Why Segmentation fault (core dumped)?

Question

I'm learning C. For a school assignment, I wrote some code to print prime numbers within a certain range. I use 50000 as the maxbyte for a bit-array. My code compiles, but it gives me an error called "segmentation fault:11" (It stops at 46337). Can someone tell me what the problem is in my code? How can I fix this error? Thank you.

#include <stdio.h>
#include <stdlib.h>

//#define MAXBYTES 1000000
#define MAXBYTES 50000


void setBit(unsigned int A[], int k);
unsigned getBit(unsigned int A[], int k);
void print_prime (int prime_num);
void sieve_Prime(unsigned int bit_arr[]);

int main (int argc, char** argv)
{
    //int bit_arr[MAXBYTES];      //This is the bit array (32 X MAXBYTES)
    unsigned int bit_arr[MAXBYTES];      //or bit_arr[MAXBYTES]
    int i;

    for (i=0; i < MAXBYTES; i++)
    {
        bit_arr[i] = 0x00;            //initialize all bits to 0s
    }

    setBit(bit_arr, 0);             //0 is not prime, set it to be 1
    setBit(bit_arr, 1);             //1 is not prime, set it to be 1

    sieve_Prime(bit_arr);
    printf("\n");

    return 0;

}

//Set the bit at the k-th position to 1
void setBit(unsigned int A[], int k)
{
    int i = k/32;
    int pos = k % 32;

    unsigned int flag = 1;      //flag = 0000 ..... 00001
    flag = flag << pos;         //flag = 0000...010...000 (shifted k positions)

    A[i] = A[i] | flag;         //Set the bit at the k-th position in A[i];
}

//get the bit at the k-th position
unsigned getBit(unsigned int A[], int k)
{
    int i =k/32;
    int pos = k % 32;

    unsigned int flag = 1;

    flag = flag << pos;

    if (A[i] & flag)
        return 1;
    else
        return 0;
}


void print_prime (int prime_num)
{
    //print a prime number in next of 8 columns
    static int numfound=0;

    if (numfound % 8 == 0)
        printf("\n");
    if (prime_num+1 < MAXBYTES*8)                       
        printf("%d\t", prime_num);
    numfound++;
}

void sieve_Prime(unsigned int bit_arr[])
{
    int i;
    int k;

    int next_prime = 2;

    print_prime(2);

    while (next_prime+1 < MAXBYTES*8)                   
    {
        k = next_prime;

        //multiples of next_prime is not primpe
        while(next_prime*k < MAXBYTES*8)                
        {
            setBit(bit_arr, next_prime*k);      //set it to be 1
            k++;     
        }

        //find next_prime by skipping non-prime bits marked 1
        next_prime++;
        while (next_prime + 1 < MAXBYTES*8 && getBit(bit_arr, next_prime))
        {
            next_prime++;
        }
        print_prime(next_prime);

    }
}

Answer 1

First thing to do is make up your mind about what MAXBYTES actually means. Since you allocate the array bit_arr to have that many ints, not bytes, you're allocating 4 times the memory you need and just wasting it. Also, you're allocating it as a local on the stack--many machines won't have that much space on the stack, which is probably why it fails with a bigger number. Better to either make it static or allocate it on the heap with malloc().

Also, you need to take more care about the ranges of the numbers used: i and k, for example, may be up to MAXBYTES*8, so you may have to make them long if you increase it. And you have an intermediate result k * next_prime that might overflow plain int.

Personally, since you are indexing the array by bits, I would set your limits in those same terms to make things clearer--have a MAXBITS, allocate the array as MAXBITS/32, compare limits and ranges in terms of bits, and make sure your bit counts fit into the variable types.

Answer 2

In while(next_prime*k < MAXBYTES*8)

The largest value for next_prime*k is (MAXBYTES*8-1)*(MAXBYTES*8-1). This large value can't hold in a signed int and may turn into a negative value causing a segfault.

Using

while(unsigned(next_prime*k) < MAXBYTES*8) 

would eliminate the segfault.