What is causing a segmentation fault? - c++

Question

I have been attempting to write a program that will determine if a number is prime or not. I have based it off of the Sieve of Eratosthenes. Anyway, my program works for small numbers (15485863 works), but if I use large numbers (ex. 17485863) I receive a segmentation fault. I am using unsigned long longs and do not think I have surpassed their maximum value. I just don't see what I have done wrong. Thank you in advance for any assistance!
#include <iostream>
#include <limits>
using namespace std;
bool soe (unsigned long long);
int main (void)
{
unsigned long long x = 17485863;
bool q = soe(x);
cout << x << " is ";
if(q)
cout << "prime." << endl;
else
cout << "not prime." << endl;
return 0;
}
bool soe(unsigned long long input)
{
unsigned long long arrayLength = input%2 + input/2;
unsigned long long index = 2;
unsigned long long greatestMult = 0;
bool array[arrayLength];
array[0] = true; //ignore true values in the array
array[1] = true;
do{
array[index] = false;
}while(++index < arrayLength);
index = 2;
do
{
if(input%index != 0)
{
greatestMult = input/index;
while(index*greatestMult > arrayLength)
greatestMult--;
do
{
array[index*greatestMult] = true;
}while(--greatestMult > 0);
do
{
if(!array[index])
break;
}while(++index < arrayLength);
}
else
{
cout << endl << input << " is divisble by " << index << endl;
return false;
}
}while(index < arrayLength);
return true;
}

Answer 1

Please note that neither long long nor using variables to dimension automatic arrays are part of C++ - they are extensions provided by gcc and should not be used if portability is an issue.
To address your problem, dimensioning an array like this:
bool array[arrayLength];
will cause a stack overflow (and thus a seg fault) if the arrayLength value is too large. Use a std::vector instead, but be aware that memory is not an infinite resource.

Answer 2

On Line 24 you have: bool array[arrayLength]; You cannot declare an array on the stack like this. The program is crashing on line 29. You need to declare this on the heap using new/delete;
Something to this effect (I may have a leak or two in there, but you get the idea);
//Beginning on Line 28
bool *array = new bool[arrayLength];
array[0] = true; //ignore true values in the array
array[1] = true;
do{
array[index] = false;
}while(++index < arrayLength);
index = 2;
do
{
if(input%index != 0)
{
greatestMult = input/index;
while(index*greatestMult > arrayLength)
greatestMult--;
do
{
array[index*greatestMult] = true;
}while(--greatestMult > 0);
do
{
if(!array[index])
break;
}while(++index < arrayLength);
}
else
{
cout << endl << input << " is divisble by " << index << endl;
delete [] array;
return false;
}
}while(index < arrayLength);
delete [] array;
return true;
}
Output
g++ -g test.cpp
gdb ./a.out
...clipped...
(gdb) run
Starting program: /Users/nextraztus/a.out
Reading symbols for shared libraries ++. done
17485863 is divisble by 3
17485863 is not prime.
Program exited normally.
(gdb)

Answer 3

It is possible for index*greatestMult to be equal to arrayLength, so you can overwrite the last element past the array end.
Also allocating large arrays on the stack like that can cause a problem depending on the operating system. Some systems will expand the stack that much, others will not be able to.