pThread线程（三） 线程同步--条件变量

pthread_cond_init (condition,attr)
pthread_cond_destroy (condition)

pthread_condattr_init (attr)
pthread_condattr_destroy (attr)

pthread_cond_wait (condition,mutex)
pthread_cond_signal (condition)
pthread_cond_broadcast (condition)

 1 /******************************************************************************
 2  * 描述：
 3  *     应用Pthreads条件变量的实例代码，主线程创建三个线程，其中两个为“count”变量做
 4  * 加法运算，第三个线程监视“count”的值。当“count”达到一个限定值，等待线程准备接收来
 5  * 自于两个加法线程中一个的信号，等待 线程唤醒后更改“count”的值。程序继续运行直到加法
 6  * 线程达到TCOUNT的值。最后，主程序打印出count的值。
 7  ******************************************************************************/
 8 #include <pthread.h>
 9 #include <stdio.h>
10 #include <stdlib.h>
11 
12 #define NUM_THREADS  3
13 #define TCOUNT 5       //单线程轮询次数
14 #define COUNT_LIMIT 7  //发送信号的次数
15 int count = 0; //全局的累加量
16 
17 pthread_mutex_t count_mutex;
18 pthread_cond_t count_threshold_cv;
19 
20 void *inc_count(void *t) {
21     int i;
22     long my_id = (long) t;
23 
24     for (i = 0; i < TCOUNT; i++) {
25         pthread_mutex_lock(&count_mutex);
26         count++;
27         /*
28          * 检查count的值，如果条件满足就发信号给等待线程
29          * 注意，此处是用信号量锁定的。
30          * */
31         if (count < COUNT_LIMIT) {
32             printf("inc_count(): thread %ld, count = %d  Threshold reached. ",
33                     my_id, count);
34             pthread_cond_signal(&count_threshold_cv);
35             printf("Just sent signal.\n");
36         }
37         printf("inc_count(): thread %ld, count = %d, unlocking mutex\n", my_id,
38                 count);
39         pthread_mutex_unlock(&count_mutex);
40 
41         /*为线程轮询互斥锁增加延时*/
42         sleep(1);
43     }
44     pthread_exit(NULL);
45 }
46 
47 void *watch_count(void *t) {
48     long my_id = (long) t;
49     printf("Starting watch_count(): thread %ld\n", my_id);
50 
51     /*锁定互斥量并等待信号，注意，pthread_cond_wait函数在等待时将自动以自动原子方式
52      * 解锁互斥量。还有，请注意，如果等待线程运行到等待函数之前已经满足COUNT_LIMIT的
53      * 条件判断，轮询会忽略掉等待函数，
54      * */
55     while (count < COUNT_LIMIT) {
56         pthread_mutex_lock(&count_mutex);
57         printf("watch_count(): thread %ld going into wait...\n", my_id);
58         pthread_cond_wait(&count_threshold_cv, &count_mutex);
59         printf("watch_count(): thread %ld Condition signal received.\n", my_id);
60 
61         printf("watch_count(): thread %ld count now = %d.\n", my_id, count);
62         pthread_mutex_unlock(&count_mutex);
63     }
64     pthread_exit(NULL);
65 }
66 
67 int main(int argc, char *argv[]) {
68     int i;
69     long t1 = 1, t2 = 2, t3 = 3;
70     pthread_t threads[3];
71     pthread_attr_t attr;
72 
73     /*初始化互斥量和条件变量对象*/
74     pthread_mutex_init(&count_mutex, NULL);
75     pthread_cond_init(&count_threshold_cv, NULL);
76 
77     /*创建线程时设为可连接状态，便于移植*/
78     pthread_attr_init(&attr);
79     pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_JOINABLE);
80     pthread_create(&threads[0], &attr, watch_count, (void *) t1);
81     pthread_create(&threads[1], &attr, inc_count, (void *) t2);
82     pthread_create(&threads[2], &attr, inc_count, (void *) t3);
83 
84     /* 等待所有线程完成*/
85     for (i = 1; i < NUM_THREADS; i++) {
86         pthread_join(threads[i], NULL);
87     }
88    /*发送信号给监听线程*/
89     pthread_cond_signal(&count_threshold_cv); 
90     pthread_join(threads[0],NULL);
91     printf("Main(): Waited on %d threads. Final value of count = %d. Done.\n",
92             NUM_THREADS, count);
93 
94     /*清除并退出 */
95     pthread_attr_destroy(&attr);
96     pthread_mutex_destroy(&count_mutex);
97     pthread_cond_destroy(&count_threshold_cv);
98     pthread_exit(NULL);
99 }