Kiểu hiệu dụng

/* 1. Doi tuong CO KHAI BAO: kieu hieu dung la kieu khai bao,
      va no KHONG BAO GIO doi */

int x;                          /* kieu hieu dung = int, vinh vien */
float *p = (float *)&x;
*p = 1.0f;                      /* UB: ghi float vao doi tuong kieu int */

/* 2. Bo nho CAP PHAT: chua co kieu hieu dung */

void *m = malloc(sizeof(int));  /* chua co kieu */
*(int *)m = 5;                  /* kieu hieu dung gio la int */
float f = *(float *)m;          /* UB: doc int qua float * */

/* 3. Ghi lai bang kieu khac: kieu hieu dung DOI */

*(float *)m = 1.0f;             /* kieu hieu dung gio la float */
float g = *(float *)m;          /* HOP LE */
int   i = *(int *)m;            /* UB */

/* Neu bo nho tu malloc CO kieu co dinh, thi khong the viet duoc
   mot bo cap phat, mot be nho, hay bat cu thu gi tai su dung bo nho.

   Chuan giai quyet bang cach noi: bo nho tu malloc co kieu hieu dung
   duoc XAC DINH LAI moi khi ban ghi vao no bang mot kieu khac.

   Nen doan nay hoan toan hop le: */

void *m = malloc(1024);

*(int *)m = 5;              /* kieu hieu dung: int */
int a = *(int *)m;          /* hop le */

*(double *)m = 3.14;        /* kieu hieu dung: gio la double */
double b = *(double *)m;    /* hop le */

SinhVien *sv = m;
*sv = (SinhVien){ .ma = 1 };  /* kieu hieu dung: gio la SinhVien */

free(m);

/* Ba lan dung lai cung mot vung nho voi ba kieu khac nhau.
   Do la ca diem cua "chua co kieu". */

#include <string.h>

/* C11: kiem tra kich thuoc luc dich */
#define DOI_BIT(T, x) __extension__({                              \
    _Static_assert(sizeof(T) == sizeof(x), "kich thuoc khong khop"); \
    T _kq;                                                          \
    memcpy(&_kq, &(x), sizeof _kq);                                 \
    _kq;                                                            \
})

float f = 1.0f;
uint32_t bits = DOI_BIT(uint32_t, f);

/* Macro nay dung bieu thuc khoi cua GNU, nen chi chay tren
   GCC va Clang. Ban chuan phai la mot ham: */

static inline uint32_t float_sang_bit(float f) {
    uint32_t u;
    memcpy(&u, &f, sizeof u);
    return u;
}

static inline float bit_sang_float(uint32_t u) {
    float f;
    memcpy(&f, &u, sizeof f);
    return f;
}

/* Va tu GCC 11 hoac Clang 14: */
uint32_t bits = __builtin_bit_cast(uint32_t, f);   /* ro nghia nhat */

unsigned char bo_dem[1024];
fread(bo_dem, 1, sizeof bo_dem, f);

/* SAI: ca ve trung bi danh lan ve can chinh */
TieuDe *td = (TieuDe *)bo_dem;

/* DUNG */
TieuDe td;
memcpy(&td, bo_dem, sizeof td);

/* Nhung chu y: cach nay van phu thuoc vao BO CUC struct cua may ban.
   Bai 53.4 se noi ve cach tuan tu hoa tung truong, la cach duy nhat
   thuc su kha chuyen. */

/* Ham bam nhan mot doi tuong bat ky va doc byte cua no */

uint64_t bam(const void *du_lieu, size_t n) {
    const unsigned char *p = du_lieu;    /* HOP LE, ngoai le char */
    uint64_t h = 14695981039346656037ULL;

    for (size_t i = 0; i < n; ++i) {
        h ^= p[i];
        h *= 1099511628211ULL;
    }
    return h;
}

/* Doc qua unsigned char * la HOP LE voi moi doi tuong.
   Do la ly do ham bam kieu nay viet duoc bang C chuan.

   NHUNG: dung bam mot struct co PHAN DEM, vi byte dem
   khong co gia tri xac dinh. Bai 53.1. */

#include <stdlib.h>
#include <stddef.h>

typedef struct {
    unsigned char *bo_nho;      /* tu malloc, nen CHUA co kieu */
    size_t         co;
    size_t         da_dung;
} Be;

int be_tao(Be *b, size_t co) {
    b->bo_nho = malloc(co);      /* quan trong: malloc, khong phai mang tinh */
    if (b->bo_nho == NULL) return -1;
    b->co = co;
    b->da_dung = 0;
    return 0;
}

void *be_cap(Be *b, size_t n, size_t can_chinh) {
    size_t vi_tri = (b->da_dung + can_chinh - 1) / can_chinh * can_chinh;

    if (vi_tri + n > b->co) return NULL;

    b->da_dung = vi_tri + n;
    return b->bo_nho + vi_tri;
}

/* Dung: */
SinhVien *sv = be_cap(&b, sizeof *sv, _Alignof(SinhVien));
if (sv != NULL) *sv = (SinhVien){ .ma = 1 };   /* dat kieu hieu dung */

/* Vi bo nho den tu malloc, no chua co kieu hieu dung, nen lan ghi
   nay hop le. Neu "bo_nho" la mot mang tinh thi day la vung xam. */