Sơ đồ phụ thuộc và kiểu mờ

/* Ve lai thanh so do, chi xet header: */

        main.c
          |
        ui.h
          |
       quanly.h
        /     \
sinhvien.h   loi.h
      ^
      |
  luutru.h

Nguyen tac: MOI mui ten di xuong. Khong co mui ten nao di len.

/* Truoc: hai module can du lieu chung, nen chung include nhau */

quanly.h  <->  luutru.h

/* Sau: dua phan chung xuong tang duoi */

quanly.h      luutru.h
     \          /
      sinhvien.h

/* Neu hai module can biet ve nhau, thuong la vi co mot khai niem
   thu ba dang bi chia doi giua chung. Tim ra khai niem do
   va cho no mot module rieng. */

/* Van de: quanly.c muon bao cho ui.c biet khi co thay doi,
   nhung quanly la tang duoi, khong duoc biet ve ui. */

/* quanly.h: khai bao mot cho cam vao, khong nhac toi ui */
typedef void (*HamBaoThayDoi)(size_t so_luong, void *ngu_canh);

void ql_dat_bao_thay_doi(QuanLy *q, HamBaoThayDoi f, void *ngu_canh);

/* ui.c: cam ham cua minh vao */
static void khi_thay_doi(size_t n, void *ngu_canh) {
    (void)ngu_canh;
    printf("Danh sach gio co %zu sinh vien\n", n);
}

void ui_khoi_tao(QuanLy *q) {
    ql_dat_bao_thay_doi(q, khi_thay_doi, NULL);
}

/* Bay gio quanly goi nguoc len duoc MA KHONG biet ui ton tai.
   Bai 13.6 goi day la ham goi lai. */

#ifndef QUANLY_H
#define QUANLY_H

#include <stddef.h>

#include "loi.h"
#include "sinhvien.h"

typedef struct QuanLy QuanLy;      /* CHI khai bao, khong dinh nghia */

QuanLy *ql_tao(void);              /* tra ve con tro, nguoi dung khong tu tao */
void    ql_huy(QuanLy *q);

MaLoi   ql_them(QuanLy *q, const SinhVien *sv);
size_t  ql_so_luong(const QuanLy *q);            /* thay cho q->n */
const SinhVien *ql_lay(const QuanLy *q, size_t i);

#endif /* QUANLY_H */

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

#include "quanly.h"

struct QuanLy {                    /* DINH NGHIA chi o day */
    SinhVien *ds;
    size_t    n, suc_chua;
    int       da_thay_doi;
};

QuanLy *ql_tao(void) {
    QuanLy *q = calloc(1, sizeof *q);   /* calloc: moi truong bang 0 */

    return q;                            /* NULL neu that bai, nguoi goi kiem */
}

void ql_huy(QuanLy *q) {
    if (!q) return;                      /* nhan NULL vo hai, giong free */

    free(q->ds);
    free(q);
}

size_t ql_so_luong(const QuanLy *q) { return q->n; }

const SinhVien *ql_lay(const QuanLy *q, size_t i) {
    return i < q->n ? &q->ds[i] : NULL;
}