如何使用rand_r（）在C中创建线程安全随机数生成器？

Question

I was asked to not use rand() because they were not "thread safe" and to also use a different seed value each time. I found examples on GitHub using a seed value like this:

unsigned int seed = time(NULL);

That only has a precision by seconds. Since the program runs in under 1 second, I end up getting the same random number every instance.

How would I fix this algorithm so that it only uses rand_r() or any other "thread safe" methods to generate 10 random numbers?

int main()
{
    for(int i = 0; i < 10; i++){
        int random;
        unsigned int seed = time(NULL);
            random = 1 + (rand_r(&seed)% 10);
        printf("%d\n",random);
    }
 return 0;
}

Answer 1

The rand_r function takes a pointer to a state variable. This is set to a seed value before rand_r is called for the first time. Then each time you call rand_r , you pass in address of this value.

To be thread safe, each thread needs to have its own state variable. You don't however want to use the same initial value for each thread's state variable, otherwise every thread will generate the same sequence of pseudo-random values.

You need to seed the state variable with data that differs for each thread, such as the thread id, plus other information such as the time and/or pid.

For example:

// 2 threads, 1 state variable each
unsigned int state[2];

void *mythread(void *p_mystate)
{
    unsigned int *mystate = p_mystate;
    // XOR multiple values together to get a semi-unique seed
    *mystate = time(NULL) ^ getpid() ^ pthread_self();

    ...
    int rand1 = rand_r(mystate);
    ...
    int rand2 = rand_r(mystate);
    ...
    return NULL;
}

int main()
{
    pthread_t t1, t2;

    // give each thread the address of its state variable
    pthread_create(&t1, NULL, mythread, &state[0]);
    pthread_create(&t2, NULL, mythread, &state[1]);
    ...
    pthread_join(t1, NULL);
    pthread_join(t2, NULL);
    return 0;
}