Mười sáu hành vi không xác định phải thuộc

int a[5];

a[5]  = 1;      /* UB: chi so hop le la 0..4 */
a[-1] = 1;      /* UB */
a[10] = 1;      /* UB */

int *p = &a[5];       /* HOP LE: dia chi ngay sau phan tu cuoi
                         duoc phep TINH RA de lam moc dung vong lap */
*p = 1;               /* UB: nhung KHONG duoc doc hay ghi qua no */

int *q = &a[6];       /* UB: ngay ca viec TINH dia chi nay cung sai */

int *p = NULL;

*p = 10;              /* UB, gan nhu chac chan sap */

if (p != NULL) *p = 10;   /* dung */

/* Chu y: kiem tra PHAI dat truoc, khong duoc dat sau.
   Bai 33.2 da cho thay kiem tra dat sau bi xoa. */

unsigned char *p = malloc(16);
memcpy(p, "xinchao!", 8);

hien("truoc free :", p);
free(p);

hien("sau free   :", p);        /* 1. dung sau khi giai phong */

unsigned char *q = malloc(16);
memcpy(q, "AAAAAAAA", 8);

hien("q moi      :", q);
hien("p doc lai  :", p);        /* 2. p va q co trung dia chi khong? */

printf("p = %p, q = %p, trung nhau: %s\n",
       (void *)p, (void *)q, p == q ? "CO" : "KHONG");

free(p);                        /* 3. giai phong hai lan */
printf("qua duoc double free\n");

free(q);

int x;
printf("%d", x);          /* UB */

int a[10];
int t = 0;
for (int i = 0; i < 10; ++i) t += a[i];   /* UB */

/* Khong phai "gia tri rac". Chuan goi day la gia tri BAT DINH,
   va doc no la UB neu bien co the nam trong thanh ghi.

   Hau qua that: trinh bien dich duoc phep cho x mang hai gia tri
   khac nhau o hai cho doc khac nhau trong cung mot ham. */

int x;
if (x == x) puts("bang");    /* KHONG bao dam in ra */

/* Cach dung: luon khoi tao. */
int x = 0;
int a[10] = { 0 };

INT_MAX + 1               /* UB */
INT_MIN - 1               /* UB */
-INT_MIN                  /* UB: |INT_MIN| khong bieu dien duoc */
INT_MIN / -1              /* UB: ket qua 2147483648 khong lot vao int.
                             Tren x86, lenh idiv sinh ngoai le phan cung. */
abs(INT_MIN)              /* UB, cung ly do */

UINT_MAX + 1u             /* HOP LE: = 0, so khong dau quan vong */

/* Do la ly do dung size_t va unsigned cho chi so va kich thuoc:
   khong co UB tran. Nhung phai canh giac voi phep tru: */

size_t n = 3, m = 5;
if (n - m > 0) { }        /* LUON dung: 3 - 5 quan vong thanh so rat lon */
if (n > m) { }            /* dung */

x / 0;    x % 0;         /* UB */

/* Voi so THUC thi khac: 1.0 / 0.0 cho ra inf, va do la hanh vi
   duoc dinh nghia neu cai dat theo IEEE 754. */

double d = 1.0 / 0.0;    /* inf, hop le tren gan nhu moi may */
double n = 0.0 / 0.0;    /* NaN, hop le */

__attribute__((noinline)) unsigned dich(unsigned x, int n) { return x << n; }

int main(void) {
    printf("dich(1, 32) luc chay = %u\n", dich(1, 32));
    printf("dich(1, 33) luc chay = %u\n", dich(1, 33));
    printf("dich(1, 64) luc chay = %u\n", dich(1, 64));
    printf("(-1) << 1 = %d\n", -1 << 1);
    return 0;
}

i = i++ + ++i;            /* UB */
a[i] = i++;               /* UB */
printf("%d %d", i++, i++);  /* UB */

/* Bai 33.5 danh ca bai cho chuyen nay. */

int *f(void) {
    int x = 42;
    return &x;            /* UB: x chet khi ham tra ve */
}

/* GCC o -O2 thay luon dia chi do bang NULL. Ban mong doi mot dia chi
   rac va nhan duoc 0.

   Cach dung: cap phat dong, hoac de nguoi goi cap bo dem. */

int *f(void) {
    int *p = malloc(sizeof *p);
    if (p) *p = 42;
    return p;                       /* nguoi goi phai free */
}

void f(int *ra) { *ra = 42; }       /* thuong tot hon: khong ai phai free */

int a[5], b[5];

if (a < b) { }            /* UB: hai mang khong lien quan */
if (a == b) { }           /* HOP LE: == va != luon dung duoc */

if (&a[0] < &a[3]) { }    /* HOP LE: cung mot mang */

/* Trong thuc te thi so sanh < gan nhu luon "chay dung" vi no
   thanh mot lenh cmp tren dia chi. Nhung chuan khong bao dam,
   va tren may co bo nho phan doan thi no that su sai. */

char *s = "abc";
s[0] = 'A';               /* UB: chuoi hang nam trong .rodata */

const char *s = "abc";    /* dung: kieu noi ro la chi doc */

char s[] = "abc";         /* dung: day la MANG, sao chep tu chuoi hang */
s[0] = 'A';               /* hop le */

/* Bat -Wwrite-strings de trinh bien dich bat ho ban. No khong nam
   trong -Wall vi no gay on voi ma cu. */

float f = 1.0f;
int   i = *(int *)&f;     /* UB. Bai 33.6 noi ky */

/* Cach dung: memcpy hoac union. Ca hai deu KHONG ton them lenh nao. */

int f(int x) {
    if (x) return 1;
}                          /* UB khi x == 0 */

/* GCC rut ca ham thanh "mov eax, 1; ret". Nen f(-5) tra ve 1.

   Ngoai le duy nhat: main khong can return, chuan quy dinh
   no ngam tra ve 0. */

printf("%d", 3.14);        /* UB */
printf("%s", 42);          /* UB, gan nhu chac chan sap */
printf("%d", (long)5);     /* UB tren may co long 64 bit */
printf("%s");              /* UB: thieu doi so */

scanf("%d", x);            /* UB: quen dau & */

/* GCC bat gan het nhung loi nay voi -Wformat, nam trong -Wall.
   Bat -Wformat=2 de nghiem hon. */

char a[20] = "0123456789";

memcpy(a, a + 1, 9);       /* UB: hai vung chong nhau */
memmove(a, a + 1, 9);      /* dung */

/* Nguyen mau noi het:

     void *memcpy(void *restrict d, const void *restrict s, size_t n);
     void *memmove(void *d, const void *s, size_t n);

   memcpy co restrict, tuc no HUA hai vung khong chong. memmove khong co,
   nen no ho tro chong nhau. Bai 32.3 da noi ky.

   Trong thuc te memcpy voi vung chong nhau thuong van "chay dung"
   voi n nho, roi hong khi n du lon de trinh bien dich vector hoa. */