想问一下在Linux内核码中的do{...}while(0)字段是什么意思啊?有没有特殊的意义

jetwins

   

skykufo

在http://www.kernelnewbies.org/faq/说：
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();

eax

没有特殊的定义，你会发现这是在宏定义里用的，因为宏在展开时会遇到一些复杂难料的情况，所以用do while(0)包括起来，作为一个独立的部分加以保护。