Re: [9fans] stdarg & va_copy

[Pietro Gagliardi <pietro10@mac.com>]

One possible use of va_copy is in an implementation of print[f]()  
that supports an option to get a specific argument. For example,

	print("%2#s %1#s", "second", "first");

prints

	first second

For example, one could maintain an array of the first n va_list  
items, save them as they are read, and then use the copies when  
necessary. The other option is
	va_end()
	va_start()
	while (newcount < count)
		va_arg()
which could flood your code.

On Jan 24, 2008, at 5:39 PM, Russ Cox wrote:

>> i didn't see anything in the definition that would make va_copy
>> wrong given the plan 9 definition of va_list.  is there a particular
>> case on plan 9 that would be a problem?
>
> it might make sense to *have* va_copy, but since plan 9
> programs don't use va_copy, there's no need to provide
> an implementation.  and honestly, the fewer people who
> use va_copy, the better.
>
>> but plan 9 does have va_start and va_end.  wouldn't make sense
>> to have va_copy as well?
>
> plan 9 makes no claims of being C99 compliant,
> although it happens to have a few of the same extensions.
> va_start, va_end, and va_arg are from an earlier standard.
> those (in particular va_arg) provide useful functionality.
> va_copy does not.
>
> like i said, if you need it, it's easy to put a #define
> in your own compatibility headers.  if you don't
> need it, don't worry about it.
>
> russ
>