do{}while(0)的意义

原文地址：http://wmnmtm.blog.163.com/blog/static/3824571420116299450188/

#define CHECK(x)  do { if ((x) < 0) return -1; } while (0)

把do while换行写，看的较明白一点，如下：

do
{
      if ((x) < 0) 
      return -1;
}
while (0)

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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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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，为什么不去掉外面的do{..}while结构呢？我曾一度在心里把它叫做“怪圈”。原来这也是非常巧妙的技巧。在工程中可能经常会引起麻烦，而上面的定义能够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();  
 
现在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，但是仔细想想。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，编译通过了，但是运行的结果一定是错误的。  
 
为了避免这个错误，我们使用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。同时因为绝大多数的编译器都能够识别do{…}while(0)这种无用的循环并进行优化，所以使用这种方法也不会导致程序的性能降低。  
 
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可是直接用{}括起来的话，最后的分号会引起麻烦的  
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但这样就一定要在最后加分号，不能当作表达式用了。  
唉，还是尽量避免用宏替换的方法，太容易出现问题了。

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FAQ FROM 优快云:

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 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) 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();