What the hell!? Crashes and memory leaks?

pobri19 · 09-26-2008, 05:25 AM

So, what the HELL is going on here... Here's the deal, I made this to find the highest prime factor of the number n (Yeah, w/e that number is, 600 billion something). The thing is, it will run, then all of a sudden it will stop working. Like I will run it say 10 times or so, then it will continuously keep crashing. Sometimes it will print 1 number, sometimes none, sometimes it will cause a runtime error, sometimes just NOTHING happens, I have no freaking idea what the hell is happening here... Also, if I change the array to type int, then run it, it will cause a crash, but then if I change it back to char it will start working again. Every time these crashes happen if I change it to int, run it, then to char it will magically start working again... My only guess is something weird is happening in memory. But I can't see what's wrong with this. Here's the code!

Code:

#include <iostream>
#include <cstdlib>
#define uint unsigned long long int
#define N 10000

using namespace std;

char s[N];

void FillSieve() {
	int x;
	int y = 2;
	int i = 0;
	for (x = 0; x < N; x++)
		s[x] = 1;

	while (y < x) {
		if (s[y] == 1) {
			i = y;
			while ( i < N ) {
				i+=y;
				s[i] = 0;
			}
		}
		y++;
	}

}

int main() {
	FillSieve();
	uint n = 600851475143;
	int temp = 0;
	for (int i = 2; i < N; i++)
		if (s[i] == 1)
			if ( !(n % i) )
				cout << i << endl;
}

Note: When it actually did work the answer was correct... What the hell? Ok cheers.

Elysia · 09-26-2008, 05:32 AM

Quote:

Originally Posted by pobri19

Code:

#include <iostream>
#include <cstdlib>
#define uint unsigned long long int
#define N 10000

using namespace std;

char s[N];

void FillSieve() {
	int x;
	int y = 2;
	int i = 0;
	for (x = 0; x < N; x++)
		s[x] = 1;

	while (y < x) {
		if (s[y] == 1) {
			i = y;
			while ( i < N ) {
				i+=y;
				s[i] = 0;
			}
		}
		y++;
	}

}

int main() {
	FillSieve();
	uint n = 600851475143;
	int temp = 0;
	for (int i = 2; i < N; i++)
		if (s[i] == 1)
			if ( !(n % i) )
				cout << i << endl;
}

You have undefined behavior on the red line.
It tries to assign to s[10000] among others.

matsp · 09-26-2008, 05:33 AM

Code:

	for (x = 0; x < N; x++)
		s[x] = 1;

	while (y < x) {

when the for-loop ends, x will be the same value as N. So why not use N?

This overflows:

Code:

		while ( i < N ) {
				i+=y;
				s[i] = 0;
			}

if i + y > N you will go outsid the loop here, as you add y to i after checking if i is below N.

As to why it only causes crashes SOMETIMES, that's just the way "undefined behviour" works - it's like running a red light: Sometimes there's a car going across the junction the other direction with a green light that hits you, at other times it's clear and you get across unhurt.

--
Mats

pobri19 · 09-26-2008, 05:45 AM

Ah I see.. Ahwell easily fixed

Cheers for the help.

09-26-2008, 05:33 AM	#3
matsp Kernel hacker Join Date: Jul 2007 Location: Farncombe, Surrey, England Posts: 15,686	Code: for (x = 0; x < N; x++) s[x] = 1; while (y < x) { when the for-loop ends, x will be the same value as N. So why not use N? This overflows: Code: while ( i < N ) { i+=y; s[i] = 0; } if i + y > N you will go outsid the loop here, as you add y to i after checking if i is below N. As to why it only causes crashes SOMETIMES, that's just the way "undefined behviour" works - it's like running a red light: Sometimes there's a car going across the junction the other direction with a green light that hits you, at other times it's clear and you get across unhurt. -- Mats __________________ Compilers can produce warnings - make the compiler programmers happy: Use them! Please don't PM me for help - and no, I don't do help over instant messengers.
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