Why is this program giving a stack overflow? - c

Question

My C merge code works when I initialize the array globally at the top of the program until the stack overflows. I'm trying to initialize the array with malloc, but when I do, the code will only read in two integers and stop running.
This program pulls random numbers from a file called alg.txt and then sorts them. Again, the code works (up until 500k integers) when defining z at the top of the program to the number of integers to be sorted, and declaring the array globally to be equal to arr[z]. How do I figure out what is going on?
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
int count = 0;
void merge(int arr[], int l, int m, int r)
{
int i, j, k;
int n1 = m - l + 1;
int n2 = r - m;
int L[n1], R[n2];
for (i = 0; i < n1; i++)
L[i] = arr[l + i];
count++;
for (j = 0; j < n2; j++)
R[j] = arr[m + 1+ j];
count++;
i = 0;
j = 0;
k = l;
while (i < n1 && j < n2)
{
if (L[i] <= R[j])
{
arr[k] = L[i];
i++;
count++;
}
else
{
arr[k] = R[j];
j++;
count++;
}
k++;
count++;
}
while (i < n1)
{
arr[k] = L[i];
i++;
k++;
count++;
}
while (j < n2)
{
arr[k] = R[j];
j++;
k++;
count++;
}
}
void mergeSort(int arr[], int l, int r)
{
if (l < r)
{
int m = l+(r-l)/2;
mergeSort(arr, l, m);
mergeSort(arr, m+1, r);
merge(arr, l, m, r);
}
}
void printArray(int A[], int size)
{
int i;
for (i=0; i < size; i++)
printf("%d ", A[i]);
printf("\n");
}
int main()
{
int i;
FILE *myFile;
myFile = fopen("alg.txt", "r");
int z;
printf("Enter the size of the array: ");
scanf("%d", &z);
int *arr = (int *)malloc(z*sizeof(int));
int n = sizeof(arr)/sizeof(arr[0]);
for(i=0; i < z; i++)
{
fscanf(myFile, "%d,", &arr[i]);
}
mergeSort(arr, 0, n - 1);
printf("\nSorted array is \n");
printArray(arr, n);
printf("count is %d\n", count);
return 0;
}

Answer 1

You are declaring the L and R array's locally (inside the merge function). They consume stack on every function call. You should initialize them globally so that they can be stored on the heap. Also, you should declare just one array (like temp_arr in the below code) as there's no need for two separate arrays like L and R.
See Local vs Global variable storage
You can improve your merge function to this:
int temp_arr[500000]; // Global declaration
void merge(int arr[], int l, int m, int r)
{
int i=l, j=m+1, k=l;
while(i<=m && j<=r)
{
if(arr[i]<arr[j])
temp_arr[++k] = arr[++i];
else temp_arr[++k] = arr[++j];
}
while(i<=m)
temp_arr[++k] = arr[++i];
while(j<=r)
temp_arr[++k] = arr[++j];
i = l;
while(i<=r)
arr[i] = temp_arr[++i]; // Storing stored array in arr
}