Prime generator using a huge array in C

Question

I'm trying to make a prime number generator using arrays. I started with a 1D array but the max limit on my computer was 250000. So I thought maybe I should make a 2d or more array to go to the next line once this limit is reached to avoid problems on my PC. But I have no idea how to do it. Any help would be welcome.

#include<stdio.h>
#include<math.h>
#include<stdbool.h>
#include<time.h>
#define LIMIT 250000ul

//prime number generator using arrays 

void prime_generator(){

    unsigned long long n; 
    printf("How many primes do you want generated : "); 
    scanf("%llu", &n);

    unsigned long long p[LIMIT];  
    unsigned long long i, j=2; 
    unsigned long long num; 
    bool isPrime; 

    unsigned long long primes_per_line, space_column; 
    printf("How many primes do you want displayed per line? : "); 
    scanf("%llu", &primes_per_line); 
    printf("Enter space : "); 
    scanf("%llu", &space_column); 

    p[0]=2, p[1]=3;//first two primes
    unsigned long long count = 2; 

    clock_t begin = clock();   
    for(num = 5;count < n; num=num+2){//only test for odd number 
        isPrime = true; 
        for(i = 1;i < j && p[i] <= sqrt(num) ;i++){
            if(num%p[i]==0){
                isPrime = false;  
                break;//break out of i loop 
            }
        }
        if(isPrime == true){
            ++count; 
            p[j] = num; //new prime found 
            ++j; 
        }
    }
    clock_t end = clock(); 
    double time_spent = (double) (end-begin)/CLOCKS_PER_SEC; 

    for(i = 0; i < n; i++){
        if(i!=0 && i % primes_per_line == 0) printf("\n"); 
        printf("%*llu", space_column, p[i]); 
    }

    printf("\nCrunching time is : %.12f", time_spent);
}

int main(void){
    prime_generator(); 
    return 0; 
}

Answer 1

Your unsigned long long p[LIMIT]; is a local variable (with automatic storage duration ) inside your prime_generator function, so it is on the call stack (which is often limited to one or a few megabytes). Declare it as a static or make it a global variable (in both cases, it gets a static storage duration ). Then it could be much larger, but your prime_generator won't be re-entrant anymore (but in your case, you don't care).

Declaring an array as multi-dimensional won't improve its memory size (so won't solve the problem of an excessive call frame size of a variable declared local ).

Better yet, use C dynamic memory allocation that is calloc (don't forget to test it against failure, when it returns NULL use perror then exit ) and free ; beware of memory leaks ; tools like valgrind could be helpful. With calloc (or malloc ) you'll then grow your virtual address space and be limited by machine and system resources (probably gigabytes).

Avoid buffer overflows . See this .

Read also wikipages on prime numbers and on primality tests . There are more efficient algorithms.

You should be able to compute a million primes (ending with 15485761 15485773 15485783 15485801 15485807 15485837 15485843 15485849 15485857 15485863) in a few seconds at most (and probably in less than half a second).