while 循环从 2 个不同的 pthread 读取相同的变量，但代码未运行

Question

I am trying to detect core to core latency in simple memory sharing. My objective is to read a global variable from two different threads. Let's say the variable is x=0 at the beginning. Now one thread will read the value and change the x to 1. Another thread is reading the same variable and as soon as it reads x=1, it makes it 0. I have written the following code:

#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
#include <sys/time.h>

double getsecs(void)
{
    struct timeval tv;
    gettimeofday(&tv, NULL);
    return tv.tv_sec + tv.tv_usec / 1.0e6;
}

int x=0;
//count=0;

void* changetoone(void *arg)
{
    //sched_setaffinity(0);
    for (int i=0; i<10000; i++){
        while(x!=1)
        { 
            x=1;
            printf("%d", x);
        }
    }
    return 0;
}

void* changetozero(void *arg){
    //sched_setaffinity(5);
    for (int i=0; i<10000; i++){
        while(x!=0)
        { 
            x=0;
            printf("%d", x);
        }
    } 
    return 0;           
} 

int main()
{
    pthread_t thread1;

    pthread_create(&thread1, NULL, changetoone, &x);

    pthread_t thread2;
    pthread_create(&thread2, NULL, changetozero, &x);    

    pthread_join(&thread1, NULL);
    pthread_join(&thread2, NULL);
}

For some reason, the code is not running. I am not familiar with using pthread and I think I made some silly mistakes. Can anybody point out the mistake for me, please?

Answer 1

The first argument to pthread_join is pthread_t , not pthread_t* . So you shouldn't use & when calling it.

    pthread_join(thread1, NULL);
    pthread_join(thread2, NULL);

The actual behavior of the program is undefined because of lack of synchronization between the threads when accessing x . But this will at least allow the threads to run.