跟我一起做面试题-linux线程编程(3)

如题所述：

编写一个程序，开启3个线程，这3个线程的ID分别为A、B、C，每个线程将自己的ID在屏幕上打印10遍，要求输出结果必须按ABC的顺序显示；如：ABCABC….依次递推。

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用pthread_cond_signal，理论上会引起本该得到信号的线程一直处于饥饿状态，但我试过了，每次运行都ok。

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <pthread.h>
#include <errno.h>

pthread_cond_t cond_a;

pthread_mutex_t mylock;
int g_flag;

#define NUM            3

void * func(void *arg)
{
    int i;
    int name = (int)arg;
    printf("my name [%d]\n", name);
    for(i = 0; i<10;i++)
    {
        if( pthread_mutex_lock(&mylock))
        {
            printf("pthread_mutex_lock error errmsg[%s]\n", strerror(errno));
            exit(0);
        }

        while(1)
        {
            if(g_flag == name)
            {
                g_flag = (g_flag+1)%NUM;
                printf("%c ", ‘A‘ + name);
                break;
            }
            else
            {
                if(pthread_cond_wait(&cond_a, &mylock))
                {
                    printf("wait error errmsg[%s]\n", strerror(errno));
                    exit(0);
                }
            }
        }
        if(pthread_mutex_unlock(&mylock))
        {
            printf("pthread_mutex_unlock error errmsg[%s]\n", strerror(errno));
            exit(0);
        }
        //if(pthread_cond_broadcast(&cond_a))
        if(pthread_cond_signal(&cond_a))
        {
            printf("pthread_cond_broadcast error errmsg[%s]\n", strerror(errno));
            exit(0);
        }
    }
    printf("\n");
    pthread_exit(NULL);
}

int appInit()
{
    g_flag = 0;
    pthread_mutex_init(&mylock, NULL);
    pthread_cond_init(&cond_a, NULL);
    return 0;
}

int appDone()
{
    pthread_cond_destroy(&cond_a);
    pthread_mutex_destroy(&mylock);
    return 0;
}

int main(int argc, char *argv[])
{

    pthread_t th[NUM];
    int ret;
    int i;
    ret = appInit();
    if(ret)
    {
        printf("appinit failure\n");
        return -1;
    }
    for( i = 0; i < NUM; i++)
    {
        ret = pthread_create(&th[i], NULL, func, (void*)i);
        if(ret)
        {
            printf("pthread_create ta error [%s]\n", strerror(errno));
            return 1;
        }
    }

    for( i = 0; i < NUM; i++)
        pthread_join(th[i], NULL);

    printf("main thread exit\n");
    ret = appDone();
    if(ret)
    {
        printf("appDone failure\n");
        return -1;
    }
    return 0;
}