Linux多线程之互斥

共要卖票20张，由命令行输入窗口数，由线程模拟窗口。每卖掉一张票，屏幕显示由几号窗口所卖，一并显示剩余票数

由于票数 ticket_cnt 是全局变量，因此每当一个线程将其减一（卖出一张票），并将其显示，应该被封装为一个原子操作。因为线程是并发执行的，可能当前线程将ticket_cnt减1后还没有来得及显示此时的剩余票数ticket_cnt，ticket_cnt已经被另一个线程减一了。此处通过互斥锁实现互斥。

创建线程：

NAME       pthread_create - create a new threadSYNOPSIS       #include <pthread.h>       int pthread_create(pthread_t *thread, const pthread_attr_t *attr,                          void *(*start_routine) (void *), void *arg);       Compile and link with -pthread.

回收线程资源，该函数为阻塞函数。

NAME       pthread_join - join with a terminated threadSYNOPSIS       #include <pthread.h>       int pthread_join(pthread_t thread, void **retval);       Compile and link with -pthread.DESCRipTION       The  pthread_join() function waits for the thread specified       by thread to terminate.  If that thread has already  termi-       nated, then pthread_join() returns immediately.  The thread       specified by thread must be joinable.       If retval is not NULL, then pthread_join() copies the  exit       status  of the target thread (i.e., the value that the tar-       get thread supplied to pthread_exit(3)) into  the  location       pointed  to by *retval.  If the target thread was canceled,       then PTHREAD_CANCELED is placed in *retval.       If multiple threads simultaneously try  to  join  with  the       same  thread,  the  results  are  undefined.  If the thread       calling pthread_join() is canceled, then the target  thread       will remain joinable (i.e., it will not be detached).RETURN VALUE       On  success, pthread_join() returns 0; on error, it returns       an error number.

解锁开锁

NAME       pthread_mutex_lock       pthread_mutex_unlock - lock and unlock a mutexSYNOPSIS       #include <pthread.h>       int pthread_mutex_lock(pthread_mutex_t *mutex);       int pthread_mutex_unlock(pthread_mutex_t *mutex);

初始化锁，销毁锁

NAME       pthread_mutex_destroy,  pthread_mutex_init  -  destroy  and initialize a mutexSYNOPSIS       #include <pthread.h>       int pthread_mutex_destroy(pthread_mutex_t *mutex);       int pthread_mutex_init(pthread_mutex_t *restrict mutex, const pthread_mutexattr_t *restrict attr);

/*************************************************************************  > File Name: ticket.c  > Author: KrisChou  > Mail:zhoujx0219@163.com   > Created Time: Mon 25 Aug 2014 07:40:38 PM CST ************************************************************************/#include <stdio.h>#include <stdlib.h>#include <string.h>#include <pthread.h>int ticket_cnt = 20;     /* 共有20张票 */typedef struct tag{    int s_id;    pthread_mutex_t *s_p;}DATA,*pDATA;void* handler(void *arg ){    int id = ((pDATA)arg)->s_id;    pthread_mutex_t *p_mutex = ((pDATA)arg)-> s_p;    PRintf("a window on !: %d /n", id);    while(1)    {        pthread_mutex_lock(p_mutex);        if(ticket_cnt == 0)        {            printf("ticket out! /n");            pthread_mutex_unlock(p_mutex);            free((pDATA)arg);            return (void*)0;        }        --ticket_cnt;        sleep(rand()%3 + 1);        printf("window: %d : a ticket sold. left : %d /n", id,ticket_cnt );        pthread_mutex_unlock(p_mutex);        sleep(rand() % 3 + 1); /* 如果不sleep，锁会一直被这个执行完的线程所占据 */    }}int main(int argc, char *argv[]){    srand(getpid());    pthread_mutex_t mutex;    pthread_mutex_init(&mutex, NULL);    int thd_cnt = atoi(argv[1]); /* 从命令行输入卖票窗口数 */    pthread_t *tds = (pthread_t*)calloc(thd_cnt,sizeof(pthread_t));    int index;    for(index = 0; index < thd_cnt; index++ )    {        pDATA p = (pDATA)calloc(1,sizeof(DATA));        p->s_id = index;        p->s_p = &mutex;        pthread_create(tds + index , NULL,handler,(void*)p);    }    printf("joining.../n");    for(index = 0; index < thd_cnt; index++)    {        pthread_join(tds[index],NULL);    }    pthread_mutex_destroy(&mutex);    return 0;}

编译运行：

[purple@localhost review]$ gcc ticket.c -lpthread[purple@localhost review]$ ./a.out 5joining...a window on !: 2a window on !: 3a window on !: 4a window on !: 1a window on !: 0window: 2 : a ticket sold. left : 19window: 3 : a ticket sold. left : 18window: 4 : a ticket sold. left : 17window: 1 : a ticket sold. left : 16window: 0 : a ticket sold. left : 15window: 2 : a ticket sold. left : 14window: 3 : a ticket sold. left : 13window: 4 : a ticket sold. left : 12window: 1 : a ticket sold. left : 11window: 0 : a ticket sold. left : 10window: 2 : a ticket sold. left : 9window: 3 : a ticket sold. left : 8window: 4 : a ticket sold. left : 7window: 1 : a ticket sold. left : 6window: 0 : a ticket sold. left : 5window: 2 : a ticket sold. left : 4window: 3 : a ticket sold. left : 3window: 4 : a ticket sold. left : 2window: 1 : a ticket sold. left : 1window: 0 : a ticket sold. left : 0ticket out!ticket out!ticket out!ticket out!ticket out!

如果有一个整型参数 a 需要在创建线程时将其传递给线程，那么以下两种传值方式，实际上是由区别的。来看代码：

方式1：

void* handler(void *arg){    int val=*(int*)arg;    printf("from main: %d/n",val);    pthread_exit((void*)"hello world");}int main(){    pthread_t thd;    int a=12345;    pthread_create(&thd,NULL,handler,(void*)&a);    printf("thd: %x/n",thd);    pthread_join(thd,(void*)&ret);    return 0;}

方式2：

void* handler(void *arg){    int val=(int)arg;    printf("from main: %d/n",val);    pthread_exit((void*)"hello world");}int main(){    pthread_t thd;    int a=12345;    pthread_create(&thd,NULL,handler,(void*)a);    printf("thd: %x/n",thd);    pthread_join(thd,(void*)&ret);    return 0;}

对于方式1，传给handler的参数是a的地址，如果a的值之后在主线程里会发生变化，那么传给handler的数值a可能就不是原来我们想传的那个了。

而对于方式2，由于是值传递，我们当时想传什么，传到handler中的就一定是什么。

由此可见，虽然方式1节省了存储空间，但是同样也容易发生错误。