Simple insertion sort

ch4-insertion-sort.c

#include <stdlib.h>
#include <stdio.h>
#include <string.h>

int issort(
        void *data,
        int size,
        int esize,
        int (*compare)(const void *key1, const void *key2)
    ) {

    char *a = data;
    void *key;
    int i, j;

    // Allocate storage for the key element.
    if ((key = (char *)malloc(esize)) == NULL) {
        return -1;
    }

    // Repeatedly insert a key element among the sorted elements. 
    for (j = 1; j < size; ++j) {
        memcpy(key, &a[j * esize], esize);
        i = j - 1;

        // Determine the position at which to insert the key element.
        while (i >= 0 && compare(&a[i * esize], key) > 0) {
            memcpy(&a[(i + 1) * esize], &a[i * esize], esize);
            --i;
        }

        memcpy(&a[(i + 1) * esize], key, esize);
    }

    // Free the storage allocated for sorting.
    free(key);

    return 0;
}

int compare_number(const void *key1, const void *key2) {
    const int n1 = (const int)key1;
    const int n2 = (const int)key2;

    if (n1 > n2) {
        return -1;
    } else if (n2 > n1) {
        return 1;
    }

    return 0;
}

void print_int_array(const int *arr, int size) {
    if (arr == NULL || size < 1) {
        return;
    }

    printf("SORTED DATA ARRAY\n"
           "=================\n\n");
    printf("[ ");

    for (int i = 0; i < size; ++i) {
        printf("%d%s", arr[i], (((i + 1) != size) ? ", " : ""));
    }

    printf(" ]\n");
}

int main(int argc, char *argv[]) {
    int original_data[] = { 0, 3, 5, 6, 8, 5, 4, 2 };
    int data[8];

    memcpy(data, original_data, 8);

    const int size = 8;
    const int esize = 8;
    int rc = issort(data, size, esize, compare_number);
    printf("Sort was a %s.\n\n", rc == 0 ? "SUCCESS" : "FAILURE");

    print_int_array(original_data, esize);

    exit(EXIT_SUCCESS);
}