linux下父子线程交替执行，有一部分不输出

Question

#include<stdlib.h>
#include<pthread.h>
#include<stdio.h>
#include<string.h>
#include<unistd.h>
const int count=15;
int flag;
void* even(void * a){
    int i;
    for (i = 1; i < count; i+=2)
    {
        if (flag==0)
        {
            flag=1;
            printf("child thread is %d\n",i);
        }
        sleep(1);
    }
    return 0;
}

int main(int argc, char const *argv[])
{
    int err;
    int * rev;
    pthread_t tid;
    err=pthread_create(&tid,NULL,even,NULL);
    if(err!=0){
        printf("create pthread failure\n");
        return 0;
    }
    for (int b = 0; b < count; b+=2)
    {
        if (flag==1)
        {
        flag=0;
        printf("main thread is %d\n",b);
        }
        sleep(1);
    }
    pthread_exit(rev);
}

上面的程序让父子线程分别交替打印偶数和奇数，结果会交替，但是有一部分数没有输出。

请大家帮忙看看上面的问题在哪里

Answer 1

//多线程当然要加锁
//for那里i+=2或b+=2部分逻辑自己好好理解下
//变量记得赋初始值

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

pthread_mutex_t mutex ;  
const int count=15;
int flag = 1;
void* even(void * a)
{
    int i;
    for (i = 1; i < count; )
    {
        if (flag==0)
        {
            pthread_mutex_lock(&mutex);
            flag=1;
            printf("child thread is %d\n",i);
            i+=2;
            pthread_mutex_unlock(&mutex);
        }
        sleep(1);
    }
    return 0;
}

int main(int argc, char const *argv[])
{
    pthread_t tid; 
    pthread_create(&tid,NULL,even,NULL);

    pthread_mutex_init(&mutex,NULL); 


    for (int b = 0; b < count; )
    {
        if (flag==1)
        {
            pthread_mutex_lock(&mutex);
            flag=0;
            printf("main thread is %d\n",b);
            b+=2;
            pthread_mutex_unlock(&mutex);
        }
        sleep(1);
    }
    pthread_join(tid,NULL);

    pthread_mutex_destroy(&mutex);  
}