do{}while(0)的意義

一直以爲Linux裏面，那些do{}while(0)只是爲了程序的源代碼看起來比較好看而已  

今天聽說他是有特殊作用的，在線請教，是什麼作用？  
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是爲了解決使用宏的時候煩人的分號問題。  
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樓說的不是很全面，我給個例子吧  
 
#define  wait_event(wq,condition)  /  
 
do{  if(condition)  break;  __wait_event(wq,condition);  }while(0)  
 
這是一個奇怪的循環，它根本就只會運行一次，爲什麼不去掉外面的do{..}while結構呢？我曾一度在心裏把它叫做“怪圈”。原來這也是非常巧妙的技巧。在工程中可能經常會引起麻煩，而上面的定義能夠保證這些麻煩不會出現。下面是解釋：  
 
假設有這樣一個宏定義  
 
#define  macro(condition)  if(condition)  dosomething();  
 
現在在程序中這樣使用這個宏：  
 
if(temp)  
             macro(i);  
else  
             doanotherthing();  
 
一切看起來很正常，但是仔細想想。這個宏會展開成：  
 
if(temp)  
             if(condition)  dosomething();  
else    
             doanotherthing();  
 
這時的else不是與第一個if語句匹配，而是錯誤的與第二個if語句進行了匹配，編譯通過了，但是運行的結果一定是錯誤的。  
 
爲了避免這個錯誤，我們使用do{….}while(0)  把它包裹起來，成爲一個獨立的語法單元，從而不會與上下文發生混淆。同時因爲絕大多數的編譯器都能夠識別do{…}while(0)這種無用的循環並進行優化，所以使用這種方法也不會導致程序的性能降低。  
 
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可是直接用{}括起來的話，最後的分號會引起麻煩的  
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但這樣就一定要在最後加分號，不能當作表達式用了。  
唉，還是儘量避免用宏替換的方法，太容易出現問題了。

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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();