Question

以下代码来自include/linux/typecheck.h：

/*
 * Check at compile time that something is of a particular type.
 * Always evaluates to 1 so you may use it easily in comparisons.
 */
#define typecheck(type,x) \
({  type __dummy; \
    typeof(x) __dummy2; \
    (void)(&__dummy == &__dummy2); \
    1;                                        \\ <---- Why here is a 1;?
})

/*
 * Check at compile time that 'function' is a certain type, or is a pointer
 * to that type (needs to use typedef for the function type.)
 */
#define typecheck_fn(type,function) \
({  typeof(type) __tmp = function; \
    (void)__tmp; \
})

1;会有什么不同吗？另外，块如何“评估为1”？

Answer 1

宏是一个语句表达式：https://gcc.gnu.org/onlinedocs/gcc/Statement-Exprs.html

最终表达式的值是宏“返回”的值。

如果typecheck不是x，则type宏会导致编译时警告。假设我宣布char *a然后尝试typecheck(char, a)并使用gcc -Wall编译：

1.c: In function 'main':
1.c:5:21: warning: comparison of distinct pointer types lacks a cast [enabled by default]
     (void)(&__dummy == &__dummy2); \
                     ^
1.c:14:2: note: in expansion of macro 'typecheck'
  typecheck(char, a);
  ^

了解linux内核中的类型检查

1 个答案: