Re: [musl] [PATCH] vfprintf: support C2x %b and %B conversion specifiers

["Jₑₙₛ Gustedt" <jens.gustedt@inria.fr>]

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Rich,

on Mon, 19 Sep 2022 14:10:39 -0400 you (Rich Felker <dalias@libc.org>)
wrote:

> On Mon, Sep 19, 2022 at 07:59:52PM +0200, Szabolcs Nagy wrote:
> > * Rich Felker <dalias@libc.org> [2022-09-19 11:09:17 -0400]:  
> > > On Mon, Sep 12, 2022 at 04:42:51PM +0200, Jₑₙₛ Gustedt wrote:  
>  [...]  
> > > 
> > > What do these entail? It looks like there's a requirement for
> > > printf to support them, so I don't see how you'd do that as a
> > > separate library. It looks like __STDC_IEC_60559_DFP__ is
> > > optional though, so maybe we could just decline to define it and
> > > leave the support sporadic at the level the compiler supports, as
> > > an extension rather than part of the standard-specified
> > > functionality?  
> > 
> > it seems there is
> > https://github.com/libdfp/libdfp/tree/master/printf-hooks
> > using glibc specific apis (register_printf_specifier)
> > 
> > i'm not sure how musl can handle this internally since
> > we dont know in advance if the user links against libdfp.  
> 
> Yeah, I don't see that as being a usable approach. It's closely tied
> to the glibc printf model that's not usable in bounded memory with
> arbitrary width and precision, and not compatible with linking
> semantics as you mention. The amount of code needed for decimal float
> printing in decimal is miniscule anyway and something we can easily do
> with no actual decimal floating point code. I thought the hard case
> was hex, but looking at the spec again, %a doesn't actually do hex for
> decimal floats, so it should be easy too.

Yes exactly. There is nothing conceptually difficult here and nothing
that should not be in some form or another already in every C library.

So yes, sorry, for the separate library part I forgot formated IO and
string functions. But the huge amount of functions that are added for
these types are math functions (I guess something like 600 or so)
stepping on user's identifier space all over.

Unfortunately, again as for complex types, the standard doesn't
properly distinguish language support for the new optional types and
library support. I really would have preferred to have the whole thing
in a separate header, but my voice echoed in the void. There are the
`__STDC_VERSION_…_H__` macros now, so this gives at least some sort of
feature test.

But for implementing the parts that are outside of math, things should
indeed not be so difficult. gcc has support for the types since long,
I think, and should also provide predefined macros that could be used
to check for language support. Then, the types themselves have clear
definition and prescribed representation, the ABI is de-facto sorted
out, so there would be not much other implementation dependency to
worry about.

Other types that come with C23, and these are mandatory, are
bit-precise integers. There the support by compilers is probably not
yet completely established. I know of an integration into llvm, but I
am not sure about the state of affairs for gcc, nor if there is a
de-facto agreement on ABI issues. In any case, these types need
support in formatted IO, too.

Also, C23, provides the possibility for extended integer types that
are wider than `[u]intmax_t` under some conditions. This is intended
in particular to allow for implementations such as gcc on x86_64 to
interface the existing 128 bit integer types properly as
`[u]int128_t`. From a C library POV, these then also would need
integration into formatted IO, but here again support in the compiler
with usable feature test macros is there for ages and the ABI should
already be sorted out.

So in summary that means that there is some work to do to make
formatted IO of C libraries become compliant with C23. Let me know if
and where I could help to make that happen for musl.


Thanks
Jₑₙₛ

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