On Fri, Mar 24, 2023 at 12:37:42PM +0100, Bruno Haible wrote: > Eric Blake wrote in > <https://lists.gnu.org/archive/html/bug-gnulib/2023-03/msg00060.html>: > > C23 admits that %#B is not available for > > portable use of outputting uppercase '0B...', you would have to > > manually write 0B%b instead. However, since glibc DOES support %B as > > the uppercase counterpart to %b, should we add that in gnulib (but > > this time under the auspices of *printf-gnu, rather than > > *printf-posix)? > > Nice idea. I was pondering what to do with the wording in ISO C23 > "Implementations that did not use an uppercase B as their own > extension before are encouraged to implement it similar to > conversion specifier b as standardized above, with the > alternative form (#B) generating 0B as prefix for nonzero values." > > "encouraged" - should we add it to the *printf-posix modules or not? >
Ah, the fun of chasing a spec still in progress > Your suggestion to put it under *printf-gnu, that being an extension > of *printf-posix, makes perfect sense, since glibc implements %B > since version 2.35. Implemented as follows. The latest draft comments on C go a step further (warning, big document): https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3091.doc Searching that for %B finds: FR-217 Make %B optional Accepted with comment: Wording from N 3072 section 15 with editorial corrections so now go check N3072: https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3072.htm#make-printf-format-specifier-b-optional which states that the PRIBPTR in <inttype.h> is now a reliable witness of whether %B has C23 semantics rather than other implementation-defined semantics. and glibc has updated their %B implementation accordingly: https://sourceware.org/git/?p=glibc.git;a=commit;h=2d4728e60621 | Update printf %b/%B C2x support | | WG14 recently accepted two additions to the printf/scanf %b/%B | support: there are now PRIb* and SCNb* macros in <inttypes.h>, and | printf %B is now an optional feature defined in normative text, | instead of recommended practice, with corresponding PRIB* macros that | can also be used to test whether that optional feature is supported. | See N3072 items 14 and 15 for details (those changes were accepted, | some other changes in that paper weren't). | | Add the corresponding PRI* macros to glibc and update one place in the | manual referring to %B as recommended. (SCNb* should naturally be | added at the same time as the corresponding scanf %b support.) Since %B is optional (but now MUST have a reliable PRIBPTR witness macro), I could see still keeping it in only the gnu-*printf variants. Or we can just state that we are going to blindly define PRIBPTR in inttypes.h if the posix-*printf variants are in use. Either way, you probably want to work on followup patches that take PRIBPTR into account. Thanks for all your work on this, by the way! -- Eric Blake, Principal Software Engineer Red Hat, Inc. +1-919-301-3266 Virtualization: qemu.org | libvirt.org
