FAQ/DoWhile0 - Linux Kernel Newbies

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 [http://gcc.gnu.org/onlinedocs/gcc-4.1.1/gcc/Statement-Exprs.html#Statement-Exprs 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;             \
    })
```