The article is written in ISO C90
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#include <stdlib.h>
#include <stdio.h>

/* Function to merge the two haves arr[l..m] and arr[m+1..r] of array arr[] */
void merge(int arr[], int l, int m, int r)
{
	int i, j, k;
	int n1 = m - l + 1;
	int n2 =  r - m;

	/* create temp arrays */
	int L[n1], R[n2];

	/* Copy data to temp arrays L[] and R[] */
	for(i = 0; i < n1; i++)
		L[i] = arr[l + i];
	for(j = 0; j < n2; j++)
		R[j] = arr[m + 1+ j];

	/* Merge the temp arrays back into arr[l..r]*/
	i = 0;
	j = 0;
	k = l;
	while (i < n1 && j < n2)
	{
		if (L[i] <= R[j])
		{
			arr[k] = L[i];
			i++;
		}
		else
		{
			arr[k] = R[j];
			j++;
		}
		k++;
	}

	/* Copy the remaining elements of L[], if there are any */
	while (i < n1)
	{
		arr[k] = L[i];
		i++;
		k++;
	}

	/* Copy the remaining elements of R[], if there are any */
	while (j < n2)
	{
		arr[k] = R[j];
		j++;
		k++;
	}
}

/* l is for left index and r is right index of the sub-array
 of arr to be sorted */
void mergeSort(int arr[], int l, int r)
{
	if (l < r)
	{
		int m = l+(r-l)/2; //Same as (l+r)/2, but avoids overflow for large l and h
		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 arr[] = {12, 11, 13, 5, 6, 7};
	int arr_size = sizeof(arr)/sizeof(arr[0]);

	printf("Given array is \n");
	printArray(arr, arr_size);

	mergeSort(arr, 0, arr_size - 1);

	printf("\nSorted array is \n");
	printArray(arr, arr_size);
	return 0;
}