Watchpoint, vũ khí mạnh nhất của GDB

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

typedef struct {
    char ten[8];
    int  diem;
} SinhVien;

static void dat_ten(SinhVien *sv, const char *ten) {
    strcpy(sv->ten, ten);          /* khong kiem tra do dai */
}

int main(void) {
    SinhVien sv;

    sv.diem = 100;
    printf("diem truoc = %d\n", sv.diem);

    dat_ten(&sv, "NguyenVanA");    /* 10 ky tu vao mang 8 byte */

    printf("diem sau   = %d\n", sv.diem);
    printf("ten        = %s\n", sv.ten);

    return 0;
}

set pagination off
set confirm off
break main
run
next
watch sv.diem
continue
bt
info locals
quit

watch sv.diem              # bien hoac truong
watch *p                   # thu con tro tro toi
watch p                    # chinh con tro
watch a[3]                 # mot phan tu mang
watch *(int *)0x61fe44     # mot dia chi cu the
watch g_dem if g_dem > 100 # co dieu kien, nhu diem dung

info watchpoints           # liet ke
delete 2                   # xoa, dung chung danh sach voi diem dung

(gdb) watch x
Hardware watchpoint 2: x

(gdb) continue
...
Watchpoint 2 deleted because the program has left the block
in which its expression is valid.

/* Xay ra khi x la bien CUC BO va ham chua no da tra ve.
   GDB tu xoa watchpoint vi bieu thuc "x" khong con nghia. */

(gdb) watch sv.diem
Hardware watchpoint 2: sv.diem            <- tot: dung thanh ghi phan cung

(gdb) watch bo_dem
Watchpoint 2: bo_dem                      <- KHONG co chu "Hardware":
                                             ban PHAN MEM, rat cham

/* Trieu chung: dung sau khi giai phong, nhung khong biet ai free */

(gdb) break malloc
(gdb) run
(gdb) finish                    /* lay dia chi vua cap phat */
Value returned is $1 = (void *) 0x602010

(gdb) watch -l *(int *)0x602010
(gdb) continue

/* Khi bo cap phat ghi so sach vao khoi do luc free, watchpoint dung,
   va backtrace chi thang vao lan free.

   Cach nay cham va thu cong. Neu dung duoc ASan thi dung ASan:
   no in ca ba cho trong mot bao cao. Nhung khi khong dich lai duoc
   thi day la cach duy nhat. */

/* Trieu chung: cay do den mat can bang sau vai nghin thao tac,
   nhung khong biet thao tac nao lam hong. */

(gdb) watch -l nut->mau
(gdb) commands
 >silent
 >printf "mau doi tai: "
 >bt 3
 >continue
 >end

/* Ket hop watchpoint voi lenh tu dong o Bai 35.2: ban duoc mot
   nhat ky moi lan truong do doi, kem ba khung goi, ma khong dung
   chuong trinh lan nao. */

/* Doi khi ban muon chung minh dieu nguoc lai: */

(gdb) watch g_cau_hinh
(gdb) continue
...
[Inferior 1 (process 1234) exited normally]

/* Chay het chuong trinh ma watchpoint khong dung lan nao
   -> bien do that su khong bi sua.

   Rat huu ich khi ban dang loai tru gia thuyet. */