学步园

互斥量进行同步

另一种用在多线程程序中的访问方法是使用互斥量。它允许 程序员锁住某个对象，使得每次只能有一个线程访问他。

为了控制对关键代码的访问，必须在进入代码之前锁住一个互斥量，然后在完成操作之后解锁它。

下面用实例来说明，本实例是经典的多线程程序，线程1接受输入字符，线程2统计字符数。它们共享一个

公共的工作区

实例：

#include <stdio.h><br /> #include <unistd.h><br /> #include <stdlib.h><br /> #include <string.h><br /> #include <pthread.h><br /> #include <semaphore.h><br /> pthread_mutex_t work_mutex;<br /> void *thread_fun(void *arg);<br /> #define WORK_SIZE 1024<br /> char work_area[WORK_SIZE];<br /> int time_to_exit = 0;<br /> int main()<br /> {<br /> int res;<br /> pthread_t a_thread;<br /> void *thread_result=NULL;<br /> int print_count1 = 0;</p> <p> //初始化互斥量<br /> if(pthread_mutex_init(&work_mutex,0)!=0)<br /> {<br /> perror("mutex initialization failed");<br /> exit (1);<br /> }</p> <p> //创建新的线程<br /> if(pthread_create(&a_thread,NULL,thread_fun,NULL)!=0)<br /> {<br /> perror("thread create failed");<br /> exit (1);<br /> }</p> <p> //给工作区枷锁，<br /> res=pthread_mutex_lock(&work_mutex);<br /> if(res!=0)<br /> {<br /> perror("mutex lock failed");<br /> exit (1);<br /> }<br /> printf("input some text.enter 'end' to finish/n");</p> <p> //读文本<br /> while(!time_to_exit)<br /> {<br /> fgets(work_area,WORK_SIZE,stdin);//从标准输入读到工作区</p> <p> //解锁，允许新线程访问他并统计字符数目<br /> res=pthread_mutex_unlock(&work_mutex);<br /> if(res!=0)<br /> {<br /> perror("mutex lock failed");<br /> exit (1);<br /> }</p> <p> while(1)<br /> {<br /> //枷锁<br /> res=pthread_mutex_lock(&work_mutex);<br /> if(res!=0)<br /> {<br /> perror("mutex lock failed");<br /> exit (1);<br /> }<br /> if(work_area[0]!='/0')<br /> {<br /> //解锁<br /> res=pthread_mutex_unlock(&work_mutex);<br /> if(res!=0)<br /> {<br /> perror("mutex unlock failed");<br /> exit (1);<br /> }<br /> sleep(1);<br /> }<br /> else<br /> break;<br /> }<br /> }<br /> //解锁<br /> res=pthread_mutex_unlock(&work_mutex);<br /> if(res!=0)<br /> {<br /> perror("mutex unlock failed");<br /> exit (1);<br /> }</p> <p> puts("/nwaiting for thread to finished.../n");<br /> if(pthread_join(a_thread,&thread_result))<br /> {<br /> perror("thread join failed");<br /> exit (1);<br /> }<br /> printf("joined,it returned %s/n",(char *)thread_result);<br /> //销毁互斥量所占用的资源<br /> pthread_mutex_destroy(&work_mutex);</p> <p> return 0;<br /> }<br /> void *thread_fun(void *arg)<br /> {<br /> int res;<br /> sleep(1);</p> <p> res=pthread_mutex_lock(&work_mutex);<br /> if(res!=0)<br /> {<br /> perror("mutex lock failed");<br /> exit (1);<br /> }<br /> while(strncmp("end",work_area,3)!=0)<br /> {<br /> //统计字符个数<br /> printf("you input %d characters/n",strlen(work_area)-1);</p> <p> work_area[0]='/0';<br /> res=pthread_mutex_unlock(&work_mutex);<br /> if(res!=0)<br /> {<br /> perror("mutex unlock failed");<br /> exit (1);<br /> }</p> <p> sleep(1);//等待主线程继续运行</p> <p> res=pthread_mutex_lock(&work_mutex);<br /> if(res!=0)<br /> {<br /> perror("mutex lock failed");<br /> exit (1);<br /> }</p> <p> while(work_area[0]=='/0')<br /> {<br /> /*周期行给互斥量枷锁，加锁成功，检查主线程是否有字符送来处理，没有解锁互斥量<br /> 继续等待；如果有，统计字符个数，再次进入循环<br /> */<br /> res=pthread_mutex_unlock(&work_mutex);<br /> if(res!=0)<br /> {<br /> perror("mutex unlock failed");<br /> exit (1);<br /> }</p> <p> sleep(1);//等待主线程继续运行</p> <p> res=pthread_mutex_lock(&work_mutex);<br /> if(res!=0)<br /> {<br /> perror("mutex lock failed");<br /> exit (1);<br /> }<br /> }</p> <p> }<br /> time_to_exit = 1;<br /> work_area[0] = '/0';</p> <p> res=pthread_mutex_unlock(&work_mutex);<br /> if(res!=0)<br /> {<br /> perror("mutex unlock failed");<br /> exit (1);<br /> }</p> <p> pthread_exit("OK!");<br /> }<br /> 运行结果：<br /> [sea@localhost ~]$ ./threadtest1<br /> input some text.enter 'end' to finish<br /> zhanghaiyang<br /> you input 12 characters<br /> zhang<br /> you input 5 characters<br />

这种通过轮询来获得结果的方法通常并不是好的编程方式，在实际的编程中，应该尽可能用信号量来避免出现这种情况