On Sunday 06 May 2007 09:34, Min-Hua Chen wrote:
> hi,
>
> A few days ago, when I was tracing linux driver code. I found some debug
> macros
> like:
>
> #define dbg(fmt, args...) do { printk(KERN_INFO fmt, ## args); } while(0)
> #define print_dbg(f, arg...) printk(KERN_DEBUG __FILE__ ": " f "\n", ##
> arg)
>
> I checked the C grammar in the appenix of "The C Programming Language,
> 2ed", and
> the variable-length arg list is like:
>
> parameter-type-list:
> parameter-list
> parameter-list, ...
>
> It looks like the "..." must be at the end of arg and there must be a ','
> before it.
>
> And I wrote a little test program:
>
> #define BUG(fmt...) do {printf(fmt);} while (0)
> the fmt is followed by "..." without a ',' before it, but the macro wroked
> well.
>
> I found it is very useful when debugging, but I have no idea why and how
> these macros work. Can someone explain this?
>
The macros are expanded by the preprocessor, thus they have nothing to do with
the C grammar.
http://gcc.gnu.org/onlinedocs/cpp/Variadic-Macros.html
tavi
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