Re: fake loop

[Crutcher Dunnavant <crutcher@datastacks.com>]

++ 03/08/01 09:57 -0600 - Andrey Ilinykh:
> Hi!
> Very often I see in kernel such code as
> do {
>   dosomthing();
> } while(0);

This is a fake function. Macros often use this to give themselves an environment
to allocate stack variables in. Example:

 #define swap(A, B) \
 { \
	int C; \
	C = (A); (A) = (B); (B) = C; \
 } while(0)

swap will now act almost exactly like a function. Please note that this macro
does NOT have a semicolon on the end, as that would cause bad things if I did:

 if (test)
	swap(a,b);
 else
	do_something();

> or even
> #define prepare_to_switch()     do { } while(0)

This is most often found in situations where a code block becomes trivial when
a config option is turned off. The spinlock functions mostly become this when
SMP is not turned on, so they get optimized out of the code.

-- 
Crutcher        <crutcher@datastacks.com>
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