DoWhile0--华硕笔试题

华硕2012校招碰到的一道题，之前在实验室讨论过，关于Linux内核里很多的宏定义都是用do{}while(0)写的，讨论没那么深入，刚好http://kernelnewbies.org/FAQ/DoWhile0这里有个总结，转过来。

Why do a lot of #defines in the kernel use do { ... } while(0)?

There are a couple of reasons:

(from Dave Miller) Empty statements give a warning from the compiler so this is why you see #define FOO do { } while(0).

空的宏定义，编译器会给warning。

(from Dave Miller) It gives you a basic block in which to declare local variables.

代码块，可以声明局部变量。

(from Ben Collins) It allows you to use more complex macros in conditional code. 

在有条件判断的地方可以应用稍微复杂点的宏定义。

Imagine a macro of several lines of code like:

#define FOO(x) \
printf("arg is %s\n", x); \
do_something_useful(x);

Now imagine using it like:

if (blah == 2)
FOO(blah);

This interprets to:

if (blah == 2)
printf("arg is %s\n", blah);
do_something_useful(blah);;

As you can see, the if then only encompasses the printf(), and the do_something_useful() call is unconditional (not within the scope of the if), like you wanted it. So, by using a block like do { ... } while(0), you would get
this:

if (blah == 2)
do {
printf("arg is %s\n", blah);
do_something_useful(blah);
} while (0);

Which is exactly what you want.

(from Per Persson)  多个语句时候 分号 也可能引发错误

As both Miller and Collins point out, you want a block statement so you can have several lines of code and declare local variables. But then the natural thing would be to just use for example:

#define exch(x,y) { int tmp; tmp=x; x=y; y=tmp; }

However that wouldn't work in some cases. The following code is meant to be an if-statement with two branches:

if (x > y)
exch(x,y);          // Branch 1
else
do_something();     // Branch 2

But it would be interpreted as an if-statement with only one branch:

if (x > y) {                // Single-branch if-statement!!!
int tmp;            // The one and only branch consists
tmp = x;            // of the block.
x = y;
y = tmp;
}
;                           // empty statement
else                        // ERROR!!! "parse error before else"
do_something();

The problem is the semi-colon (;) coming directly after the block. The
solution for this is to sandwich the block between do and while (0). Then we have a single statement with the capabilities of a block, but not considered as being a block statement by the
compiler. Our if-statement
now becomes:

if (x > y)
do {
int tmp;
tmp = x;
x = y;
y = tmp;
} while(0);
else
do_something();

(from Bart Trojanowski) gcc adds Statement-Expressions which
provide an alternative to the do-while-0 block. They provide the above mentioned benefits and are slightly more legible.

#define FOO(arg) ({         \
typeof(arg) lcl; \
lcl = bar(arg);  \
lcl;             \
})