信号量的概念
-信号量是特殊的线程锁
-信号量允许N个线程同时访问临界资源
-Qt中直接支持信号量(QSemaphore)
线程锁是用来保护临界资源的，每个线程锁每次只允许一个线程进行访问临界资源。
QSemaphore sem(1)定义了一个信号量，并且N的值为1，意味着一次只允许一个线程去访问临界资源。
sem.acquire()当前线程尝试着获取这个特殊的线程锁，首先到该函数中查看n的值是不是大于0的，如果是的话，就可以获取这把特殊的线程锁。如果n的值为0，那就必须等待。
再论生产消费者的问题：
在前面的实验中已经做过一个生产者，一个消费者，一个仓库。在本课程中实现n个生产者，n个消费者，n个仓库。如何做到并发高效性呢，此时就需要使用QSemphore信号量了。
#include <QCoreApplication>
#include <QThread>
#include <QDebug>
#include <QSemaphore>
const int SIZE = 5; //用来定义仓库的个数
unsigned char g_bufer[SIZE] = {0};
QSemaphore g_sem_free(SIZE); //表示5个仓库都是空闲的
QSemaphore g_sem_used(0); //初始值为0，表示有多少个仓库已经被使用了
class Producer : public QThread
{
protected:
void run()
{
while(true)
{
int value = qrand()%100;
g_sem_free.acquire();
for(int i=0; i<SIZE; i++)
{
if( !g_bufer[i] )
{
g_bufer[i] = value;
qDebug() << objectName() << "generate (" << i << "," << value << ")" << endl;
break;
}
}
g_sem_used.release();
sleep(2);
}
}
};
class Consumer : public QThread
{
protected:
void run()
{
while(true)
{
g_sem_used.acquire();
for(int i=0; i<SIZE; i++)
{
if( g_bufer[i] )
{
int value = g_bufer[i];
g_bufer[i] = 0;
qDebug() << objectName() << "consume (" << i << "," << value << ")" <<endl;
break;
}
}
g_sem_free.release();
sleep(2);
}
}
};
int main(int argc, char *argv[])
{
QCoreApplication a(argc, argv);
Producer p1;
Producer p2;
Producer p3;
Consumer c1;
Consumer c2;
p1.setObjectName("p1");
p2.setObjectName("p2");
p3.setObjectName("p3");
c1.setObjectName("c1");
c2.setObjectName("c2");
p1.start();
p2.start();
p3.start();
c1.start();
c2.start();
return a.exec();
}
打印结果如下：
"p1" generate ( 1 , 41 )
"p3" generate ( 2 , 41 )
"p2" generate ( 0 , 41 )
"c2" consume ( 1 , 41 )
"c1" consume ( 0 , 41 )
"p2" generate ( 0 , 67 )
"p3" generate ( 1 , 67 )
"p1" generate ( 3 , 67 )
"c1" consume ( 1 , 67 )
"c2" consume ( 0 , 67 )
"p2" generate ( 0 , 34 )
"p3" generate ( 1 , 34 )
"p1" generate ( 4 , 34 )
"c1" consume ( 1 , 34 )
"c2" consume ( 0 , 34 )
"p2" generate ( 0 , 0 )
"p3" generate ( 0 , 0 )
"c1" consume ( 2 , 41 )
"c2" consume ( 3 , 67 )
"p1" generate ( 0 , 0 )
"p2" generate ( 0 , 69 )
"c1" consume ( 0 , 69 )
"p3" generate ( 0 , 69 )
"c2" consume ( 0 , 69 )
"p1" generate ( 0 , 69 )
"c1" consume ( 0 , 69 )
"p2" generate ( 0 , 24 )
"c2" consume ( 0 , 24 )
"p3" generate ( 0 , 24 )
"c1" consume ( 0 , 24 )
"p1" generate ( 0 , 24 )
"c2" consume ( 0 , 24 )
"p2" generate ( 0 , 78 )
"c1" consume ( 0 , 78 )
"p3" generate ( 0 , 78 )
"c2" consume ( 0 , 78 )
"p2" generate ( 0 , 58 )
"c1" consume ( 0 , 58 )
"p1" generate ( 0 , 78 )
"c2" consume ( 0 , 78 )
"p3" generate ( 0 , 58 )
"c1" consume ( 0 , 58 )
""p2" generate ( 0 , 62 )
c2" consume ( 4 , 34 )
"p1" generate ( 1 , 58 )
"c1" consume ( 0 , 62 )
"p3" generate ( 0 , 62 )
"c2" consume ( 0 , 62 )
"p1" generate ( 0 , 62 )
"c1" consume ( 0 , 62 )
"p2" generate ( 0 , 64 )
"c2" consume ( 0 , 64 )
"p3" generate ( 0 , 64 )
"c1" consume ( 0 , 64 )
"p1" generate ( 0 , 64 )
"c2" consume ( 0 , 64 )
"p3" generate ( 0 , 5 )
"c1" consume ( 0 , 5 )
"p2" generate ( 0 , 5 )
"c2" consume ( 0 , 5 )
"p1" generate ( 0 , 5 )
"c1" consume ( 0 , 5 )
"p3" generate ( 0 , 45 )
"c2" consume ( 0 , 45 )
"p2" generate ( 0 , 45 )
"c1" consume ( 0 , 45 )
"p1" generate ( 0 , 45 )
"c2" consume ( 0 , 45 )
"p3" generate ( 0 , 81 )
