_set()) {
throw thread_interrupted();
}
}
template <typename Lockable>
void interrupt_flag::wait(std::condition_variable_any& cv, Lockable& lk) {
struct custom_lock {
interrupt_flag* self;
// (1) What is this lk for? Why is lk should be already locked when it is used in costume_lock constructor?
Lockable& lk;
custom_lock(interrupt_flag* self_, std::condition_variable_any& cond,
Lockable& lk_)
: self(self_), lk(lk_) {
self->set_clear_mutex.lock();
self->thread_cond_any = &cond;
}
void unlock() {
lk.unlock();
self->set_clear_mutex.unlock();
}
void lock() { std::lock(self->set_clear_mutex, lk); }
~custom_lock() {
self->thread_cond_any = 0;
self->set_clear_mutex.unlock();
}
};
custom_lock cl(this, cv, lk);
interruption_point();
cv.wait(cl);
interruption_point();
}
class interruptible_thread {
std::thread internal_thread;
interrupt_flag* flag;
public:
template <typename FunctionType>
interruptible_thread(FunctionType f) {
std::promise<interrupt_flag*> p;
internal_thread = std::thread([f, &p] {
p.set_value(&this_thread_interrupt_flag);
f();
});
flag = p.get_future().get();
}
void interrupt() {
if (flag) {
flag->set();
}
};
void join() { internal_thread.join(); };
void detach();
bool joinable() const;
};
template <typename Lockable>
void interruptible_wait(std::condition_variable_any& cv, Lockable& lk) {
this_thread_interrupt_flag.wait(cv, lk);
}
void foo() {
// (2) This is my implementation of how to use interruptible wait. Is it correct?
std::condition_variable_any cv;
std::mutex m;
std::unique_lock<std::mutex> lk(m);
try {
interruptible_wait(cv, lk);
} catch (...) {
std::cout << "interrupted" << std::endl;
}
}
int main() {
std::cout << "Hello" << std::endl;
interruptible_thread th(foo);
th.interrupt();
th.join();
}
Your custom lock type acquires the lock on the internal
set_clear_mutex when it’s constructed 1, and then sets the
thread_cond_any pointer to refer to the std:: condition_variable_any
passed in to the constructor 2.
The Lockable reference is stored for later; this must already be
locked. You can now check for an interruption without worrying about
races. If the interrupt flag is set at this point, it was set before
you acquired the lock on set_clear_mutex. When the condition variable
calls your unlock() function inside wait(), you unlock the Lockable
object and the internal set_clear_mutex 3.
This allows threads that are trying to interrupt you to acquire the
lock on set_clear_mutex and check the thread_cond_any pointer once
you’re inside the wait() call but not before. This is exactly what you
were after (but couldn’t manage) with std::condition_variable.
Once wait() has finished waiting (either because it was notified or
because of a spurious wake), it will call your lock() function, which
again acquires the lock on the internal set_clear_mutex and the lock
on the Lockable object 4. You can now check again for interruptions
that happened during the wait() call before clearing the
thread_cond_any pointer in your custom_lock destructor 5, where you
also unlock the set_clear_mutex.
First, I couldn't understand what is the purpose of Lockabel& lk in mark (1) and why it is already locked in constructor of custom_lock. (It could be locked in the very custom_lock constructor. )
Second there is no example in this book of how to use interruptible wait, so foo() {} in mark (2) is my guess implementation of how to use it. Is it correct way of using it ?
You need a mutex-like object (lk in your foo function) to call the interruptiple waiting just as you would need it for the plain std::condition_variable::wait function.
What's problematic (I also read the book and I have doubts about this example) is that the flag member points to a memory location inside the other thread which could finish right before calling flag->set(). In this specific example the thread only exists after we set the flag so that is okay, but otherwise this approach is limited in my opinion (correct me if I am wrong).
class test
{
void thread1()
{
int i = 0;
while(true){
for(unsigned int k = 0;k < mLD.size(); k++ )
{
mLD[k] = i++;
}
}
}
void thread2()
{
std::cout << "thread2 address : " << &mLD << "\n";
C();
}
void B()
{
std::cout << "B address : " << &mLD << "\n";
for(unsigned int k = 0;k < mLD.size(); k++ )
{
if(mLD[k]<=25)
{
}
}
}
void C()
{
B();
std::cout << "C address : " << &mLD << "\n";
double distance = mLD[0]; // <---- segmetation fault
}
std::array<double, 360> mLD;
};
cout result --->
thread2 address : 0x7e807660
B address : 0x7e807660
C address : 0x1010160 (sometimes 0x7e807660 )
Why mLD's address changed ....?
even i change std::array to std::array<std::atomic<double>360>, the result is the same.
Most probably, the object you referred is destroyed at the point of call to C, which points to a synchronization issue. You need to extend the lifetime of the object referred by thread(s), until the threads done executing their routine. To accomplish this, you can have something like this;
#include <thread>
#include <array>
#include <iostream>
struct foo{
void callback1(){
for(auto & elem: storage){
elem += 5;
}
}
void callback2(){
for(const auto & elem: storage){
std::cout << elem << std::endl;
}
}
std::array<double, 300> storage;
};
int main(void){
foo f;
std::thread t1 {[&f](){f.callback1();}};
std::thread t2 {[&f](){f.callback2();}};
// wait until both threads are done executing their routines
t1.join();
t2.join();
return 0;
}
The instance of foo, f lives in scope of main() function, so its' lifetime is defined by from the line it defined to end of the main's scope. By joining both threads, we block main from proceeding further until both threads are done executing their callback functions, hence the lifetime of f extended until callbacks are done.
The second issue is, the code needs synchronization primitives, because storage variable is shared between two independent execution paths. The final code with proper synchronization can look like this;
#include <thread>
#include <array>
#include <iostream>
#include <mutex>
struct foo{
void callback1(){
// RAII style lock, which invokes .lock() upon construction, and .unlock() upon destruction
// automatically.
std::unique_lock<std::mutex> lock(mtx);
for(auto & elem: storage){
elem += 5;
}
}
void callback2(){
std::unique_lock<std::mutex> lock(mtx);
for(const auto & elem: storage){
std::cout << elem << std::endl;
}
}
std::array<double, 300> storage;
// non-reentrant mutex
mutable std::mutex mtx;
};
int main(void){
foo f;
std::thread t1 {[&f](){f.callback1();}};
std::thread t2 {[&f](){f.callback2();}};
// wait until both threads are done executing their routines
t1.join();
t2.join();
return 0;
}
I am trying to construct a std::thread with a member function that takes no arguments and returns void. I can't figure out any syntax that works - the compiler complains no matter what. What is the correct way to implement spawn() so that it returns a std::thread that executes test()?
#include <thread>
class blub {
void test() {
}
public:
std::thread spawn() {
return { test };
}
};
#include <thread>
#include <iostream>
class bar {
public:
void foo() {
std::cout << "hello from member function" << std::endl;
}
};
int main()
{
std::thread t(&bar::foo, bar());
t.join();
}
EDIT:
Accounting your edit, you have to do it like this:
std::thread spawn() {
return std::thread(&blub::test, this);
}
UPDATE: I want to explain some more points, some of them have also been discussed in the comments.
The syntax described above is defined in terms of the INVOKE definition (§20.8.2.1):
Define INVOKE (f, t1, t2, ..., tN) as follows:
(t1.*f)(t2, ..., tN) when f is a pointer to a member function of a class T and t1 is an object of type T or a reference to an object of
type T or a reference to an object of a type derived from T;
((*t1).*f)(t2, ..., tN) when f is a pointer to a member function of a class T and t1 is not one of the types described in the previous
item;
t1.*f when N == 1 and f is a pointer to member data of a class T and t 1 is an object of type T or a
reference to an object of type T or a reference to an object of a
type derived from T;
(*t1).*f when N == 1 and f is a pointer to member data of a class T and t 1 is not one of the types described in the previous item;
f(t1, t2, ..., tN) in all other cases.
Another general fact which I want to point out is that by default the thread constructor will copy all arguments passed to it. The reason for this is that the arguments may need to outlive the calling thread, copying the arguments guarantees that. Instead, if you want to really pass a reference, you can use a std::reference_wrapper created by std::ref.
std::thread (foo, std::ref(arg1));
By doing this, you are promising that you will take care of guaranteeing that the arguments will still exist when the thread operates on them.
Note that all the things mentioned above can also be applied to std::async and std::bind.
Since you are using C++11, lambda-expression is a nice&clean solution.
class blub {
void test() {}
public:
std::thread spawn() {
return std::thread( [this] { this->test(); } );
}
};
since this-> can be omitted, it could be shorten to:
std::thread( [this] { test(); } )
or just (deprecated)
std::thread( [=] { test(); } )
Here is a complete example
#include <thread>
#include <iostream>
class Wrapper {
public:
void member1() {
std::cout << "i am member1" << std::endl;
}
void member2(const char *arg1, unsigned arg2) {
std::cout << "i am member2 and my first arg is (" << arg1 << ") and second arg is (" << arg2 << ")" << std::endl;
}
std::thread member1Thread() {
return std::thread([=] { member1(); });
}
std::thread member2Thread(const char *arg1, unsigned arg2) {
return std::thread([=] { member2(arg1, arg2); });
}
};
int main(int argc, char **argv) {
Wrapper *w = new Wrapper();
std::thread tw1 = w->member1Thread();
std::thread tw2 = w->member2Thread("hello", 100);
tw1.join();
tw2.join();
return 0;
}
Compiling with g++ produces the following result
g++ -Wall -std=c++11 hello.cc -o hello -pthread
i am member1
i am member2 and my first arg is (hello) and second arg is (100)
#hop5 and #RnMss suggested to use C++11 lambdas, but if you deal with pointers, you can use them directly:
#include <thread>
#include <iostream>
class CFoo {
public:
int m_i = 0;
void bar() {
++m_i;
}
};
int main() {
CFoo foo;
std::thread t1(&CFoo::bar, &foo);
t1.join();
std::thread t2(&CFoo::bar, &foo);
t2.join();
std::cout << foo.m_i << std::endl;
return 0;
}
outputs
2
Rewritten sample from this answer would be then:
#include <thread>
#include <iostream>
class Wrapper {
public:
void member1() {
std::cout << "i am member1" << std::endl;
}
void member2(const char *arg1, unsigned arg2) {
std::cout << "i am member2 and my first arg is (" << arg1 << ") and second arg is (" << arg2 << ")" << std::endl;
}
std::thread member1Thread() {
return std::thread(&Wrapper::member1, this);
}
std::thread member2Thread(const char *arg1, unsigned arg2) {
return std::thread(&Wrapper::member2, this, arg1, arg2);
}
};
int main() {
Wrapper *w = new Wrapper();
std::thread tw1 = w->member1Thread();
tw1.join();
std::thread tw2 = w->member2Thread("hello", 100);
tw2.join();
return 0;
}
Some users have already given their answer and explained it very well.
I would like to add few more things related to thread.
How to work with functor and thread.
Please refer to below example.
The thread will make its own copy of the object while passing the object.
#include<thread>
#include<Windows.h>
#include<iostream>
using namespace std;
class CB
{
public:
CB()
{
cout << "this=" << this << endl;
}
void operator()();
};
void CB::operator()()
{
cout << "this=" << this << endl;
for (int i = 0; i < 5; i++)
{
cout << "CB()=" << i << endl;
Sleep(1000);
}
}
void main()
{
CB obj; // please note the address of obj.
thread t(obj); // here obj will be passed by value
//i.e. thread will make it own local copy of it.
// we can confirm it by matching the address of
//object printed in the constructor
// and address of the obj printed in the function
t.join();
}
Another way of achieving the same thing is like:
void main()
{
thread t((CB()));
t.join();
}
But if you want to pass the object by reference then use the below syntax:
void main()
{
CB obj;
//thread t(obj);
thread t(std::ref(obj));
t.join();
}
Is there a way to start two (or more) C++11 threads and join() the first one that is finished?
An example scenario:
#include <iostream>
#include <thread>
using namespace std;
void prepare_item1() {std::cout << "Preparing 1" << std::endl;}
void consume_item1() {std::cout << "Consuming 1" << std::endl;}
void prepare_item2() {std::cout << "Preparing 2" << std::endl;}
void consume_item2() {std::cout << "Consuming 2" << std::endl;}
int main()
{
std::thread t1(prepare_item1);
std::thread t2(prepare_item2);
t1.join();
consume_item1();
t2.join();
consume_item2();
return 0;
}
I would have liked to do something like that instead:
int main()
{
std::thread t1(prepare_item1);
std::thread t2(prepare_item2);
finished_id=join_any(t1,t2)
if (finished_id==1)
{
consume_item1();
...
}
else if (finished_id==2)
{
consume_item2();
...
}
return 0;
}
Also, I would like the solution to be blocking, similar to the t.join() function.
Note: The real reason I need this is that I have two different blocking functions from which I receive commands, and whenever any of them is ready I would like to process the first command that arrives and continue to the next one when it is done. (sequential processing of commands from two parallel sources)
Thanks!
Here is a thread-safe multi-producer multi-consumer queue:
template<class T>
struct safe_queue {
std::deque<T> data;
std::atomic<bool> abort_flag = false;
std::mutex guard;
std::condition_variable signal;
template<class...Args>
void send( Args&&...args ) {
{
std::unique_lock<std::mutex> l(guard);
data.emplace_back(std::forward<Args>(args)...);
}
signal.notify_one();
}
void abort() {
abort_flag = true; // 1a
{ std::unique_lock<std::mutex>{guard}; }
signal.notify_all(); // 1b
}
std::experimental::optional<T> get() {
std::unique_lock<std::mutex> l(guard);
signal.wait( l, [this]()->bool{ // 2b
return !data.empty() || abort_flag.load(); // 2c
});
if (abort_flag.load()) return {};
T retval = std::move(data.front());
data.pop_front();
return retval;
}
};
have the threads shove data into the queue, and the main thread do a .get() on it.
If abort() is called, all waiting threads are woken up with an "empty" value from .get().
It uses std::experimental::optional, but you can replace that with something else (throw on abort? Whatever).
Code modified slightly from this other answer. Note that I think the other answer has some errors in it, which I corrected above, and attempts to solve a different problem.
The message you send could be the id of the thread that is ready to be waited upon, for example, or the work it has completed, or whatever.
I tried yesterday to use std::thread correctly, but it's very dark for me.
My program implementation with pthread works well I don't have any problem with it. I would like to have the same solution with std::thread (if possible).
Solution with pthread:
void *MyShell(void *data) {
std::string str;
while(1) {
std::cin >> str;
std::cout << str << std::endl;
}
}
void mainloop() {
pthread_t thread;
pthread_create(&thread, NULL, aed::map::shell::Shell, this);
...
pthread_cancel(thread);
}
And now the solution which doesn't work everytime, with std::thread:
class ShellThreadInterrupFlag {
public:
void interrupt() {
throw std::string("Thread interruption test\n");
}
};
class ShellThread {
public:
template<typename FunctionType, typename ParamsType>
ShellThread(FunctionType f, ParamsType params) {
std::promise<ShellThreadInterrupFlag *> p[3];
_internal_thread = new std::thread(f, p, params);
_flag = p[0].get_future().get();
_internal_thread->detach();
p[1].set_value(_flag); // tell the thread that we detached it
p[2].get_future().get(); // wait until the thread validates the constructor could end (else p[3] is freed)
}
~ShellThread() {
delete _internal_thread;
}
void interrupt() {
_flag->interrupt();
}
private:
std::thread *_internal_thread;
ShellThreadInterrupFlag *_flag;
};
void Shell(std::promise<ShellThreadInterrupFlag *> promises[3],
aed::map::MapEditor *me)
{
ShellThreadInterrupFlag flag;
promises[0].set_value(&flag); // give the ShellThread instance the flag adress
promises[1].get_future().get(); // wait for detaching
promises[2].set_value(&flag); // tell ShellThread() it is able to finish
while(1) {
std::cin >> str;
std::cout << str << std::endl;
}
}
void mainloop()
{
ShellThread *shell_thread;
shell_thread = new ShellThread(Shell, this);
... // mainloop with opengl for drawing, events gestion etc...
shell_thread->interrupt();
}
Sometimes, when I launch the program, the std::cin >> str is called and the mainloop is blocked.
Does anyone know why the thread is blocking my mainloop ? And how could I avoid this problem ?