merge sort by C

my_msort.c

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

#define DATA_FILE "./rand.txt"
#define N 1000000

void my_msort(int* ary, int stack_level, int sidx, int eidx) {
    int i, j, tmp;
    int len = eidx - sidx + 1;

    if (len <= 20 || 30 <= stack_level) {
        // do bubble sort
        for (i = sidx; i <= eidx; i++) {
            for (j = i+1; j <= eidx; j++) {
                if (ary[j] < ary[i]) {
                    tmp = ary[i];
                    ary[i] = ary[j];
                    ary[j] = tmp;
                }
            }
        }
    } else {
        int half_len = len / 2;
        int slen = half_len;
        int glen = len-slen;

        int s_sidx = sidx;
        int s_eidx = sidx + slen - 1;

        int g_sidx = sidx + slen;
        int g_eidx = eidx;

        my_msort(ary, stack_level+1, s_sidx, s_eidx);
        my_msort(ary, stack_level+1, g_sidx, g_eidx);

        int* smallers = (int*)malloc(sizeof(int)*slen);
        int* greaters = (int*)malloc(sizeof(int)*glen);

        memcpy(smallers, ary+s_sidx, sizeof(int)*slen);
        memcpy(greaters, ary+g_sidx, sizeof(int)*glen);

        // merge
        i = sidx;
        int si = 0, gi = 0;
        while (i <= eidx) {
            if (si < slen) {
                if (gi < glen) {
                    if (smallers[si] < greaters[gi]) {
                        ary[i++] = smallers[si++];
                    } else {
                        ary[i++] = greaters[gi++];
                    }
                } else {
                    ary[i++] = smallers[si++];
                }
            } else if (gi < glen) {
                ary[i++] = greaters[gi++];

            }
        }

        free(smallers);
        free(greaters);
    }
}

int main(int argc, char** argv) {
    int i;
    int* vec;
    char buf[128];
    FILE* fp = fopen(DATA_FILE, "r");

    vec = (int*)malloc(sizeof(int) * N);
    for (i = 0; i < N; i++) {
        fgets(buf, 128, fp);
        vec[i] = atoi(buf);
    }
    fclose(fp);

    //my_qsort(vec, 1, 0, N-1);
    my_msort(vec, 1, 0, N-1);

    for (i = 0; i < N; i++) {
        printf("%d\n", vec[i]);
    }
    free(vec);
}