A little explanation needed

A little explanation needed

[Joshua Stewart]

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

Re: A little explanation needed

[Greg KH]

On Sun, Dec 22, 2002 at 12:36:32AM -0500, Joshua Stewart wrote:
> 
> 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.

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

Hope that helps,

greg k-h

Re: A little explanation needed

[Joshua Stewart]

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

On Sun, 2002-12-22 at 00:35, Greg KH wrote:
> On Sun, Dec 22, 2002 at 12:36:32AM -0500, Joshua Stewart wrote:
> > 
> > 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.
> 
> http://www.kernelnewbies.org/faq/index.php3#dowhile
> 
> Hope that helps,
> 
> greg k-h
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Re: A little explanation needed

[DervishD]

    Hi Joshua :)

> 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.

    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