为什么使用 do {} while(0) - yanbin's Blog
为什么使用 do {} while(0)
有些宏定义含有多行代码，一般使用 do {} while(0) 把代码放在 'do' 和 'while' 之间的 '{}' 中。
#define foorbar(msg, callback) do {\
struct Task __task = msg_to_task((msg)); \
if (__task != NULL) { \
process(__task, (callback)); \
char *__msg = task_to_msg(__task); \
if (__msg != NULL) \ {
send_msg(__msg); \
free(__msg); \
} \
destroy_task(__task); \
} while (0)
这样用的原因是:
1.符合 C/C++ 语法习惯。
每条语句后面都一个';', 而 do {} while 语句后面也要加一个 ';'.
2.避免出现语法错误。
不用 do {} while(0) 把多行代码包起来，在 if else 语句中就会有语法错误，例如:
#define foorbar(a, b) foor((a)); bar((b)) if (something) /* 以下有语法错误 */ foorbar(3, 2); else // do something
仅仅使用 '{}' 把多行代码包起来，如果在调用宏之后加 ';', 也会有语法错误。
#define foorbar(a, b) {\
foor((a)); bar((b));\
}
foorbar(3, 2); // 此处有语法错误
/* 编译器提示:
* error: expected ‘;’ before ‘}’ token
* 如果不加 ';', 不会有语法错误但是这样不符合 C/C++ 的语法习惯
*/
3.do {} while(0) 可以根据条件跳出执行。
#define foorbar() do {\
struct condition __cond; \
if (__cond.wait_cond()) \
break; // 条件发生退出执行 \
// 条件没有发生 do something
} while(0)
4.私以为 do {} while(0) 可以保证代码执行并且只执行一次。
5.需要注意的地方。
(a)宏定义时用 '\' 连接多行语句;
(b)宏定义中定义变量，注意与外部变量名字冲突，不然原本希望用外面的变量，却用了新定义的变量。
(c)有些编译器会优化掉 do {} while(0); 直接展开 '{}' 内的代码, 如(b)所描述，此时会出现语法错误。
FIXME: 如果内部有 'break' 并且 'break' 的执行依赖运行时条件，编译器就不会优化掉 do {} while(0); 了。
举例:
#define foorbar() do {\
struct condition cond; \
if (cond.wait_cond()) \
break; // 条件发生退出执行 \
// 条件没有发生 do something
} while(0)
struct condition cond;
// do something
foobar(); // 到底用的是哪一个 cond?
#define foorbar(a, b) do {\
const char *something = get_something(a, b); \
} while(0)
const char *something;
// do something
foorbar(3, 9); // 如果编译器优化掉了 do {} while(0); 这里有语法错误。
感谢 老猫，mike2，MovableType@源赖朝 三位网友。
参考:
do { … } while (0) — what is it good for?
do{}while(0) 的作用
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