Thủ thuật và cách gỡ lỗi macro

#define DOI_CHO(a, b) do { int _t = (a); (a) = (b); (b) = _t; } while (0)

int _t = 5, y = 10;
DOI_CHO(_t, y);              /* im lang khong lam gi, Bai 54.1 */

/* Cach chua: sinh ten bien theo SO DONG */

#define NOI2(a, b) a##b
#define NOI(a, b)  NOI2(a, b)          /* hai tang, Bai 54.2 */
#define DUY_NHAT(x) NOI(x, __LINE__)

#define DOI_CHO(a, b) do {                              \
    int DUY_NHAT(_dc) = (a);                            \
    (a) = (b);                                          \
    (b) = DUY_NHAT(_dc);                                \
} while (0)

int _t = 5, y = 10;
DOI_CHO(_t, y);              /* sinh bien "_dc42" neu o dong 42 */

/* Mau "mo, lam viec, tu dong dong" bang mot vong lap chay mot lan */

#define VOI_TEP(bien, ten, che_do)                                  \
    for (FILE *bien = fopen(ten, che_do), *DUY_NHAT(_c) = bien;     \
         DUY_NHAT(_c);                                              \
         fclose(bien), DUY_NHAT(_c) = NULL)

VOI_TEP(f, "a.txt", "r") {
    char dong[256];
    while (fgets(dong, sizeof dong, f)) fputs(dong, stdout);
}
/* fclose tu chay khi ra khoi khoi */

/* CANH BAO ba dieu:
     1. "break" trong khoi se thoat vong lap MA KHONG chay fclose
     2. "return" trong khoi cung vay
     3. fopen that bai thi bien la NULL va khoi khong chay,
        nen ban khong biet la co loi

   Nen mau nay tien nhung NGUY HIEM. C khong co destructor,
   va moi no bat chuoc deu co lo hong.

   Cach an toan hon: goto va mot nhan don dep, Bai 20.6. */

/* 1. _Generic, C11 */

#define KIEM_KIEU(x, T) \
    _Static_assert(_Generic((x), T: 1, default: 0), "sai kieu")

void f(int x) {
    KIEM_KIEU(x, int);       /* dat */
    KIEM_KIEU(x, double);    /* error: static assertion failed: "sai kieu" */
}

/* 2. __builtin_types_compatible_p, GNU */

#define CUNG_KIEU(a, b) \
    __builtin_types_compatible_p(__typeof__(a), __typeof__(b))

_Static_assert(CUNG_KIEU(x, y), "hai bien phai cung kieu");

/* 3. Thu thuat "1 ? a : b", chuan hoan toan */

#define EP_AN_TOAN(T, x) ((T)(1 ? (x) : (T)0))

/* Neu x khong chuyen duoc sang T thi hai nhanh cua ?: khong co
   kieu chung, va trinh bien dich bao loi.

   Bai 53.3 da dung ky thuat nay cho container_of. */

/* 4. Kiem tra con tro khong phai void * */

#define KHONG_VOID(p) \
    _Static_assert(!__builtin_types_compatible_p(__typeof__(p), void *), \
                   "can con tro co kieu")

# 1. Xem ma sau khi tien xu ly
gcc -E file.c                  # ke ca #line va noi dung header
gcc -E -P file.c               # BO dong #line, de doc hon
gcc -E -P file.c | grep -A5 MACRO_CUA_BAN

# Meo: dua qua mot bo dinh dang cho de doc
gcc -E -P file.c | clang-format

# 2. Liet ke MOI macro da dinh nghia
gcc -dM -E file.c              # ke ca macro cua he thong
gcc -dM -E - < /dev/null       # chi macro dinh nghia san
gcc -dM -E file.c | grep MY_   # loc theo tien to

# 3. Canh bao khi dung macro chua dinh nghia trong #if
gcc -Wundef file.c
# warning: "CO_TINH_NANG" is not defined, evaluates to 0

# 4. Trong GDB, voi -g3
(gdb) info macro TOI_DA
(gdb) macro expand BINH(3)
(gdb) print TOI_DA

$ gcc -g3 -O0 -o prog prog.c
$ gdb -q ./prog

(gdb) info macro TOI_DA
Defined at /home/a/prog.c:3
#define TOI_DA 100

(gdb) macro expand BINH(3)
expands to: ((3) * (3))

(gdb) print TOI_DA
$1 = 100

/* Voi -g thuong thi ca ba lenh nay deu bao "no symbol".
   Chi -g3 moi luu thong tin macro. Bai 35.1. */