do while(0)宏定义巧用

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();

(from Bart Trojanowski) gcc adds Statement-Expressions which provide an alternative to the do-while-0 block. They provide the above mentioned benefits and are slightly more legible.

#define FOO(arg) ({         \

           typeof(arg) lcl; \

           lcl = bar(arg);  \

           lcl;             \

    })

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