PI Generation from Random Numbers

#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <math.h>
#include <pthread.h>

long long int perThreadOperations = 0;

unsigned short gcd(unsigned short u, unsigned short v) {
    unsigned short shift = 0;

    if (u == 0) return v;
    if (v == 0) return u;
    while (((u | v) & 1) == 0) {
        shift++;
        u >>= 1;
        v >>= 1;
    }

    while ((u & 1) == 0)
        u >>= 1;

    do {
        while ((v & 1) == 0)
            v >>= 1;

        if (u > v) {
            unsigned short t = v; v = u; u = t;
        }

        v -= u;
    } while (v != 0);

    return u << shift;
}

void *genPairs(void * arg) {

    long long amount = *(long long int *)arg;
    long long coprimeCount = 0;

    srandom(time(NULL));

    int mngflFrct = amount*0.01;

    for(long long int a = 0 ; a < amount; ++a) {
        if(a % mngflFrct*5) printf("\r%5.2f%%", 100.0*((float) a/ (float) amount));
        unsigned short rand1 = random(), rand2 = random();

        if(gcd(rand1,rand2) == 1)
            coprimeCount++;
    }

    pthread_exit((void *)coprimeCount);
}

int main(int argc, char **argv) {
    if(argc != 3) {
        printf("Usage: RandomPi <Thread Count> <Pair Ammount>");
        return -4;
    }

    long long pairCount = atoll(argv[2]);
    char threadCount = atoi(argv[1]);

    printf("Threads: %d\r\n", threadCount);

    pthread_t threads[threadCount];

    long long mainCoprimeCounter = 0;

    //TODO - Implement a better way to split load between threads

    perThreadOperations = pairCount/threadCount;
    printf("Operations per single thread: %lld\r\n", perThreadOperations);

    for(int a = 0 ; a < threadCount ; ++a) {
        pthread_create(&threads[a], NULL, genPairs, &perThreadOperations);
    }

    for(int a = 0 ; a < threadCount ; ++a) {
        void *coprimeCount;
        pthread_join(threads[a], &coprimeCount);
        mainCoprimeCounter+=(long long)coprimeCount;
    }

    long long cofactors = perThreadOperations * threadCount - mainCoprimeCounter;

    printf("Statistics:\r\n\tCoprime pairs: %lld\r\n\tCofactor pairs: %lld\n\tPairs altogether: %lld\r\n", mainCoprimeCounter, cofactors, mainCoprimeCounter + cofactors);

    long double piApprox;

    if(mainCoprimeCounter != 0 && cofactors != 0) {
        long double coprimeShare = (long double)mainCoprimeCounter/((long double)pairCount);
        piApprox = sqrtl((long double)6/coprimeShare);

        printf("\r\n\tPi Approximation: %.15Lf", piApprox);
    } else {
        printf("One of the critical variables is zero!\r\nApproximating PI impossible.");
    }
}