Fibonacci program c pthread

/*============================================================================
   Description :The Fibonacci sequence
  ============================================================================ */
#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>

int n;                          // size of fibonacci sequence.
int *fibseq;                    // arry holds the value of each fibonacci term.
int i;                          // counter for the threads.

void *runn(void *arg);

int main(int argc, char *argv[])
{
    if (argc != 2)
    {
        printf("format is:./a.out <intgervalue>\n");
        return -1;
    }                           // valdiate num of args.

    if (atoi(argv[1]) < 0)
    {
        printf("%d must be>=0\n", atoi(argv[1]));
        return -1;
    }                           // valdiate value of arg1.

    n = atoi(argv[1]);
    fibseq = (int *)malloc(n * sizeof(int));
    pthread_t *threads = (pthread_t *) malloc(n * sizeof(pthread_t));
    pthread_attr_t attr;        // set of thread attribute

    pthread_attr_init(&attr);

    for (i = 0; i < n; i++)
    {
        pthread_create(&threads[i], &attr, runn, NULL);
    }                           // End of creating threads.

    int j;

    for (j = 0; j < n; j++)
    {
        pthread_join(threads[j], NULL);
    }                           // End of wating the threads to exit.

    // printing fibseq.
    printf("The Fibonacci sequence.:");
    int k;

    for (k = 0; k < n; k++)
    {
        printf("%d,", fibseq[k]);
    }                           // End of printing fibseq.
    return 0;
}                               // End of main.

void *runn(void *arg)
{
    if (i == 0)
    {
        fibseq[i] = 0;
        pthread_exit(0);
    }                           // first fib term

    if (i == 1)
    {
        fibseq[i] = 1;
        pthread_exit(0);
    }                           // seconed fib term
    else
    {
        fibseq[i] = fibseq[i - 1] + fibseq[i - 2];
        // printf("fibseq[%d]%d,\n",i,fibseq[i]);
        pthread_exit(0);        // thread exit.
    }                           // End of else
}                               // End of run.