C/C++ 之多线程机制（六）

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本文详细介绍了C/C++中的多线程操作，包括线程的创建与结束、互斥与同步机制、信号量控制等核心概念。通过实例演示了如何使用pthread库进行线程管理，展示了线程互斥锁和信号量控制线程执行顺序的应用。

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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

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
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
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
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.
pthread_setcancelstate()：设置本线程对cancle线程的反应
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
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、信号量线程控制机制

C/C++在使用信号量机制的时候，默认的信号量为匿名信号量。

1. sem_init(sem)：初始化一个定位在sem的匿名信号量

2. sem_wait()：把信号量减1操作，如果信号量的当前值为0则进入阻塞，为原子操作

sem_trywait()：如果信号量的当前值为0则返回错误而不是阻塞调用(errno=EAGAIN),其实是sem_wait()的非阻塞版本

4. sem_post()：给信号量的值加1，它是一个“原子操作”，即同时对同一个信号量做加1,操作的两个线程是不会冲突的

sem_getvalue(sval)：把sem指向的信号量当前值放置在sval指向的整数上
sem_destory(sem)：销毁由sem指向的匿名信号量

五、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\n", thrd_num);
    for (count = 0; count < REPEAT_NUMBER; count++) {
        delay_time = (int)(rand() * DELAY_TIME_LEVELS/(RAND_MAX)) + 1;
        sleep(delay_time);
        printf("\tThread %d: job %d delay = %d\n", thrd_num, count, delay_time);
    }
 
    printf("Thread %d finished\n", 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\n", no);
               exit(res);
          }
     }
 
     printf("Create treads success\n Waiting for threads to finish...\n");
     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\n", no);
          } else {
            printf("Thread %d join failed\n", 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>
 
#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\n", thrd_num);
          pthread_exit(NULL);
     }
     printf("Thread %d is starting\n", thrd_num);
     for (count = 0; count < REPEAT_NUMBER; count++) {          
         delay_time = (int)(rand() * DELAY_TIME_LEVELS/(RAND_MAX)) + 1;
         sleep(delay_time);
         printf("\tThread %d: job %d delay = %d\n", 
                                      thrd_num, count, delay_time);
     }
     printf("Thread %d finished\n", thrd_num);
     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\n", no);
              exit(res);
          }
     }     
     printf("Create treads success\n Waiting for threads to finish...\n");
     for (no = 0; no < THREAD_NUMBER; no++) {
          res = pthread_join(thread[no], &thrd_ret);
          if (!res) {
                printf("Thread %d joined\n", no);
          } else  {
              printf("Thread %d join failed\n", no);
          }
     }   
     /****互斥锁解锁***/
     pthread_mutex_unlock(&mutex);
     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，于是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：

$gcc thread_mutex.c -lpthread
$ ./a.out 
Create treads success
 Waiting for threads to finish...
Thread 0 is starting
        Thread 0: job 0 delay = 9
        Thread 0: job 1 delay = 4
        Thread 0: job 2 delay = 7
Thread 0 finished
Thread 0 joined
Thread 1 is starting
        Thread 1: job 0 delay = 6
        Thread 1: job 1 delay = 4
        Thread 1: job 2 delay = 7
Thread 1 finished
Thread 1 joined
Thread 2 is starting
        Thread 2: job 0 delay = 3
        Thread 2: job 1 delay = 1
        Thread 2: job 2 delay = 6
Thread 2 finished
Thread 2 joined

3. 使用信号量控制线程的执行顺序sem_post

修改上面例程，上面的是使用pthread_mutex_lock互斥锁控制线程执行顺序，
使用另外一种线程执行顺序的控制：

/* thread_sem.c */
#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
#include <semaphore.h>
 
#define THREAD_NUMBER       3
#define REPEAT_NUMBER       3
#define DELAY_TIME_LEVELS   10.0
 
sem_t sem[THREAD_NUMBER];
 
void * thrd_func(void *arg) {
    int thrd_num = (int)arg;
    int delay_time = 0;
    int count = 0;
    sem_wait(&sem[thrd_num]);
    printf("Thread %d is starting\n", thrd_num);
    for (count = 0; count < REPEAT_NUMBER; count++) {
        delay_time = (int)(rand() * DELAY_TIME_LEVELS/(RAND_MAX)) + 1;
        sleep(delay_time);
        printf("\tThread %d: job %d delay = %d\n", thrd_num, count, delay_time);
    }
    printf("Thread %d finished\n", 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++) {
        sem_init(&sem[no], 0, 0);
        res = pthread_create(&thread[no], NULL, thrd_func, (void*)no);
        if (res != 0) {
            printf("Create thread %d failed\n", no);
            exit(res);
        }
    }
 
    printf("Create treads success\n Waiting for threads to finish...\n");
    sem_post(&sem[THREAD_NUMBER - 1]);
    for (no = THREAD_NUMBER - 1; no >= 0; no--) {
        res = pthread_join(thread[no], &thrd_ret);
        if (!res) {
            printf("Thread %d joined\n", no);
        } else {
            printf("Thread %d join failed\n", no);
        }
        sem_post(&sem[(no + THREAD_NUMBER - 1) % THREAD_NUMBER]);           
    }
 
    for (no = 0; no < THREAD_NUMBER; no++) {
        sem_destroy(&sem[no]);      
    }
    return 0;        
}

执行结果，仍然是建立3条线程，每条线程执行时休眠随机时长：

$ gcc thread_sem.c -lpthread 
$ ./a.out 
Create treads success
 Waiting for threads to finish...
Thread 2 is starting
        Thread 2: job 0 delay = 9
        Thread 2: job 1 delay = 9
        Thread 2: job 2 delay = 5
Thread 2 finished
Thread 2 joined
Thread 1 is starting
        Thread 1: job 0 delay = 5
        Thread 1: job 1 delay = 7
        Thread 1: job 2 delay = 4
Thread 1 finished
Thread 1 joined
Thread 0 is starting
        Thread 0: job 0 delay = 3
        Thread 0: job 1 delay = 9
        Thread 0: job 2 delay = 8
Thread 0 finished
Thread 0 joined

执行结果与第2个例程非常相似，只不过教材中进行倒序执行而已；
那么这种方式其实与使用互斥锁相比，代码量可读性基本持平不相上下；
原文链接https://www.cnblogs.com/renhui/p/9900509.html
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