csdn's FAQ:
一直以为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
今天听说他是有特殊作用的，在线请教，是什么作用？
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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 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 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();
