why isn't my Sieve of erathosthenes loop executing?

I just used Sieve of erathosthenes algorithm, at beginning with single int passing value, it was working just fine, but further, loop won't execute at all. Please help .. this is the problem where the following code belongs. http://ift.tt/1MVUNPA

include

include

void sieve_algo(long in, long fi){

bool prime_initial[in], prime[fi+1];
memset(prime, true, sizeof(prime));
memset(prime, false, sizeof(prime_initial));

for(long i=in; i*i<=fi; i++ ){   // this loop is not executing

    if(prime[i] == true){
        for(long p=i*2; p<=fi; p+=i)                
            prime[p]= false;

    }
}
for(long k=in; k<=fi; k++){
    if(prime[k]==true){
        printf("%ld\n", k);
    }
}

}

int main(){

int tc;
scanf("%d", &tc);
for(int i=0; i<tc; i++){
    long int m,n;
    scanf("%ld %ld", &m, &n);
    sieve_algo(m,n);
    printf("\n");
}
return 0;

}