thread互斥测试

编译运行附件中的代码，并说明程序的功能
根据自己的理解，提交不少于3张图片

代码

#include  <stdio.h>
#include  <stdlib.h>
#include  <pthread.h>
#include  <ctype.h>
 
struct arg_set {
        char *fname;
        int  count;
};
 
struct arg_set  *mailbox = NULL;
pthread_mutex_t lock = PTHREAD_MUTEX_INITIALIZER;//互斥锁
pthread_cond_t  flag = PTHREAD_COND_INITIALIZER;//条件变量
 
void *count_words(void *);
int main(int argc, char *argv[])
{
    pthread_t t1, t2;
    struct arg_set args1, args2;   
    int reports_in = 0;
    int total_words = 0;
 
    if ( argc != 3 ){
        printf("usage: %s file1 file2\n", argv[0]);
        exit(1);
    }
 
    args1.fname = argv[1];
    args1.count = 0;
    pthread_create(&t1, NULL, count_words, (void *) &args1);
 
    args2.fname = argv[2];
    args2.count = 0;
    pthread_create(&t2, NULL, count_words, (void *) &args2);
 
    pthread_mutex_lock(&lock);//拿到互斥锁，进入临界区
    while( reports_in < 2 ){
        printf("MAIN: waiting for flag to go up\n");
        pthread_cond_wait(&flag, &lock); //令线程等待在条件变量上
        printf("MAIN: Wow! flag was raised, I have the lock\n");
        printf("%7d: %s\n", mailbox->count, mailbox->fname);
        total_words += mailbox->count;
        if ( mailbox == &args1)
            pthread_join(t1,NULL);//等待线程t1执行结束
        if ( mailbox == &args2)
            pthread_join(t2,NULL);//等待线程t2执行结束
        mailbox = NULL;
        pthread_cond_signal(&flag); //通知等待在条件变量上的线程
        reports_in++;
    }
    pthread_mutex_unlock(&lock);//释放互斥锁
     
    printf("%7d: total words\n", total_words);
}
void *count_words(void *a)
{
    struct arg_set *args = a;
    FILE *fp;
    int  c, prevc = '\0';
     
    if ( (fp = fopen(args->fname, "r")) != NULL ){
        while( ( c = getc(fp)) != EOF ){
            if ( !isalnum(c) && isalnum(prevc) )
                args->count++;
            prevc = c;
        }
        fclose(fp);
    } else
        perror(args->fname);
    printf("COUNT: waiting to get lock\n");
    pthread_mutex_lock(&lock);//拿到互斥锁，进入临界区
    printf("COUNT: have lock, storing data\n");
    if ( mailbox != NULL ){
        printf("COUNT: oops..mailbox not empty. wait for signal\n");
        pthread_cond_wait(&flag,&lock);
    }
    mailbox = args;        
    printf("COUNT: raising flag\n");
    pthread_cond_signal(&flag); //通知等在条件变量上的线程
    printf("COUNT: unlocking box\n");
    pthread_mutex_unlock(&lock);   
    return NULL;
}

 此为thread互斥的测试，设计到锁与等待的问题，功能是互斥进行查看两个文件中字符串的数量，一个空格分开算两个，第一个1.txt文件先获得锁，第二个2.txt文件则需要等待，当第一个完成之后再进行第二个文件的统计操作，最后输出总结果。

互斥锁，是一种信号量，常用来防止两个进程或线程在同一时刻访问相同的共享资源。