pi_multi-thread.c [Avoid race condition]

#include <stdlib.h>
#include <stdio.h>
#include <time.h>
#include <math.h>
#include <pthread.h>
const int NUM_THREADS = 4;
 clock_t start, end;
 struct countEachThread
 {
 int limit;
 int count;
 };
 _Thread_local unsigned int seed = 0;

void* circle_cnt(void* argp)
{   int count = 0;
    struct countEachThread* ptr = (struct countEachThread*)argp;
     for(int i=0;i < ptr->limit;i++)
    {


        int n = rand_r(&seed);
        double x=(double)n/((double)RAND_MAX);

        n = rand_r(&seed);
        double y=(double)n/((double)RAND_MAX);

        double distance= 1.0*x*x+1.0*y*y;

        if(distance<=1) count++;


    }
    ptr->count = count;
    pthread_exit(NULL);
}
int main(int argc, char*argv[])
{
        pthread_t tid[NUM_THREADS];

    struct countEachThread calc[NUM_THREADS];

    int n = atoi(argv[1]); // input

    srand(time(NULL));

    start = clock(); // init start time.

    int i = 0;   // init loop variable

    for (i = 0; i < NUM_THREADS; ++i) // create NUM_THREADS threads to calculate
    {
        pthread_attr_t attr;
        pthread_attr_init(&attr);
        calc[i].limit = n / NUM_THREADS;
        pthread_create(&tid[i], &attr, circle_cnt,&calc[i]);
    }
    int countTotal = 0;

    for (i = 0; i < NUM_THREADS; ++i) // join threads together.
    {
        pthread_join(tid[i], NULL);
        countTotal = countTotal + calc[i].count;
    }

    double pi = 4.0 * countTotal / n;

        end = clock();

        double exe_time = ((double)(end - start)) / ( (double)(CLOCKS_PER_SEC) );

    printf("Pi: %lf\n",pi);

        printf("Time: %g seconds", exe_time);

    return 0;
}