C++11 没有提供信号量，但是可以使用条件变量和互斥锁很容易的实现信号量。信号量是用来在多线程中进行资源同步的。信号量内部维护资源的数量，并且提供2个操作——wait和signal，wait的时候获取资源并减少计数器，signal的时候释放资源并增加计数器。只有当计数器的数目>0的情况下去wait才能够获取到资源。
注意：
使用信号量的一个原则是只有获取资源才wait，释放资源才调用signal！！
信号量实现
#include <condition_variable>
#include <thread>
class semaphore
{
public:
semaphore(int count) :m_counter(count){}
~semaphore(){}
void wait()
{
std::unique_lock<std::mutex> lock(m_mutex);
m_counter--;
while (m_counter < 0)// 防止假唤醒
{
m_cv.wait(lock);
}
}
void signal()
{
std::unique_lock<std::mutex> lock(m_mutex);
if (++m_counter <= 0) // 在执行++之后<=0，表明有其他线程在等待资源，因此执行唤醒
{
m_cv.notify_one();
}
}
int m_counter;
std::condition_variable m_cv;
std::mutex m_mutex;
};
测试代码
#include <algorithm>
#include <vector>
#include <atomic>
#include <condition_variable>
#include <thread>
#include <chrono>
#include <iostream>
#include <chrono>
#include "fxtime.h"
int main()
{
semaphore s(5);
int task_count = 1000;
int i = 0;
std::vector<int> queue(task_count);
std::transform(queue.begin(), queue.end(), queue.begin(), [&i](int &n) {i++; return i; });
int thread_count = 3;
std::vector<std::thread> threads;
std::mutex m;
int sub_index = 0;
fxtime t;
for (int i = 0; i < thread_count; ++i)
{
auto t = std::thread([&queue, &sub_index,&s,&m, task_count]() {
do
{
s.wait();
std::unique_lock<std::mutex> lock(m);
if (sub_index >= task_count)
{
std::cout << "thread return" << std::endl;
return;
}
std::cout << ",q[" << sub_index << "]=" << queue[sub_index] << ";counter = [" << s.m_counter << "]"<< std::endl;
sub_index++;
lock.unlock();
std::this_thread::sleep_for(std::chrono::milliseconds(std::rand() % 100+10));
s.signal();
} while (true);
});
threads.push_back(std::move(t));
//s.wait(); // 这里进行wait是一个明显的错误，因为这里并没有获取资源！！！
}
std::cout << ";counter = [" << s.m_counter << "]" << std::endl;
int exit_thread_count = 0;
for (auto& t : threads)
{
t.join();
exit_thread_count++;
std::cout << "exit thread count = " << exit_thread_count << std::endl;
}
std::cout << "dont wait>time espaced:" << t.escaped_miliseconds() << "ms" << std::endl;
std::cout << ";counter = [" << s.m_counter << "]" << std::endl;
return 0;
}