why do we use while (0)?

why do we use while (0)?
2007-12-26 16:02:05

今天在Linux Kernel Newbie的Mail List上看到一个问题,原文如下:

Recently I started looking into linux kernel and trying to understand the code.
I am working with linux-2.6.9.
in file include/llinux/list.h - I found something like this.

#define INIT_LIST_HEAD(ptr) do { \
(ptr)->next = (ptr); (ptr)->prev = (ptr); \
} while (0)


My question is why do we use a loop when we actually know that it is not going to execute more than once? Cannot we simply do -

#define INIT_LIST_HEAD(ptr) {(ptr)->next = (ptr); (ptr)->prev = (ptr)}

Do we get some kind of optimization by using while (0)?

觉得很有意思,开始关注这个帖子,很快就有人回复了,比较权威的是kernelnewbies.org的FAQ:
http://kernelnewbies.org/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();

*(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; \
})