Level 2: Quản lý sinh viên

typedef struct {
    char  mssv[16], ho_ten[64], lop[16];
    int   nam_sinh;
    char  gioi_tinh;
    float diem[5], diem_tb;
    char  xep_loai[16];
} SinhVien;

typedef struct { SinhVien *pt; size_t co, suc; } DsSV;

static int ds_them(DsSV *d, SinhVien sv) {
    if (d->co == d->suc) {
        size_t moi = d->suc ? d->suc * 2 : 4;       /* gap doi, bat dau 4 */
        SinhVien *p = realloc(d->pt, moi * sizeof *p);
        if (!p) return -1;                          /* het bo nho, GIU cai cu */
        d->pt = p;
        d->suc = moi;
    }
    d->pt[d->co++] = sv;
    return 0;
}

static int theo_diem_giam(const void *a, const void *b) {
    float x = ((const SinhVien *)a)->diem_tb;
    float y = ((const SinhVien *)b)->diem_tb;
    return (x < y) - (x > y);           /* giam dan, KHONG dung x - y */
}

static int theo_ten(const void *a, const void *b) {
    return strcmp(((const SinhVien *)a)->ho_ten,
                  ((const SinhVien *)b)->ho_ten);
}

/* Dung: */
qsort(d.pt, d.co, sizeof *d.pt, theo_diem_giam);
qsort(d.pt, d.co, sizeof *d.pt, theo_ten);