C/C++ 多线程机制

一、C/C++多线程操作说明

C/C++多线程基本操作如下： 
1. 线程的建立结束 
2. 线程的互斥和同步 
3. 使用信号量控制线程 
4. 线程的基本属性配置 

在C/C++代码编写时，使用多线程机制，首先需要做的事情就是声明引用，具体如下：

#include "pthread.h"

二、线程基本操作方法

基本线程操作：

1. pthread_create()：创建线程开始运行相关线程函数，运行结束则线程退出

2. pthread_eixt()：因为exit()是用来结束进程的，所以则需要使用特定结束线程的函数

3. pthread_join()：挂起当前线程，用于阻塞式地等待线程结束，如果线程已结束则立即返回，0=成功

4. pthread_cancel()：发送终止信号给thread线程，成功返回0，但是成功并不意味着thread会终止

5. pthread_testcancel()：在不包含取消点，但是又需要取消点的地方创建一个取消点，以便在一个没有包含取消点的执行代码线程中响应取消请求.

6. pthread_setcancelstate()：设置本线程对cancle线程的反应

7. pthread_setcanceltype()：设置取消状态 继续运行至下一个取消点再退出或者是立即执行取消动作

8. pthread_setcancel()：设置取消状态

三、线程互斥与同步机制 

基本的互斥与同步的操作方法：

1. pthread_mutex_init()：互斥锁的初始化

2. pthread_mutex_lock()：锁定互斥锁，如果尝试锁定已经被上锁的互斥锁则阻塞至可用为止

3. pthread_mutex_trylock()：非阻塞的锁定互斥锁

4. pthread_mutex_unlock()：释放互斥锁

5. pthread_mutex_destory()：互斥锁销毁函数

四、多线程实践

1. 基本的线程及建立运行

下面的代码是C/C++开发的基本的线程的运行，使用的就是最基本的pthread.h：

/* thread.c */
#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
 
#define THREAD_NUMBER       3                 /*线程数*/
#define REPEAT_NUMBER       5                 /*每个线程中的小任务数*/
#define DELAY_TIME_LEVELS  10.0             /*小任务之间的最大时间间隔*/
//
void *thrd_func(void *arg) { 
    /* 线程函数例程 */
    int thrd_num = (int)arg;
    int delay_time = 0;
    int count = 0;
    printf("Thread %d is starting
", thrd_num);
    for (count = 0; count < REPEAT_NUMBER; count++) {
        delay_time = (int)(rand() * DELAY_TIME_LEVELS/(RAND_MAX)) + 1;
        sleep(delay_time);
        printf("	Thread %d: job %d delay = %d
", thrd_num, count, delay_time);
    }
 
    printf("Thread %d finished
", thrd_num);
    pthread_exit(NULL);
}
 
int main(void) {
     pthread_t thread[THREAD_NUMBER];
     int no = 0, res;
     void * thrd_ret;
     srand(time(NULL));    
     for (no = 0; no < THREAD_NUMBER; no++) {
          /* 创建多线程 */
          res = pthread_create(&thread[no], NULL, thrd_func, (void*)no);
          if (res != 0) {
               printf("Create thread %d failed
", no);
               exit(res);
          }
     }
 
     printf("Create treads success
 Waiting for threads to finish...
");
     for (no = 0; no < THREAD_NUMBER; no++) {
          /* 等待线程结束 */
          res = pthread_join(thread[no], &thrd_ret);
          if (!res) {
            printf("Thread %d joined
", no);
          } else {
            printf("Thread %d join failed
", no);
          }
     }
     return 0;        
}

例程中循环3次建立3条线程，并且使用pthread_join函数依次等待线程结束； 
线程中使用rand()获取随机值随机休眠5次，随意会出现后执行的线程先执行完成； 
运行结果：

$ gcc thread.c -lpthread
$ ./a.out 
Create treads success
 Waiting for threads to finish...
Thread 0 is starting
Thread 1 is starting
Thread 2 is starting
Thread 1: job 0 delay = 2
Thread 1: job 1 delay = 2
Thread 0: job 0 delay = 8
Thread 2: job 0 delay = 10
Thread 2: job 1 delay = 3
Thread 1: job 2 delay = 10
Thread 0: job 1 delay = 8
Thread 0: job 2 delay = 3
Thread 0: job 3 delay = 1
Thread 2: job 2 delay = 8
Thread 1: job 3 delay = 8
Thread 1: job 4 delay = 1
Thread 1 finished
        Thread 2: job 3 delay = 6
        Thread 0: job 4 delay = 7
Thread 0 finished
Thread 0 joined
Thread 1 joined
        Thread 2: job 4 delay = 10
Thread 2 finished
Thread 2 joined

可以看到，线程1先于线程0执行，但是pthread_join的调用时间顺序，先等待线程0执行； 
由于线程1已经早结束，所以线程0被pthread_join等到的时候，线程1已结束，就在等待到线程1时，直接返回； 

2. 线程执行的互斥和同步pthread_mutex_lock

下面我们在上面的程序中增加互斥锁：

/*thread_mutex.c*/
#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
#include <unistd.h>
 
#define THREAD_NUMBER        3            /* 线程数 */
#define REPEAT_NUMBER        3            /* 每个线程的小任务数 */
#define DELAY_TIME_LEVELS 10.0         /*小任务之间的最大时间间隔*/
pthread_mutex_t mutex;
 
void *thrd_func(void *arg) {
     int thrd_num = (int)arg;
     int delay_time = 0, count = 0;
     int res;
     /* 互斥锁上锁 */
     res = pthread_mutex_lock(&mutex);
     if (res) {
          printf("Thread %d lock failed
", thrd_num);
          pthread_exit(NULL);
     }
     printf("Thread %d is starting
", thrd_num);
     for (count = 0; count < REPEAT_NUMBER; count++) {          
         delay_time = (int)(rand() * DELAY_TIME_LEVELS/(RAND_MAX)) + 1;
         sleep(delay_time);
         printf("	Thread %d: job %d delay = %d
", 
                                      thrd_num, count, delay_time);
     }
     printf("Thread %d finished
", thrd_num);
     /****互斥锁解锁***/
     pthread_mutex_unlock(&mutex);
     pthread_exit(NULL);
}
 
int main(void) {
     pthread_t thread[THREAD_NUMBER];
     int no = 0, res;
     void * thrd_ret;
 
     srand(time(NULL));
     /* 互斥锁初始化 */
     pthread_mutex_init(&mutex, NULL);
     for (no = 0; no < THREAD_NUMBER; no++) {
          res = pthread_create(&thread[no], NULL, thrd_func, (void*)no);
          if (res != 0) {
              printf("Create thread %d failed
", no);
              exit(res);
          }
     }     
     printf("Create treads success
 Waiting for threads to finish...
");
     for (no = 0; no < THREAD_NUMBER; no++) {
          res = pthread_join(thread[no], &thrd_ret);
          if (!res) {
                printf("Thread %d joined
", no);
          } else  {
              printf("Thread %d join failed
", no);
          }
     }   
     pthread_mutex_destroy(&mutex);          
     return 0;        
}

在上面的例程中直接添加同步锁pthread_mutex_t； 
在线程中加入，程序在执行线程程序时，调用pthread_mutex_lock上锁，发现上锁时候后进入等待，等待锁再次释放后重新上锁； 
所以线程程序加载到队列中等待，等待成功上锁后继续执行程序代码； 
运行结果如下：

Create treads success
 Waiting for threads to finish...
Thread 0 is starting
    Thread 0: job 0 delay = 7
    Thread 0: job 1 delay = 2
    Thread 0: job 2 delay = 9
Thread 0 finished
Thread 2 is starting
Thread 0 joined
    Thread 2: job 0 delay = 6
    Thread 2: job 1 delay = 7
    Thread 2: job 2 delay = 10
Thread 2 finished
Thread 1 is starting
    Thread 1: job 0 delay = 3
    Thread 1: job 1 delay = 5
    Thread 1: job 2 delay = 2
Thread 1 finished
Thread 1 joined
Thread 2 joined