How to divide stupidly big numbers in C

Am learning C and thought that Project Euler problems would be a fun and interesting way to learn (and would kill 2 birds with 1 stone because it would keep me thinking about maths as well) but I've hit a snag.

I have (what I think is) a good (if simple) algorithm for findi开发者_高级运维ng the largest prime factor of a number. It works (as far as I have tested it), but the PE question uses 600851475143 as it's final question. I have tried to use doubles and such, but I can never seem to find both a modulo and a division operator. Any help would be greatly appreciated.

Code attached is before I started to make it work with doubles (or any other type):

#include<stdio.h>
#include <math.h>

void main() {
    int target, divisor, answer;
    target = 375;
    divisor = 2;
    answer = -1;

    answer = factorise (target,divisor);

    printf("Answer to Euler Problem 3: %i\n", answer);
}

int factorise(number, divisor) {
    int div;
    while (divisor < number) {
        div = divide(number,divisor);
        if (div) {number = div;}
        else {divisor++;}
    }
    return divisor;
}

int divide(a,b) {
    if (a%b) {return 0;}
    else {return a/b;}
}

Have you tried long or long long? Depending on your compiler, those might work. But you'll eventually need a bigint library for other PE problems. There are some on the internet, but since you're doing this to learn I'd suggest writing your own.

The C Standard specifies the lower limit for integral types:

char: 127 (2^7 - 1)
short: 32767 (2^15 - 1)
int: 32767 (2^15 - 1)
long: 2147483647 (2^31 - 1)
long long (C99): 9223372036854775807 (2^63 - 1)

If Project Euler uses a C99 compiler you're guaranteed with long long.

Also, these are minimum values. I think Project Euler's longs are 64 bits, so long should also work for C89.

The biggest integral type in C99 is long long, you can try this one.

You cannot make precise integral calculations with double as it's imprecise with big numbers.