Segmentation Fault in Mergesort While assigning an array - c

Question

I have written a Merge Sort program in c but getting a segmentation fault. After trying to debug my code i got to know the particular line in which i am getting the error but i wanted to know what is the reason??

Here is my code:

void Mergesort(struct record r[],int n)
{
    int k;
    if(n>1)
    {
        int i,j;
        struct record r1[n/2];
        struct record r2[n/2];
        for(i=0,j=n/2;i<n/2,j<n;i++,j++)
        {
            r1[i]=r[i];
            r2[i]=r[j]; // this is the line where i am getting the error.
        }
        Mergesort(r1,n/2);
        Mergesort(r2,n/2);
        r=Merge(r1,r2,r,n);
    }

}

struct record * Merge(struct record r1[],struct record r2[],struct record r[],int n)
{
    int i=0,j=0,k=0;
    while(i<n/2 && j<n/2)
    {
        if (strcmp(r1[i].a,r2[j].a)<=0)
        {
            r[k]=r1[i];
            i=i+1;
        }
        else
        {
            r[k]=r2[j];
            j=j+1;
        }
        k=k+1;
    }
    if(i==n/2)
    {
        for(j;j<n/2,k<n;j++,k++)
        {
            r[k]=r2[j];
        }

    }
    else
    {
        for(i;i<n/2,k<n;i++,k++)
        {
            r[k]=r1[i];
        }
    }
    return r;
}

One more thing : I am getting segmentation fault only when the no. of entries which i want to sort are not a power of 2. The code is running properly for entries such as 2,4,8,16.

There is no need to declare arrays at all inside the recursive function. You need a pointer to the beginning of the segment and a pointer to the end. — Lundin, Feb 02 '16 at 15:09
`struct record r1[n/2];` .. `i<=n/2` .. `r1[i]=r[i];` occurs out of bounds. — BLUEPIXY, Feb 02 '16 at 15:12
I rolled back the question so the answers make sense. Please don't change the question after you accept an answer. Ask a new question instead. — 2501, Feb 02 '16 at 17:32

CiaPan · Answer 1 · 2016-02-03T09:29:12.197

1

For n==3 you get n/2 == 1 and you have no place to store all 3 items. The simplest solution (can't say if the best one) is to use n/2 and n-n/2 to size sub-arrays.

edited Feb 03 '16 at 09:29

answered Feb 02 '16 at 15:21

CiaPan

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score 1 · Answer 2 · edited May 23 '17 at 11:52

1

Certainly an error because the multiple conditions of the for loop separated by a comma: the condition i<n/2 is simply ignored. See here for details.

edited May 23 '17 at 11:52

Community

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answered Feb 02 '16 at 15:22

Puck

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score 1 · Accepted Answer · answered Feb 02 '16 at 15:25

1

The error is because when you have odd number of entries, you under allocate.
Example:
when n = 7, n/2 is 3. so you have both r1 and r2 of size 3 for a total of 6. In the for loop, the i is less than 3 (i.e, 0,1,2) and j has initial value 3 and max value 6 (so 3,4,5,6). so j needs 4 but r2 has only memory for 3 elements. So, it crashes.

It does not happen for powers of 2, because even when you divided the array by 2, they still remain powers of 2 and even except for 1. When there is only one element, you don't have to do anything.

I think you need to allocate n - n/2 for r2.

answered Feb 02 '16 at 15:25

phoenix

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**Thanks Guys for helping me out. No more Segmentation Problem.!! But there is a weird problem now. The code does not even sort the records if any number other than power of 2 is given, instead it just puts them in a random order.** PLEASE HELP..!! – Devansh Bansal Feb 02 '16 at 16:07
1

raise a new question with details. – phoenix Feb 02 '16 at 16:31
As you said I have raised a new question for this problem. Link: https://stackoverflow.com/questions/35158903/weird-output-of-merge-sort-for-no-of-entries-not-being-a-power-of-2-c – Devansh Bansal Feb 02 '16 at 16:38

Segmentation Fault in Mergesort While assigning an array - c

3 Answers3