Re: Extending vfprintf and family

[Rich Felker <dalias at libc dot org>]

On Thu, Aug 14, 2014 at 11:45:07AM -0700, Russ Allbery wrote:
> Philip Prindeville <philipp_subx@redfish-solutions.com> writes:
> 
> > When I was at Cisco in the IOS group, there was a way to register
> > on-the-fly additional format specifiers that *printf() could then
> > leverage; some of the formatting helpers were quite sophisticated,
> > understanding ASN.1 or X.509 certs, etc.  Not sure we need to be that
> > complex, though I wouldnât reject it if it were offered, either.
> 
> glibc supports exactly this.  See:
> 
> https://www.gnu.org/software/libc/manual/html_node/Customizing-Printf.html#Customizing-Printf

But this feature really should not be used except possibly for
implementing in-progress extensions to the base language, like
decimal-float. There's a very small space of characters available for
use, risk of colliding with use by other code, and various
thread-safety, dlclose-safety, etc. issues with registration.

There's a very simple solution to the temp buffer problem: compound
literals. Doing something like this works:

static char *do_fmt_sockaddr(void *sa, socklen_t sl, char *buf, size_t size)
{
	getnameinfo(sa, sl, buf, size, 0, 0, NI_NUMERICHOST);
	return buf;
}
#define fmt_sockaddr(sa, sl)  do_fmt_sockaddr(sa, sl, (char[HOSTMAX]){0}, HOSTMAX)

...

printf("%s\n", fmt_sockaddr(sa, sl));

Perhaps most importantly, this allows you to keep the logic for how a
particular type should be formatted local to the calling code rather
than imposing global state for the choice of formatting.

Note that for this example a socklen was needed, which is ugly; you
could fix that by just having a lookup table of the right lengths for
each address family. I've used getnameinfo rather than inet_ntop
because the latter fails to show link ids for link-local ipv6
addresses despite their being mandatory, thus making it unsuitable for
general use.

Rich