Re: RFC: changing printf(1) behavior on %b

[Chet Ramey]

On 8/31/23 11:35 AM, Eric Blake wrote:
> In today's Austin Group call, we discussed the fact that printf(1) has
> mandated behavior for %b (escape sequence processing similar to XSI
> echo) that will eventually conflict with C2x's desire to introduce %b
> to printf(3) (to produce 0b000... binary literals).
>
> For POSIX Issue 8, we plan to mark the current semantics of %b in
> printf(1) as obsolescent (it would continue to work, because Issue 8
> targets C17 where there is no conflict with C2x), but with a Future
> Directions note that for Issue 9, we could remove %b entirely, or
> (more likely) make %b output binary literals just like C.

I doubt I'd ever remove %b, even in posix mode -- it's already been there
for 25 years.

> But that
> raises the question of whether the escape-sequence processing
> semantics of %b should still remain available under the standard,
> under some other spelling, since relying on XSI echo is still not
> portable.
>
> One of the observations made in the meeting was that currently, both
> the POSIX spec for printf(1) as seen at [1], and the POSIX and C
> standard (including the upcoming C2x standard) for printf(3) as seen
> at [3] state that both the ' and # flag modifiers are currently
> undefined when applied to %s.

Neither one is a very good choice, but `#' is the better one. It at least
has a passing resemblence to the desired functionality.

Why not standardize another character, like %B? I suppose I'll have to look
at the etherpad for the discussion. I think that came up on the mailing
list, but I can't remember the details.

> Is there
> any interest in a patch to coreutils or bash that would add such a
> synonym, to make it easier to leave that functionality in place for
> POSIX Issue 9 even when %b is repurposed to align with C2x?

It's maybe a two or three line change at most.

--
``The lyf so short, the craft so long to lerne.'' - Chaucer
                 ``Ars longa, vita brevis'' - Hippocrates
Chet Ramey, UTech, CWRU    chet@case.edu    http://tiswww.cwru.edu/~chet/