merge.c

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

int * merge(int* array, int length) {
    if (length < 2)
        return array;

    int midpoint = length / 2;
    int* front = merge(array, midpoint);
    int* back = merge(array + midpoint, length - midpoint);

    static int* out;
    out = (int*) malloc(length * sizeof(int));
    int outIndex = 0;
    int frontIndex = 0;
    int backIndex = 0;
    while (frontIndex < midpoint && backIndex < length - midpoint) {
        if (*(front + frontIndex) < *(back + backIndex)) {
            *(out + outIndex) = *(front + frontIndex);
            frontIndex++;
        } else {
            *(out + outIndex) = *(back + backIndex);
            backIndex++;
        }
        outIndex++;
    }

    for (; frontIndex < midpoint; frontIndex++) {
        *(out + outIndex) = *(front + frontIndex);
        outIndex++;
    }

    for (; backIndex < length - midpoint; backIndex++) {
        *(out + outIndex) = *(back + backIndex);
        outIndex++;
    }
    return out;
}

int main() {
    int myArray[] = {3, 1, 4, 2, 6};
    int* result = merge(myArray, 5);
    for (int i = 0; i < 5; i++)
        printf("%d ", *(result + i));
    printf("\n");
    return 0;
}

Adapted from my Java implementation :)