C++ OpenMP merge sort - c++

Question

I have written parallel implementation of merge sort but for some reasons that I cannot understand sometimes it finishes properly, and sometimes it crashes. Whats more interesting following code sometimes seems to crash AFTER it has sorted the array (the array gets printed and then code stops working).
Could anyone help me diagnose a problem? I guess I messed something with threads and they are not ending properly?
void printArray (int* a, int elements) {
for(int i = 0; i < elements; i++) {
printf("%d\t", a[i]);
}
}
void merge2(int a[], int l, int middle, int r) {
int leftIterator = l;
int rightIterator = middle+1;
int k = 0;
int* tmp = new int[(r - l)];
while (leftIterator <= middle && rightIterator <= r) {
if (a[leftIterator] < a[rightIterator]) {
tmp[k] = a[leftIterator];
leftIterator++;
}
else {
tmp[k] = a[rightIterator];
rightIterator++;
}
k++;
}
if (leftIterator <= middle) {
while (leftIterator <= middle) {
tmp[k] = a[leftIterator];
leftIterator++;
k++;
}
}
else {
while (rightIterator <= r) {
tmp[k] = a[rightIterator];
rightIterator++;
k++;
}
}
for (int i = 0; i <= (r - l); i++)
a[l + i] = tmp[i];
}
void merge_sort(int a[], int l, int r) {
if (l != r) {
int middle= (l + r) / 2; //find middle element
#pragma omp parallel sections
{
#pragma omp section
merge_sort(a, l, middle);
#pragma omp section
merge_sort(a, middle + 1, r);
}
merge2(a, l, middle, r);
}
}
int main(void) {
int elements = 10;
int a[10] = { 10,2,1,5,6,3,4,9,8,7 };
merge_sort(a, 0, elements-1);
printArray(a, elements); // print sorted array
printf("\n");
return 0;
}