Linux 线程 条件变量

　　下面是一个多线程，生产者消费者问题，一个队列放暂存的数据：

 1 #include <iostream>
 2 #include <queue>
 3 #include <stdlib.h>
 4 #include <unistd.h>
 5 #include <pthread.h>
 6
 7 using std::cout;
 8 using std::endl;
 9 using std::queue;
10
11 #define N 100
12 #define ST 10
13
14 pthread_mutex_t lock = PTHREAD_MUTEX_INITIALIZER;
15 pthread_cond_t ready = PTHREAD_COND_INITIALIZER;
16
17 queue<int> que;
18
19 void* threadProducer(void* arg)
20 {
21     while(true)
22     {
23         sleep(rand() % ST);
24
25         cout << "Produce try in...\n";
26         pthread_mutex_lock(&lock);
27         cout << "Produce in!\n";
28         int source = rand() % N;
29         cout << "Produce " << source << endl;
30         que.push(source);
31         pthread_mutex_unlock(&lock);
32         cout << "Produce out\n";
33
34         pthread_cond_signal(&ready);
35     }
36 }
37
38 void* threadConsumer(void* arg)
39 {
40     while(true)
41     {
42         sleep(rand() % ST);
43
44         cout << "Consum try in...\n";
45         pthread_mutex_lock(&lock);
46         cout << "Consum in!\n";
47         while(que.empty())
48         {
49             pthread_cond_wait(&ready, &lock);
50             cout << "Consum from sleep\n";
51         }
52         cout << "Consum " << que.front() << endl;
53         que.pop();
54         pthread_mutex_unlock(&lock);
55         cout << "Consum out\n\n";
56     }
57 }
58
59 int main(void)
60 {
61     pthread_t tProducer, tConsumer;
62     pthread_create(&tProducer, NULL, threadProducer, NULL);
63     pthread_create(&tConsumer, NULL, threadConsumer, NULL);
64
65     pthread_join(tProducer, NULL);
66     pthread_join(tConsumer, NULL);
67
68     exit(0);
69 }

生消

看到倒数的三四行，消费者进去了，发现没有数据了，则睡眠了，然后生产者进去生产了。

　　下面是一个多线程的小例子，线程1打印非3的倍数，线程2打印3的倍数：

#include <iostream>
#include <stdlib.h>
#include <unistd.h>
#include <pthread.h>

using std::cout;
using std::endl;

pthread_mutex_t lock = PTHREAD_MUTEX_INITIALIZER;
pthread_cond_t ready = PTHREAD_COND_INITIALIZER;

int data = 0;

void* threadProducer(void* arg)
{
    int i;
    for(i = 1; i < 22; i++)
    {
        sleep(1);

        if(i % 3 != 0)
        {
            cout << "thread1:" << i << endl;
        }
        else
        {
            pthread_mutex_lock(&lock);
            data = i;
            pthread_mutex_unlock(&lock);

            pthread_cond_signal(&ready);
        }
    }
}

void* threadConsumer(void* arg)
{
    while(true)
    {
        pthread_mutex_lock(&lock);
        while(data == 0)    //no data
            pthread_cond_wait(&ready, &lock);
        cout <<"thread2:" << data << endl;
        if(data == 21)
            break;
        else
            data = 0;    //empty data
        pthread_mutex_unlock(&lock);
    }
}

int main(void)
{
    pthread_t tProducer, tConsumer;
    pthread_create(&tProducer, NULL, threadProducer, NULL);
    pthread_create(&tConsumer, NULL, threadConsumer, NULL);

    pthread_join(tProducer, NULL);
    pthread_join(tConsumer, NULL);

    exit(0);
}

3打印

　　程序大致这样：线程1中的循环，如果i不是3的倍数就自己打印了，如果是的话，把这个数放到一个地方（由于这个地方可以被线程2发现，所以要加锁访问），然后唤醒等待数据的线程2（如果线程2还没有在等待，那么这个唤醒则丢失，这是个bug，见下），线程2被唤醒后，消费了这个3的倍数，清空数据区。

　　上面提到，如果唤醒线程2的消息没有被收到，则bug。看下面的代码，也就多了38一行，让线程2睡了一会，就在它睡觉的那么一会，线程1把3的倍数往那里一扔就走了，自己再继续下两个不是3倍数的数字，这就直接输出了下面两个数字，又到了3倍数，又扔过去覆盖了之前数字：

 1 #include <iostream>
 2 #include <stdlib.h>
 3 #include <unistd.h>
 4 #include <pthread.h>
 5
 6 using std::cout;
 7 using std::endl;
 8
 9 pthread_mutex_t lock = PTHREAD_MUTEX_INITIALIZER;
10 pthread_cond_t ready = PTHREAD_COND_INITIALIZER;
11
12 int data = 0;
13
14 void* threadProducer(void* arg)
15 {
16     int i;
17     for(i = 1; i < 22; i++)
18     {
19         sleep(1);
20
21         if(i % 3 != 0)
22         {
23             cout << "thread1:" << i << endl;
24         }
25         else
26         {
27             pthread_mutex_lock(&lock);
28             data = i;
29             pthread_mutex_unlock(&lock);
30
31             pthread_cond_signal(&ready);
32         }
33     }
34 }
35
36 void* threadConsumer(void* arg)
37 {
38     sleep(20);
39     while(true)
40     {
41         pthread_mutex_lock(&lock);
42         while(data == 0)    //no data
43             pthread_cond_wait(&ready, &lock);
44         cout <<"thread2:" << data << endl;
45         if(data == 21)
46             break;
47         else
48             data = 0;    //empty data
49         pthread_mutex_unlock(&lock);
50     }
51 }
52
53 int main(void)
54 {
55     pthread_t tProducer, tConsumer;
56     pthread_create(&tProducer, NULL, threadProducer, NULL);
57     pthread_create(&tConsumer, NULL, threadConsumer, NULL);
58
59     pthread_join(tProducer, NULL);
60     pthread_join(tConsumer, NULL);
61
62     exit(0);
63 }

bug

　　从上面可以总结出下面的条件变量的生产者消费者代码模型：

//下面是生产者

pthread_mutex_lock(&lock);    //加锁访问临界区
/*在这里生产数据*/
pthread_mutex_unlock(&lock);    //解锁

pthread_cond_signal(&ready);    //通知消费者

//下面是消费者

pthread_mutex_lock(&lock);    //加锁访问临界区
while(没有待消费数据)
        pthread_cond_wait(&ready, &lock);    //睡在这里，等待被唤醒
/*被叫醒了，在这里消费数据*/
pthread_mutex_unlock(&lock);    //解锁

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