In c, how to make a thread wait for other threads to finish

I have a program where I created two threads. In one thread I assigned a value to integers a and b. In the second thread, I want to access a and b, to change their values.

#include <stdio.h>
#include <pthread.h>

struct data {
    int a;
    int b;
};

struct data temp;

void *assign(void *temp)
{
    struct data *new;

    new = (struct data *) temp;
    new->a = 2;
    new->b = 2;
    printf("You are now in thread1..\n The value of a and b is: %d, %d", new->a + 1, new->b + 1);
    printf("\n");
    pthread_exit(NULL);
}

void *add(void *temp1)
{
    struct data *new1;
    new1 = (struct data *) temp1;
    printf("You are now in thread 2\nValue of a and b is: %d, %d\n", new1->a - 1, new1->b - 1);
    pthread_exit(NULL);
}

int main()
{
    pthread_t threads[2];
    pthread_attr_t attr;
    void *status;
    int rc, t;
    pthread_attr_init(&attr);
    pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_JOINABLE);
    pthread_create(&threads[0], NULL, assign, (void *) &temp);
    pthread_create(&threads[1], NULL, add, (void *) &temp);
    pthread_attr_destroy(&attr);
    for (t = 0; t < 2; t++) {
        rc = pthread_join(threads[t], &status);
        if (rc) {
            printf("ERROR; return code from pthread_join() is %d\n", rc);
            exit(-1);
        }
        printf("Main: completed join with thread %ld having a status of %ld\n", t, (long) status);
    }
    pthread_exit(NULL);
    return 0;
}

But the above program executes the two threads simultaneously. Sometimes I get

thread1..
The value of a and b is: 3, 3
thread 2
Value of a and b is: 1, 1

and sometimes I get

thread 2
Value of a and b is: -1, -1
You are now in thread1..
The value of a and b is: 3, 3

I want to make thread-2(add) to wait for thread-1(assign) to finish and exit. How can I implement it?