DoWhile0

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 1else          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 statementelse                        // 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;             \    })