l i n u x - u s e r s - g r o u p - o f - d a v i s
Next Meeting:
July 7: Social gathering
Next Installfest:
Latest News:
Jun. 14: June LUGOD meeting cancelled
Page last updated:
2007 Jan 18 17:31

The following is an archive of a post made to our 'vox-tech mailing list' by one of its subscribers.

Report this post as spam:

(Enter your email address)
Re: [vox-tech] Nifty macro expansion bug
[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: [vox-tech] Nifty macro expansion bug

On Thu, Jan 18, 2007 at 04:14:22PM -0800, Bill Kendrick wrote:
> So at work, a coworker noticed an issue with the following code:
>   for (i = 0; i < n; i++)
>     myFREE(foo[i]);
> which went away when he wrapped it in braces:
>   for (i = 0; i < n; i++)
>   {
>     myFREE(foo[i]);
>   }
> Figured maybe it was a weird compiler bug.  Being author of the myFREE()
> code -- which happens to be a macro -- I realized immediately what I did
> wrong.  (And continue to be annoyed with C syntax ;) )
> See, myFREE() is actually a macro that looks like this:
>   #define myFREE(x) if ((x) != NULL) FREE(x); x = NULL;
> Two statements... hence the need for {} around the call to the macro.
> (Or, more sensibly, including {}s around what the macro expands to.)
> That was a neat catch. :)

I believe the standard way of doing macros in the linux kernel is to use a
bogus do-while, ie:

#define CRASHMACHINE() do { laughtAtUser(); crashMachine(); } while(0);

that way, any expansion like:

doesn't end up being expanded as:

	laughAtUser(); crashMachine();

	               ^ oops, outside of if scope

but as

	do ( laughAtUser(); crashMachine(); } while(0);

which will execute once and only once.

I saw a good post on this once, but I can't really remember where.

vox-tech mailing list

LUGOD Group on LinkedIn
Sign up for LUGOD event announcements
Your email address:
LUGOD Group on Facebook
'Like' LUGOD on Facebook:

Hosting provided by:
Sunset Systems
Sunset Systems offers preconfigured Linux systems, remote system administration and custom software development.

LUGOD: Linux Users' Group of Davis
PO Box 2082, Davis, CA 95617
Contact Us

LUGOD is a 501(c)7 non-profit organization
based in Davis, California
and serving the Sacramento area.
"Linux" is a trademark of Linus Torvalds.

Sponsored in part by:
EDGE Tech Corp.
For donating some give-aways for our meetings.