Quy trình sáu bước và tệp .gdbinit

# Co dinh cac yeu to ngau nhien de loi tai hien duoc

# 1. So ngau nhien: dat hat co dinh
srand(12345);              /* thay cho srand(time(NULL)) */

# 2. Dia chi cap phat: tat ngau nhien hoa khong gian dia chi
setarch -R ./prog          # Linux
(gdb) set disable-randomization on    # GDB tat san, mac dinh

# 3. Bo nho chua khoi tao: bat cho no mot mau co dinh
MALLOC_PERTURB_=42 ./prog  # glibc: dien 42 vao bo nho moi cap
                           # va 213 vao bo nho vua free

# 4. Da luong: giam so luong
./prog --so-luong=1        # neu chuong trinh cho phep

# 5. Moi truong: chay trong moi truong toi thieu
env -i ./prog

# Tep dau vao 10000 dong lam chuong trinh sap.

head -5000 vao.txt > a.txt && ./prog a.txt     # sap? -> loi o nua dau
tail -5000 vao.txt > b.txt && ./prog b.txt     # sap? -> loi o nua sau

# Lap lai. Sau 14 lan chia doi tu 10000 dong, ban con 1 dong.

# Cong cu tu dong hoa viec nay:
#   ddmin, delta debugging
#   creduce, cvise: rut gon MA NGUON C giu nguyen mot trieu chung

# "Thang truoc con chay dung, gio thi sap." Git tim ho ban:

git bisect start
git bisect bad                    # ban hien tai co loi
git bisect good v1.2.0            # ban nay chay dung

# Git checkout mot ban o giua. Ban chay thu roi bao:
git bisect good                   # hoac
git bisect bad

# Lap lai. Voi 1000 ban ghi, chi can 10 lan thu.

git bisect reset                  # xong, quay ve nhanh cu

# Tu dong hoan toan, neu ban co mot lenh kiem tra:
git bisect run ./kiem_tra.sh      # tra ve 0 la tot, khac 0 la loi

/* Xoa dan cho toi khi loi bien mat. Moi buoc phai kiem tra lai. */

/* Vong 1: bo mot nua so ham goi trong main */
/* Vong 2: bo mot nua con lai */
/* ...

   Cach nay tho nhung hieu qua. Khi ban con hai muoi dong ma van
   sap, viec doc ma tro nen de dang.

   Meo: dung "#if 0 ... #endif" thay vi xoa that, de con khoi phuc.
   Bai 19.6 da noi. */

/* Va mot bien the tinh te hon: chia doi CO DICH */

gcc -O2 ...        # sap
gcc -O1 ...        # chay dung  -> loi lien quan toi toi uu o -O2
gcc -O2 -fno-strict-aliasing ...   # chay dung -> tim ra thu pham

/* Dung khung kiem thu ba macro cua Bai 32.4 */

#include "test.h"
#include "sinhvien.h"

static void thu_loi_ghi_de(void) {
    NHOM("loi #142: ten qua dai ghi de len diem");

    SinhVien sv = { "", 100 };

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

    KIEM_BANG(sv.diem, 100);           /* diem KHONG duoc doi */
    KIEM_BANG(sv.ten[7], 0);           /* co ky tu ket thuc chuoi */
    KIEM_CHUOI(sv.ten, "NguyenV");     /* bi cat dung bay ky tu */
}

# --- hien thi ---
set print pretty on              # struct xuong dong, thut le
set print array on               # mang moi phan tu mot dong
set print array-indexes on       # kem chi so: [0] = 5
set print elements 200           # so phan tu toi da in ra
set print repeats 10             # gom gia tri lap lai
set print null-stop on           # chuoi dung o ky tu ket thuc
set print union on               # in ca thanh vien union

# --- tien lam viec ---
set pagination off               # khong hoi "Press enter to continue"
set confirm off                  # khong hoi lai khi quit hay delete
set history save on              # nho lenh giua cac phien
set history size 10000
set history filename ~/.gdb_history

# --- hop ngu ---
set disassembly-flavor intel     # cu phap Intel, de doc hon AT&T

# --- go loi ---
set disable-randomization on     # mac dinh da bat, ghi ro cho chac
set backtrace past-main off      # khong hien ruot cua libc truoc main

# --- cho phep nap .gdbinit trong thu muc du an ---
add-auto-load-safe-path ~/duan

# --- khoi dong nhanh ---
define bm
  break main
  run
end
document bm
Dat diem dung o main va chay ngay.
end

# --- in danh sach lien ket ---
define in_ds
  set $p = $arg0
  set $i = 0
  while $p != 0
    printf "[%d] ma=%-4d ten=%s\n", $i, $p->ma, $p->ten
    set $p = $p->sau
    set $i = $i + 1
  end
  printf "tong cong %d nut\n", $i
end
document in_ds
In toan bo danh sach lien ket. Vi du: in_ds dau
end

# --- in mot sinh vien cho de doc ---
define in_sv
  printf "SinhVien{ ma=%d ten=%s diem=%d }\n", \
         $arg0->ma, $arg0->ten, $arg0->diem
end

# --- do bo nho mot khoi ---
define do_khoi
  printf "=== %d byte tu %p ===\n", $arg1, $arg0
  x/$arg1xb $arg0
end
document do_khoi
Do n byte tu mot dia chi. Vi du: do_khoi p 64
end

# --- ngu canh day du khi dung ---
define nc
  printf "=== vet goi ===\n"
  bt
  printf "=== bien cuc bo ===\n"
  info locals
  printf "=== tham so ===\n"
  info args
end
document nc
In vet goi, bien cuc bo va tham so cua khung hien tai.
end

# Trong .gdbinit hoac mot tep .py rieng

python
import gdb

class InDanhSach(gdb.Command):
    """In danh sach lien ket. Cu phap: in_ds <nut_dau>"""

    def __init__(self):
        super().__init__("in_ds", gdb.COMMAND_USER)

    def invoke(self, doi_so, tu_man_hinh):
        p = gdb.parse_and_eval(doi_so)
        i = 0

        while p != 0:
            print(f"[{i}] ma={int(p['ma']):<4} ten={p['ten'].string()}")
            p = p["sau"]
            i += 1

            if i > 10000:
                print("qua 10000 nut, co the co vong")
                break

        print(f"tong cong {i} nut")

InDanhSach()
end

# Python trong GDB con lam duoc:
#   - pretty printer: doi cach GDB hien mot kieu do ban dinh nghia
#   - doc va ghi file, goi ra ngoai
#   - phan tich cau truc du lieu phuc tap
#
# Moi ban GDB hien dai deu co san Python. Kiem tra:
#   (gdb) python print(gdb.VERSION)