How to use sort an array with struct entries using mergesort?

This is the code that I have written for the merge function

void merge(struct record *record_arr, int low, int mid, int high) {
    int i, j, k;
    int n1 = mid - low + 1;
    int n2 = high - mid;
    struct record *tmp1 = (struct record *)allocate_memory((n1 + 1) * sizeof(struct record));
    struct record *tmp2 = (struct record *)allocate_memory((n2 + 1) * sizeof(struct record));
    for (i = 0; i <= n1; i++) {
        tmp1[i] = record_arr[low + i];
    }
    for (j = 0; j <= n2; j++) {
        tmp2[j] = record_arr[mid + 1 + j];
    }
    tmp1[i + 1] = (struct record){ 0 };
    tmp2[j + 1] = (struct record){ 0 };
    i = j = 0;
    for (k = low; k <= high; k++) {
        if ((cmp_record(tmp1 + i, tmp2 + j) == 0) || (cmp_record(tmp1 + i, tmp2 + j) == -1)) {
            record_arr[k] = tmp1[i];
            i++;
        } else {
            record_arr[k] = tmp2[j];
            j++;
        }
    }
}

After implementing it, the array is not being sorted and I am getting this error

Running TEST2 for 10 inputs
array is not sorted
test2: lib.c:527: check_array_is_sorted_by_uid: Assertion `0' failed.
Aborted

Your implementation relies on an error prone technique called sentinels where a value known to be larger than all valid values is inserted at the end of the slices. This is not a good idea, you should instead compare the index values and only compare the structures inside the slices.

Note also that only the left slice needs saving, structures in the right slice are never overwritten before they are compared.

Furthermore, you call the comparison function twice and test for explicit return values 0 and -1. You should just test for <= 0 to determine which structure to copy.

Memory allocated for the copies must be freed at the end of the merge function, otherwise the memory is lost.

Here is a modified version:

void mergesort(struct record *record_arr, int low, int high) {
    if (low < high) {
        int mid = low + (high - low) / 2;
        mergesort(record_arr, low, mid);
        mergesort(record_arr, mid + 1, high);
        merge(record_arr, low, mid, high);
    }
}

void merge(struct record *record_arr, int low, int mid, int high) {
    int i, j, k;
    int n1 = mid - low + 1;
    int n2 = high - mid;

    // save structures from the left slice
    struct record *tmp1 = (struct record *)allocate_memory(n1 * sizeof(struct record));
    for (i = 0; i < n1; i++) {
        tmp1[i] = record_arr[low + i];
    }

    i = 0;
    j = mid + 1;
    k = low;
    while (i < n1 && j <= high) {
        if (cmp_record(tmp1 + i, record_arr + j) <= 0) {
            record_arr[k++] = tmp1[i++];
        } else {
            record_arr[k++] = record_arr[j++];
        }
    }
    while (i < n1) {
        record_arr[k++] = tmp1(i++];
    }
    free_memory(tmp1);
}