pthread intervals prime check

#define _GNU_SOURCE
#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
#include <string.h>
#include <sched.h>
#include <stdbool.h>
#include <unistd.h>

#define LINE_SIZE (1 << 12)

static int a, b, n;

bool isPrime(int x) {
    if (x < 2) {
        return false;
    }
    if (x == 2) {
        return true;
    }
    for (int div = 2; div * div <= x; div++) {
        if (x % div == 0) {
            return false;
        }
    }
    return true;
}

void *routine(void *arg) {
    int *pair = (int*)arg;
    for (int x = pair[0]; x <= pair[1]; x++) {
        if (isPrime(x)) {
            printf("%d\n", x);
        }
    }
    return NULL;
}

int main(int argc, char *argv[]) {
    if (argc != 4) {
        fprintf(stderr, "WRONG USAGE!\n");
        exit(1);
    }

    a = atoi(argv[1]);
    b = atoi(argv[2]);
    n = atoi(argv[3]);
    if (!a || !b || !n) {
        fprintf(stderr, "WRONG Params!\n");
        exit(1);
    }

    cpu_set_t cpuset;
    CPU_ZERO(&cpuset);
    CPU_SET(0, &cpuset);
    CPU_SET(1, &cpuset);
    CPU_SET(2, &cpuset);

    pthread_attr_t attr;
    pthread_attr_init(&attr);
    pthread_attr_setaffinity_np(&attr, sizeof(cpu_set_t), &cpuset);

    int nrElems = b - a + 1;
    if (nrElems % n) {
        fprintf(stderr, "WRONG values!\n");
        exit(1);
    }
    int intSize = (nrElems / n);

    pthread_t thread[n];
    int pairs[n][2];
    for (int i = 0; i < n; i++) {
        pairs[i][0] = a + i * intSize;
        pairs[i][1] = pairs[i][0] + intSize - 1;
    }

    int r = 0;
    for (int i = 0; i < n; i++) {
        if ((r = pthread_create(&thread[i], &attr, routine, pairs[i])) != 0) {
            fprintf(stderr, "pthread_create: %s\n", strerror(r));
            exit(1);
        }
    }

    for (int i = 0; i < n; i++) {
        if ((r = pthread_join(thread[i], NULL)) != 0) {
            fprintf(stderr, "pthread_join: %s\n", strerror(r));
            exit(1);
        }
    }

    pthread_attr_destroy(&attr);

    return 0;
}