视频教程：https://www.bilibili.com/video/av94487439
本文主要介绍多线程中数据同步的方法，技术包括：线程锁，同步变量，原子变量，消息处理等；以及三种同步队列的实现方法。
std::unique_lock
- 与std:::lock_gurad基本一致，但更加灵活的锁管理类模板，构造时是否加锁是可选的，在对象析构时如果持有锁会自动释放锁，所有权可以转移。对象生命期内允许手动加锁和释放锁。但提供了更好的上锁和解锁控制接口(lock，try_lock，try_lock_for，try_lock_until 和unlock)
条件变量
- 条件变量可以阻塞一个或多个线程，直到收到另外一个线程发出的通知，或者超时了才会唤醒当前阻塞的线程。
类型
- condition_variable，配合std::unique_lock
- condition_variable_any，配合任意带有lock，unlock语义的mutex进行操作
- 比较灵活，更通用，对所有的锁都适用
- 效率比condition_variable差
成员函数
- notify_one 通知一个等待线程(public)
- notify_all 通知所有等待线程(public)
notify_one()和notify_all()都是Object对象用于通知处在等待该对象的线程的方法，但notify_one是通知一个线程获取锁，notify_all是通知所有相关的线程去竞争锁。
- wait 阻塞当前线程直至条件变量被唤醒(public)
- wait_for 阻塞当前线程直至条件变量被唤醒或超时(public)
- wait_until 阻塞当前线程直至条件变量被唤醒或直到指定的时间点(public)
执行过程
- 拥有条件变量的线程首先获取互斥量
- 然后循环检查某个条件，如果条件不满足，释放互斥量，同时阻塞该线程直到条件满足；如果条件满足，则向下执行。
- 另一个线程获取互斥量，执行完成后调用条件变量的notify_one或notify_all唤醒一个或者所有等待线程。
简洁写法及wait机制
std::unique_lock lck(m_mtRun);
m_cvRun.wait(lck, [this]{ return m_runDown; });
- 条件变量首先检查判断式是否满足条件，例如上例中的m_runDown是否为true
- 如果不满足条件，释放mutex，将线程置为wait状态，继续等待唤醒
- 如果满足条件，重新获取mutex，线程结束wait状态，继续向下执行
- 这里需要注意的是，wait状态的线程被唤醒，但判断式不满足条件，****即假唤醒****，条件变量将继续释放mutex，将线程置为wait状态，继续等待下一次的唤醒
基本示例-wait, wait_for和假唤醒
#include
#include
#include
#include
#include
#include
bool completed;
std::mutex mtRun;
std::condition_variable cvRun;
void Wait()
{
std::unique_lock lck(mtRun);
std::cout <<"Thread_"< lck(mtRun);
std::cout << "Thread_" << std::this_thread::get_id() << " is waiting..." << std::endl;
if (!cvRun.wait_for(lck, std::chrono::seconds(4), []() {
//if (!cvRun.wait_for(lck, std::chrono::seconds(2), []() {
return completed;
}))
{
std::cout << "Thread_" << std::this_thread::get_id() << " time out!" << std::endl;
}
else
{
std::cout << "Thread_" << std::this_thread::get_id() << " is completed" << std::endl;
}
}
void Completed()
{
{
std::cout << "Thread_" << std::this_thread::get_id() << " set completed" << std::endl;
std::unique_lock lck(mtRun);
completed = true;
}
cvRun.notify_all();
}
void FakeCompleted()
{
{
std::cout << "Thread_" << std::this_thread::get_id() << " not set completed" << std::endl;
std::unique_lock lck(mtRun);
completed = false;
}
cvRun.notify_all();
}
int main()
{
//Wait
completed = false;
std::thread thWait(Wait);
thWait.detach();
Sleep(3000);
std::thread thCompleted(Completed);
thCompleted.join();
Sleep(3000);
//Waitfor
//completed = false;
//std::thread thWait(Wait_For);
//thWait.detach();
//Sleep(3000);
//std::thread thCompleted(Completed);
//thCompleted.join();
//Sleep(3000);
//Fake
/*completed = false;
std::thread thWait(Wait_For);
thWait.detach();
Sleep(3000);
std::thread thCompleted(FakeCompleted);
thCompleted.join();
Sleep(3000);*/
return 0;
}
原子变量
- 使用原子变量不需要使用互斥量来保护这个变量，使用更简洁。
- C++11提供个原子类型std::atomic
, 可以使用任意类型作为参数模板，同时也内置了基础类型的原子变量。
typedef atomic atomic_bool;
typedef atomic atomic_char;
typedef atomic atomic_schar;
typedef atomic atomic_uchar;
typedef atomic atomic_short;
typedef atomic atomic_ushort;
typedef atomic atomic_int;
typedef atomic atomic_uint;
typedef atomic atomic_long;
typedef atomic atomic_ulong;
typedef atomic atomic_llong;
typedef atomic atomic_ullong;
typedef atomic atomic_char16_t;
typedef atomic atomic_char32_t;
typedef atomic atomic_wchar_t;
typedef atomic atomic_int8_t;
typedef atomic atomic_uint8_t;
typedef atomic atomic_int16_t;
typedef atomic atomic_uint16_t;
typedef atomic atomic_int32_t;
typedef atomic atomic_uint32_t;
typedef atomic atomic_int64_t;
typedef atomic atomic_uint64_t;
typedef atomic atomic_int_least8_t;
typedef atomic atomic_uint_least8_t;
typedef atomic atomic_int_least16_t;
typedef atomic atomic_uint_least16_t;
typedef atomic atomic_int_least32_t;
typedef atomic atomic_uint_least32_t;
typedef atomic atomic_int_least64_t;
typedef atomic atomic_uint_least64_t;
typedef atomic atomic_int_fast8_t;
typedef atomic atomic_uint_fast8_t;
typedef atomic atomic_int_fast16_t;
typedef atomic atomic_uint_fast16_t;
typedef atomic atomic_int_fast32_t;
typedef atomic atomic_uint_fast32_t;
typedef atomic atomic_int_fast64_t;
typedef atomic atomic_uint_fast64_t;
typedef atomic atomic_intptr_t;
typedef atomic atomic_uintptr_t;
typedef atomic atomic_size_t;
typedef atomic atomic_ptrdiff_t;
typedef atomic atomic_intmax_t;
typedef atomic atomic_uintmax_t;
//
typedef signed char int8_t;
typedef short int16_t;
typedef int int32_t;
typedef long long int64_t;
typedef unsigned char uint8_t;
typedef unsigned short uint16_t;
typedef unsigned int uint32_t;
typedef unsigned long long uint64_t;
typedef signed char int_least8_t;
typedef short int_least16_t;
typedef int int_least32_t;
typedef long long int_least64_t;
typedef unsigned char uint_least8_t;
typedef unsigned short uint_least16_t;
typedef unsigned int uint_least32_t;
typedef unsigned long long uint_least64_t;
typedef signed char int_fast8_t;
typedef int int_fast16_t;
typedef int int_fast32_t;
typedef long long int_fast64_t;
typedef unsigned char uint_fast8_t;
typedef unsigned int uint_fast16_t;
typedef unsigned int uint_fast32_t;
typedef unsigned long long uint_fast64_t;
typedef long long intmax_t;
typedef unsigned long long uintmax_t;
- 以下写法是一样
std::atomic_int m_standbyIdIndex;
std::atomic m_standbyIdIndex;
call_once&once_flag
- 如果多个线程需要同时调用某个函数，std::call_once 可以保证多个线程对该函数只调用一次。
- 需要一个std::once_flag作为std::call_once的入参
std::once_flag m_flag;
std::call_once(m_flag, [this](){StopExecute(); });
同步队列
基本同步队列
#include
#include
#include
#include
#include
#include
class SyncQueue
{
public:
SyncQueue()
{
}
void Push(const int& x)
{
{
std::unique_lock lck(m_mutex);
m_queue.push_back(x);
}
m_notEmpty.notify_all();
}
void Pop(int& x)
{
std::unique_lock lck(m_mutex);
m_notEmpty.wait(lck, [this]() {
return !m_queue.empty();
});
x = m_queue.front();
m_queue.pop_front();
}
bool Empty()
{
std::lock_guard lck(m_mutex);
return m_queue.empty();
}
size_t Size()
{
std::lock_guard lck(m_mutex);
return m_queue.size();
}
private:
std::list m_queue;
std::mutex m_mutex;
std::condition_variable m_notEmpty;
};
SyncQueue queue;
void GetData()
{
int x = 0;
while (queue.Empty())
{
queue.Pop(x);
std::cout << "Thread_" << std::this_thread::get_id() << "---- Pop " << x << std::endl;
if (x == 0)
{
break;
}
}
std::cout << "Thread_" << std::this_thread::get_id() << "---- Pop End!" << std::endl;
}
void SetData()
{
for (int i = 10; i >= 0; i--)
{
Sleep(1000);
std::cout << "Thread_" << std::this_thread::get_id() << "---- Push " << i << std::endl;
queue.Push(i);
}
Sleep(500);
std::cout << "Thread_" << std::this_thread::get_id() << "---- Push End!" << std::endl;
}
int main()
{
std::thread thGet(GetData);
thGet.detach();
std::thread thSet(SetData);
thSet.join();
return 0;
}
//output
Thread_27072---- Push 1
Thread_26712---- Pop 1
Thread_27072---- Push 2
Thread_26712---- Pop 2
Thread_27072---- Push 3
Thread_26712---- Pop 3
Thread_27072---- Push 4
Thread_26712---- Pop 4
Thread_27072---- Push 5
Thread_26712---- Pop 5
Thread_27072---- Push 6
Thread_26712---- Pop 6
Thread_27072---- Push 7
Thread_26712---- Pop 7
Thread_27072---- Push 8
Thread_26712---- Pop 8
Thread_27072---- Push 9
Thread_26712---- Pop 9
Thread_27072---- Push 10
Thread_26712---- Pop 10
Thread_26712---- Pop End!
Thread_27072---- Push End!
带外部控制的同步队列
#include
#include
#include
#include
#include
#include
#include
class SyncQueue
{
public:
SyncQueue()
{
}
void Push(const int& x)
{
{
std::unique_lock lck(m_mutex);
m_queue.push_back(x);
}
m_notEmpty.notify_all();
}
void Pop(int& x)
{
std::unique_lock lck(m_mutex);
m_notEmpty.wait(lck, [this]() {
return !m_queue.empty();
});
x = m_queue.front();
m_queue.pop_front();
}
bool Empty()
{
std::lock_guard lck(m_mutex);
return m_queue.empty();
}
size_t Size()
{
std::lock_guard lck(m_mutex);
return m_queue.size();
}
private:
std::list m_queue;
std::mutex m_mutex;
std::condition_variable m_notEmpty;
};
SyncQueue queue;
std::atomic_bool getStop = false;
void GetData()
{
int x = 0;
while (queue.Empty())
{
if (getStop)
{
break;
}
queue.Pop(x);
std::cout << "Thread_" << std::this_thread::get_id() << "---- Pop " << x << std::endl;
}
std::cout << "Thread_" << std::this_thread::get_id() << "---- Pop End!" << std::endl;
}
void SetData()
{
for (int i = 10; i >= 0; i--)
{
Sleep(1000);
std::cout << "Thread_" << std::this_thread::get_id() << "---- Push " << i << std::endl;
queue.Push(i);
if (i == 5)
{
getStop = true;
}
}
std::cout << "Thread_" << std::this_thread::get_id() << "---- Push End!" << std::endl;
}
int main()
{
std::thread thGet(GetData);
thGet.detach();
std::thread thSet(SetData);
thSet.join();
return 0;
}
//output
Thread_29616---- Push 10
Thread_30076---- Pop 10
Thread_29616---- Push 9
Thread_30076---- Pop 9
Thread_29616---- Push 8
Thread_30076---- Pop 8
Thread_29616---- Push 7
Thread_30076---- Pop 7
Thread_29616---- Push 6
Thread_30076---- Pop 6
Thread_29616---- Push 5
Thread_30076---- Pop 5
Thread_30076---- Pop End!
Thread_29616---- Push 4
Thread_29616---- Push 3
Thread_29616---- Push 2
Thread_29616---- Push 1
Thread_29616---- Push 0
Thread_29616---- Push End!
带超时的同步队列
#include
#include
#include
#include
#include
#include
#include
class SyncQueue
{
public:
SyncQueue()
{
}
void Push(const int& x)
{
{
std::unique_lock lck(m_mutex);
m_queue.push_back(x);
}
m_notEmpty.notify_all();
}
bool Pop(int& x)
{
std::unique_lock lck(m_mutex);
if (m_notEmpty.wait_for(lck, std::chrono::seconds(1), [this]() {
return !m_queue.empty();
}))
{
x = m_queue.front();
m_queue.pop_front();
return true;
}
else
{
return false;
}
}
bool Empty()
{
std::lock_guard lck(m_mutex);
return m_queue.empty();
}
size_t Size()
{
std::lock_guard lck(m_mutex);
return m_queue.size();
}
private:
std::list m_queue;
std::mutex m_mutex;
std::condition_variable m_notEmpty;
};
SyncQueue queue;
std::atomic_bool getStop = false;
void GetData()
{
int x = 0;
while (queue.Empty())
{
if (getStop)
{
break;
}
if (queue.Pop(x))
{
std::cout << "Thread_" << std::this_thread::get_id() << "---- Pop " << x << std::endl;
}
else
{
std::cout << "Thread_" << std::this_thread::get_id() << "---- Get Data Time out" << std::endl;
}
}
std::cout << "Thread_" << std::this_thread::get_id() << "---- Pop End!" << std::endl;
}
void SetData()
{
for (int i = 10; i >= 0; i--)
{
Sleep(100);
std::cout << "Thread_" << std::this_thread::get_id() << "---- Push " << i << std::endl;
queue.Push(i);
if (i <= 5)
{
Sleep(2000);
}
}
getStop = true;
Sleep(500);
std::cout << "Thread_" << std::this_thread::get_id() << "---- Push End!" << std::endl;
}
int main()
{
std::thread thGet(GetData);
thGet.detach();
std::thread thSet(SetData);
thSet.join();
return 0;
}
//output
Thread_18908---- Push 10
Thread_2204---- Pop 10
Thread_18908---- Push 9
Thread_2204---- Pop 9
Thread_18908---- Push 8
Thread_2204---- Pop 8
Thread_18908---- Push 7
Thread_2204---- Pop 7
Thread_18908---- Push 6
Thread_2204---- Pop 6
Thread_18908---- Push 5
Thread_2204---- Pop 5
Thread_2204---- Get Data Time out
Thread_2204---- Get Data Time out
Thread_18908---- Push 4
Thread_2204---- Pop 4
Thread_2204---- Get Data Time out
Thread_2204---- Get Data Time out
Thread_18908---- Push 3
Thread_2204---- Pop 3
Thread_2204---- Get Data Time out
Thread_2204---- Get Data Time out
Thread_18908---- Push 2
Thread_2204---- Pop 2
Thread_2204---- Get Data Time out
Thread_2204---- Get Data Time out
Thread_18908---- Push 1
Thread_2204---- Pop 1
Thread_2204---- Get Data Time out
Thread_2204---- Get Data Time out
Thread_18908---- Push 0
Thread_2204---- Pop 0
Thread_2204---- Get Data Time out
Thread_2204---- Get Data Time out
Thread_2204---- Pop End!
Thread_18908---- Push End!