On Fri, Aug 03, 2001 at 09:57:34AM -0600, Andrey Ilinykh wrote:
> #define prepare_to_switch() do { } while(0)
>
> Who can explain me a reason for these fake loops?
http://kernelnewbies.org/faq/index.php3#dowhile.xml
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.
Have a nice day
--
Martin Mačok
underground.cz
openbsd.cz
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