I don't understand why the kernel contains code like this.

[phil128]

Hi all.

Throughout the kernel I keep on finding code like this:

include/linux/preempt.h
46 #define preempt_enable() 
47 do { 
48  preempt_enable_no_resched(); 
49  preempt_check_resched(); 
50 } while (0)

why is this within a do-while loop, even though its expression is evaluated to false. This code will only execute once.

Really weird. Could it redundant code? i.e the programmers couldn't be bothered to remove the do while? or could it be for testing etc.

Thanks.

[elija]

Have a look at this pdf. Page 4 seems to be relavent.

[JohnGraham]

why is this within a do-while loop, even though its expression is evaluated to false. This code will only execute once.

This is a fairly common way to define a macro, since it means the macro avoids several pitfalls of macros. Since all the macro code is within a block, it can go anywhere a single statement can, and since it requires a semicolon after it, it does not confuse some language constructs. It's basically a way for a macro to safely masquerade as a function call.

See this helpful page for details.

[elija]

Excellent link. Thanks.