Segmentation fault when using malloc and realloc with arrays

Question

I'm fairly new to c and I'm trying to understand and grasp malloc. My program takes an integer x input, then loops until x is met while also taking other integer inputs. I then do various calculations. However I'm getting a segmentation fault and I don't understand why.

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

int main(void)
{

    calculations();
}

void calculations()
{

    int i;
    int x;
    int total = 0;
    double squareRoot;
    double overall;

    scanf("%d", &x);

    int* array = malloc(x * sizeof(int));

    if (!array) {
        printf("There isn't enough memory \n");
        return;
    }

    int c = 0;

    while (c < x) {

        scanf("%d", &array[i]);

        total = array[i] * array[i];
        c++;
    }

    squareRoot = sqrt(total);
    array = realloc(array, x * sizeof(int));

    int b = 0;

    while (b < x) {

        overall = array[i] / squareRoot;
        printf("%.3f ", overall);

        b++;
    }
}

Do you have a debugger? That would tell you which line triggered the segmentation fault. — yellowantphil
– yellowantphil, Commented Mar 1, 2017 at 18:27
In array = realloc(array, x* sizeof(int)); if you intended to extend - and then index - the memory allocation, it does not give any more memory, being the same allocation as the original array = malloc(x * sizeof(int)); — Weather Vane
– Weather Vane, Commented Mar 1, 2017 at 18:32
Valgrind will help you narrow down memory access errors like this. — Toby Speight
– Toby Speight, Commented Mar 1, 2017 at 18:32
@SouravGhosh Sorry I forgot to post my indented code. I've edited it now. — LookingWest
– LookingWest, Commented Mar 1, 2017 at 18:35

Sourav Ghosh · Accepted Answer · 2017-03-01 18:30:52Z

3

The problem is with

 scanf("%d", &array[i])

where, the value of i is indeterminate.

To elaborate, i is an uninitialized local variable and unless initialized explicitly, the contents remain indeterminate. Attempt to use that value, in this scenario, would lead to invokes undefined behavior.

Even if you initialize i, you never operated on i, so all the changes will be overwritten on a fixed index. You got to take care of this case, too.

Solution: Looking at the code, it appears, you may want to use

scanf("%d", &array[c]);

instead.

answered Mar 1, 2017 at 18:30

Sourav Ghosh

135k17 gold badges192 silver badges271 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

LookingWest Over a year ago

Thanks this fixed my issue! I changed all the array[i] to array[c] however when I now run my program I only get 0.000 as my output? Any idea why this is?

Collectives™ on Stack Overflow

Segmentation fault when using malloc and realloc with arrays

1 Answer 1

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related