why I am getting segmentation fault with scanf at large inputs - c

Question

I have written c code to merge sort a array of input
and I tried the code for 2 3 5 10 100 1000 10,000 and 1,000,000
it worked as desired in all cases except the 10,000 and 1,000,000
after debugging I found that the error is within scanf() code
Program received signal SIGSEGV, Segmentation fault.
0x00000000004006a7 in main () at merge.c:10
10 scanf("%i",&array[i]);
but the code is perfectly correct as far as I am concerned
int len;
scanf("%i",&len);
int array[len];
for(int i=0; i<len;i++)
{
scanf("%i",&array[i]);
}
In case you need the full code for clearification here it is
#include <stdio.h>
void mergesort(int* list , int len);
int main()
{
int len;
scanf("%i",&len);
int array[len];
for(int i=0; i<len;i++)
{
scanf("%i",&array[i]);
}
mergesort(array,len);
for(int i=0; i<len;i++) printf("%i\n",array[i]);
printf("\b");
}
void mergesort (int* list, int len)
{
if(len == 1) return;
int i = len/2, j = len-i;
int list1[i], list2[j];
for(int k=0;k<i;k++)
{
list1[k]= list[k];
list2[k]= list[i+k];
}
if(len%2!=0) list2[j-1] = list[len-1];
mergesort(list1 , i);
mergesort(list2 , j);
int k=0,l=0;
// k represent counter over elements in list1
// l represent counter over elements in list2
// k+l represent counte over total elements in list
while(k+l!=len)
{
if(k==i)
{
for(;l<j;l++) list[k+l] = list2[l];
return;
}
else if (l==j)
{
for(;k<i;k++) list[k+l] = list1[k];
}
else if(list1[k]<list2[l])
{
list[k+l]=list1[k];
k++;
}
else if(list1[k]>list2[l])
{
list[k+l] = list2[l];
l++;
}
else
{
//handles dublication
list[k+l] = list1[k];
k++;
list[k+l] = list[l];
l++;
}
}
}
EDIT:
in case you believe that the problem platform dependent I use ubuntu 14.04
and gnu c compiler 4.8.2 and I compile the code with flags -std=c99 -Wall
and it is error and warning free

Answer 1

Merge sort eats plenty of memory, and you're allocating additional memory in what c programmers call stack ( int list1[i], list2[j]; ). And there's actually little room in stack. Doing it over and over again for large values exceeds the limit.
You must use heap allocations instead - dynamic allocations. Heap has plenty of memory available for allocation and is used for large arrays among other purposes. You do it like this:
//C example with malloc, ALWAYS use sizeof
int *array = (int*) malloc( length * sizeof(int));
//C++ way
int *array = new int[length];
But memory remains allocated after the program ends, unless you free it. Which can be a problem in thid case. Make sure to unallocate it when you no longer need it.
//C way
free(array);
//C++ way for arrays
delete[] array;
//C++ way for single heap values
delete value;
This is the right way so you should get used to it as youll need dynamic allocation very much in future.