zz:do{}while(0)的意义

csdn's FAQ:

一直觉得Linux里面，那些do{}while(0)只是为了程序的源代码看起来比较好看而已  
今天据说他是有特殊做用的，在线请教，是什么做用？  
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是为了解决使用宏的时候烦人的分号问题。  
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楼说的不是很全面，我给个例子吧  
 
#define  wait_event(wq,condition)  /  
 
do{  if(condition)  break;  __wait_event(wq,condition);  }while(0)  
 
这是一个奇怪的循环，它根本就只会运行一次，为何不去掉外面的do{..}while结构呢？我曾一度在内心把它叫作“怪圈”。原来这也是很是巧妙的技巧。在工程中可能常常会引发麻烦，而上面的定义可以保证这些麻烦不会出现。下面是解释：  
 
假设有这样一个宏定义  
 
#define  macro(condition)  if(condition)  dosomething();  
 
如今在程序中这样使用这个宏：  
 
if(temp)  
             macro(i);  
else  
             doanotherthing();  
 
一切看起来很正常，可是仔细想一想。这个宏会展开成：  
 
if(temp)  
             if(condition)  dosomething();  
else    
             doanotherthing();  
 
这时的else不是与第一个if语句匹配，而是错误的与第二个if语句进行了匹配，编译经过了，可是运行的结果必定是错误的。  
 
为了不这个错误，咱们使用do{….}while(0)  把它包裹起来，成为一个独立的语法单元，从而不会与上下文发生混淆。同时由于绝大多数的编译器都可以识别do{…}while(0)这种无用的循环并进行优化，因此使用这种方法也不会致使程序的性能下降。  
 
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但是直接用{}括起来的话，最后的分号会引发麻烦的  
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但这样就必定要在最后加分号，不能看成表达式用了。  
唉，仍是尽可能避免用宏替换的方法，太容易出现问题了。

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FAQ FROM CSDN:

FAQ/DoWhile0

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).

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

• (from Ben Collins) It allows you to use more complex macros in conditional code. 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 likedo { ... } 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) 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();