Array declaration fails while trying to declare int array

Question

I've been learning & coding sorting algorithms for some time and recently I've coded merge sort in C, and I've also coded a sort_test function to test the function that I write. In the sort test function, I'm declaring an array and assigning random values to it, but when the array size gets to 1,000,000 the program crashes. Why is that happening?

sort_test.c

#include <stdio.h>
#include <stdlib.h>
#include <assert.h>
#include "merge_sort.h"
#include "sort_test.h"

// test size
#define MIN 10
#define MAX 1000000

// int comparator

int cmpInt(const void *elem1,const void * elem2){
    int e1 = *(int *)elem1; // i-1
    int e2 = *(int *)elem2; // i
    if(e2 < e1){
        return -1;
    } else if(e2 > e1){
        return 1;
    } else {
        return 0;
    }
}

// double comparator

int cmpDouble(const void *elem1,const void *elem2){
    double e1 = *(double *)elem1;
    double e2 = *(double *)elem2;
    if(e2 < e1){
        return -1;
    } else if(e2 > e1){
        return 1;
    } else {
        return 0;
    }
}

void initSeed(){
    srand(time(NULL));
}

void intSortTest(){
    initSeed();
    for(size_t i = MIN;i <= MAX;i *=10){
        int arr[i];
        for(size_t j = 0; j < i;j++){
            arr[j] = rand();
        }
        // sorting the array
        mergesort(arr,0,i);
        // checking if sorted array hold the 
        // condition i[0] <= i[1] ... <= i[n].
        for(size_t j = 1;j < i;j++){
            int *e1 = &arr[j-1];
            int *e2 = &arr[j];
        assert(cmpInt(e2,e1) <= 0);
        }
        printf("INT TEST : %7d\tPASSED\n",i);
    }
    printf("\n");
}

void doubleSortTest(){
    initSeed();
    for(int i = MIN; i <= MAX; i *= 10){
        double arr[i];
        for(int j = 0 ; j < i;j++){
            arr[j] = (double)(rand() % 100) + 1.0;
        }
        // perform sort
        //insertion_sort(arr,sizeof (double),i,cmpDouble);
        for(int j = 1; j < i;j++){
            double *e1 = &arr[j-1];
            double *e2 = &arr[j];
            assert(cmpDouble(e2,e1) <= 0);
        }
        printf("Double Test : %5d\tPASSED\n",i);
    }
    printf("\n");
}

sort_test.h

#ifndef SORT_TEST_H
#define SORT_TEST_H

void initSeed();
void intSortTest();
void doubleSortTest();
int cmpDouble(const void *elem1,const void *elem2);
int cmpInt(const void *elem1,const void * elem2);

#endif // SORT_TEST_H

merge_sort.h

#ifndef MERGE_SORT_H
#define MERGE_SORT_H

void mergesort(int *arr,int start,int end);
void merge(int *arr,int start,int med,int end);

#endif // MERGE_SORT_H

merge_sort.c

#include <stdio.h>
#include "sort_test.h"
#include "merge_sort.h"

int main(){
    intSortTest();
    return 0;
}

void mergesort(int *arr,int start,int end){
    if(start < end){
        int median = (end + start) / 2;
        mergesort(arr,start,median);
        mergesort(arr,median+1,end);
        merge(arr,start,median,end);
    }
}

void merge(int *arr,int start,int median,int end){
    int i = start; int j = median+1;
    int copy[end+1];
    int cIndex = 0;

    while(i <= median && j <= end) {
        if(arr[j] <= arr[i]){
            copy[cIndex++] = arr[j++];
        } else {
            copy[cIndex++] = arr[i++];
        }
    }

    while(i <= median){
        copy[cIndex++] = arr[i++];
    }
    while(j <= end){
        copy[cIndex++] = arr[j++];
    }
    for(int k = 0; k < cIndex; k++){
        arr[start++] = copy[k];
    }
}

Answer 1

It is because you are allocating the arrays on the stack. Try the following code instead.

void intSortTest(){
    initSeed();
    for(size_t i = MIN;i <= MAX;i *=10){
        int *arr = malloc(i*sizeof(int));  // <-- changed this
        for(size_t j = 0; j < i;j++){
            arr[j] = rand();
        }
        // sorting the array
        mergesort(arr,0,i);
        // checking if sorted array hold the 
        // condition i[0] <= i[1] ... <= i[n].
        for(size_t j = 1;j < i;j++){
            int *e1 = &arr[j-1];
            int *e2 = &arr[j];
            assert(cmpInt(e2,e1) <= 0);
        }
        printf("INT TEST : %7d\tPASSED\n",i);
        free(arr);   // <-- added this
    }
    printf("\n");
}

EDIT

Also the merge algorithm is incorrect. More precisely, you have a problem with the value list boundaries.

When you define the start and end index of a value list, the values are in arr[start] to arr[end-1], not arr[end]. The number of values is then end-start. With this convention, you have an empty list when start == end.

As a consequence, the function mergesort becomes:

void mergesort(int *arr,int start,int end){
    if (start+1 >= end) 
        return; // a list with 0 or 1 values is already sorted
    int median = (end + start) / 2;
    mergesort(arr,start,median);
    mergesort(arr,median,end);
    merge(arr,start,median,end);
}

The merge function then become as follow:

void merge(int *arr,int start,int median,int end){
    int i = start; int j = median;
    int *copy = malloc((end-start)*sizeof(int)); // use malloc for huge arrays
    int cIndex = 0;

    while(i < median && j < end) {  // not i <= median && j <= end
        if(arr[j] <= arr[i]){
            copy[cIndex++] = arr[j++];
        } else {
            copy[cIndex++] = arr[i++];
        }
    }

    while(i < median){ // not i <= median
        copy[cIndex++] = arr[i++];
    }
    while(j < end){  // not j <= median
        copy[cIndex++] = arr[j++];
    }
    for(int k = 0; k < cIndex; k++){
        arr[start++] = copy[k];
    }
    free(copy);
}

As you can see, there are only minor differences.

With this code, your program runs without error.

Answer 2

Now that the code is visible, it is fairly easy to see that you are indeed blowing the stack as I suggested in one of my many comments.

In merge(), you have:

int copy[end+1];

as well as in intSortTest() having:

int arr[i];

where i reaches 1,000,000.

When end is 1,000,000 — it is set from i — you have an array of one million int values being sorted, and a copy with another one million int values (plus 1), so you attempt to place two million 4-byte int values on the stack — and 8,000,000 bytes blows the stack limits. Since 800,000 bytes (the previous size) fits on the stack in both Unix and Windows, it isn't 100% clear which you are using. There isn't much margin for error on Unix/Linux; the limit is thoroughly blown on Windows because neither 4 MB array fits on the stack.

The recommended fix is to use dynamic memory allocation (malloc() et al) instead of stack allocation — in both the sort test function and in the main merge() code.