A little explanation needed

Can anyone explain the purpose of this #define...

#define __cpu_raise_softirq(cpu, nr) do { softirq_pending(cpu) |= 1UL <<
(nr); } while (0) // from interrupt.h

...versus the more "plain"...

#define __cpu_raise_softirq(cpu,nr) softirq_pending(cpu |= 1UL << (nr).

In otherwords, what's the use of a do{X}while(0) "loop" instead of just X. I'm not the world's best trained C programmer, so forgive me if I sound stupid.

Josh

http://www.kernelnewbies.org/faq/index.php3#dowhile

Hope that helps,

greg k-h

Thanks for the help. It all makes perfect sense after thinking about it from a preprecessor point of view. I'm just not used to using so many macros and so I don't realize all the implications of preprocessing quite yet. But, I'm learning.

Josh

Hi Joshua :)

First, you do not sound stupid at all.

Second. The do...while use in macros is to avoid the 'swallow
semicolon' effect ;)) In other words, it makes the entire macro appear as a single statement. This avoids problems with 'else' constructs with macros that expand to multiple statements.

Someplace in the GNU cpp documentation you can find a far better
explanation of this effect and why the do...while helps (it makes the macro a single statement...).

Hope that helps :)
Raúl