c/c++: 多线程编程基础讲解（六）

上篇说了下互斥量的用法，今儿说一下条件信号量的用法，这两种多线程变量的用法其实取决于情景，需要体会，见文：

#include <iostream>
#include <pthread.h>//带头文件
#include <stdio.h>

using namespace std;

#define BOUNDARY 5

int tasks = 10;

pthread_mutex_t tasks_mutex;//因为两个线程要修改一个全局变量，需要互斥量；
pthread_cond_t tasks_cond;//因为两个线程间有条件关系：当tasks>5时，hello2处理它，处理一次减少1；反之hello1处理，直到tasks减为零；

void* say_hello2(void* args)//hello2处理函数
{
pthread_t pid = pthread_self();//打印当前线程id便于跟踪
cout << "["<< pid << "] hello in thread " << *((int*)args) << endl;

bool is_signaled = false;//随便一个标志位
while(1)//无限循环
{
pthread_mutex_lock(&tasks_mutex);//要修改了，加锁
if (tasks > BOUNDARY)//>5才修改
{
cout << "["<< pid << "] take task:  "<< tasks << " in thread "<< *((int*)args) << endl;
--tasks;//减少1
}
else if (!is_signaled)
{
cout << "["<< pid << "] pthread_cond_signal in thread " << *((int*)args) << endl;
pthread_cond_signal(&tasks_cond);//表明已经不是>5了告诉hello1进程去处理：发送信号；
is_signaled = true;//表示信号已经发送了
}
pthread_mutex_unlock(&tasks_mutex);//操作完解锁

if (tasks == 0) break;//必须等待tasks全部减为零即hello1完成操作，才跳出循环结束这个进程
}
}

void* say_hello1(void* args)//<=5处理函数
{
pthread_t pid = pthread_self();
cout << "["<< pid << "] hello in thread " << *((int*)args) << endl;</p><p>    while(1)
{
pthread_mutex_lock(&tasks_mutex);
if (tasks > BOUNDARY)//如果>5说明需要hello2处理，那么该线程就需要等待
{
cout << "["<< pid << "] pthread_cond_wait in thread " << *((int*)args) << endl;
pthread_cond_wait(&tasks_cond, &tasks_mutex);//等待信号量生效，当hello2发出信号，这里就跳出wait，执行后续；
}
else
{
cout << "["<< pid << "] take task:  "<< tasks << " in thread "<< *((int*)args) << endl;
--tasks;//<=5就--
}
pthread_mutex_unlock(&tasks_mutex);
if (tasks == 0) break;//为零时退出，同hello2一样
}
int main()
{
pthread_attr_t attr;//线程创建为joinable的，使得主进程可以和两个线程同步，两个线程完成工作退出后，主进程再退出；
pthread_attr_init(&attr);
pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_JOINABLE);
pthread_mutex_init(&tasks_mutex, NULL);//初始化互斥量
pthread_cond_init(&tasks_cond, NULL);//初始化条件信号量
pthread_t tid1, tid2;//用于保存两个线程的id号
int index1 = 1;
int ret = pthread_create( &tid1, &attr, say_hello1, (void *)&index1);
if (ret != 0)
{
cout << "pthread_create error: error_code=" << ret << endl;
}
int index2 = 2;
ret = pthread_create( &tid2, &attr, say_hello2, (void *)&index2);
if (ret != 0)
{
cout << "pthread_create error: error_code=" << ret << endl;
}
pthread_join(tid1, NULL);//连接两个线程
pthread_join(tid2, NULL);
pthread_attr_destroy(&attr);//该销毁的销毁
pthread_mutex_destroy(&tasks_mutex);
pthread_cond_destroy(&tasks_cond); //正常退出
}

惯例：g++ -lpthread -o ex_cond ex_cond.cpp

执行结果：

[cpp] view
plaincopy

[cpp@node2 pthread]$ ./ex_cond   

[140009886947088] hello in thread 2  

[140009886947088] take task:  10 in thread 2  

[140009886947088] take task:  9 in thread 2  

[140009886947088] take task:  8 in thread 2  

[140009886947088] take task:  7 in thread 2  

[140009886947088] take task:  6 in thread 2  

[140009886947088] pthread_cond_signal in thread 2  

[140009897436944] hello in thread 1  

[140009897436944] take task:  5 in thread 1  

[140009897436944] take task:  4 in thread 1  

[140009897436944] take task:  3 in thread 1  

[140009897436944] take task:  2 in thread 1  

[140009897436944] take task:  1 in thread 1