Re: [PATCH v2] Support C2X printf %b, %B

["H.J. Lu" <hjl.tools@gmail.com>]

On Tue, Oct 12, 2021 at 5:38 PM Joseph Myers <joseph@codesourcery.com> wrote:
>
> C2X adds a printf %b format (see
> <http://www.open-std.org/jtc1/sc22/wg14/www/docs/n2630.pdf>, accepted
> for C2X), for outputting integers in binary.  It also has recommended
> practice for a corresponding %B format (like %b, but %#B starts the
> output with 0B instead of 0b).  Add support for these formats to
> glibc.
>
> One existing test uses %b as an example of an unknown format, to test
> how glibc printf handles unknown formats; change that to %v.  Use of
> %b and %B as user-registered format specifiers continues to work (and
> we already have a test that covers that, tst-printfsz.c).
>
> Note that C2X also has scanf %b support, plus support for binary
> constants starting 0b in strtol (base 0 and 2) and scanf %i (strtol
> base 0 and scanf %i coming from a previous paper that added binary
> integer literals).  I intend to implement those features in a separate
> patch or patches; as discussed in the thread starting at
> <https://sourceware.org/pipermail/libc-alpha/2020-December/120414.html>,
> they will be more complicated because they involve adding extra public
> symbols to ensure compatibility with existing code that might not
> expect 0b constants to be handled by strtol base 0 and 2 and scanf %i,
> whereas simply adding a new format specifier poses no such
> compatibility concerns.
>
> Note that the actual conversion from integer to string uses existing
> code in _itoa.c.  That code has special cases for bases 8, 10 and 16,
> probably so that the compiler can optimize division by an integer
> constant in the code for those bases.  If desired such special cases
> could easily be added for base 2 as well, but that would be an
> optimization, not actually needed for these printf formats to work.
>
> Tested for x86_64 and x86.  Also tested with build-many-glibcs.py for
> aarch64-linux-gnu with GCC mainline to make sure that the test does
> indeed build with GCC 12 (where format checking warnings are enabled
> for most of the test).
>

Binaries using such C2X specifiers should have a dependency on
the glibc version which supports them.  Ideally this dependency
should be added only when C2X specifiers are used.

-- 
H.J.