c++ - Iterative mergesort from index a to index b -

i'm trying sort of mergesort going it'll mergesort(int[] a, int start, int stop) start index start sorting , stop index stop sorting. idea can have 4 different threads sorting 4 subsections of array @ same time. can call mergesort(a, 0, length/4); mergesort(a, length/4, length/2); mergesort(a, length/2, (3/4)*length); etc. on different threads.

i've come across bunch of iterative algorithms go 0 length when edit bit try them work subsections of array segfaults on inputs large , bunch of silent errors when looking in valgrind.

here's have right now:

void mergesort(int *a, int *b, int low, int high) {     int pivot;     if (low < high) {         pivot = (low + high)/2;         if(level < maxlevel) { // if have not reached max threads spawn             mergesort(a, b, low, pivot); // need change these spawn threads             mergesort(a, b, pivot + 1, high);             merge(a, b, low, pivot, high);         } else {             level += 2; // tell when can stop spawning threads             (int curr_size=1; curr_size<=(high-low)-1; curr_size = 2*curr_size) {                 // pick starting point of different subarrays of current size                 (int left_start=low; left_start<high; left_start += 2*curr_size) {                     // find ending point of left subarray. mid+1 starting                      // point of right                     int mid = left_start + curr_size - 1;                     int right_end = min(left_start + 2*curr_size - 1, high);                     // merge subarrays arr[left_start...mid] & arr[mid+1...right_end]                     merge(a, b, left_start, mid, right_end);                 }             }         }     } } 

the output sorted array if doesn't segfault. valgrind there 1 instance of "invalid write of size 4" error: https://puu.sh/s7zb2.png

here's merge function i'm using:

void merge(int *a, int *b, int low, int pivot, int high) {     cout << "merging " << low << " " << high << endl;     int h, i, j, k;     h = low;     = low;     j = pivot + 1;      while ((h <= pivot) && (j <= high)) {         if (a[h] <= a[j]) {             b[i] = a[h]; // store element of left half in temporary array             h++; // shift index of array element copied temporary          }         else {             b[i] = a[j];             j++; // shift index of array element copied temporary          }         i++;     }     if (h > pivot) {         (k = j; k <= high; k++) {             b[i] = a[k];             i++;         }     } else {         (k = h; k <= pivot; k++) {             b[i] = a[k];             i++;         }     }     (k = low; k <= high; k++) a[k] = b[k]; // recopy values temporary original array. } 

what's going wrong? have feeling it's cause of merge function trying access value it's not supposed to, can't seem locate happening.

thanks