Re: printf doesn't respect locale

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

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Hello Rich,

On Wed, 11 Sep 2019 07:44:37 -0400 Rich Felker <dalias@libc.org> wrote:

> On Wed, Sep 11, 2019 at 12:07:22PM +0200, Jens Gustedt wrote:

> > I think that WG14 would be happy to hear any suggestions how we
> > could get out of this trap, a proposal for C2x would even be
> > better.  
> 
> The obvious solution is a modifier character to printf/scanf format
> strings that applies to numeric conversions and means "always
> format/interpret this as if in the C locale". However this is hard to
> test for at build time unless there's a macro declaring its
> availability, so ideally WG14 would also adopt the sort of
> fine-grained feature availability macros some of us have been
> proposing for extensions.

If such a proposal would be made, it would have to be based on a
reference implementation in the field. Would musl be willing to be
such a reference implementation?

In addition, I would think that it should not switch off all locale
feature but should leave the encoding properties such as UTF-8
functional.

> An alternative/additional solution, which I actually might like
> better, is having a function which sets a thread-local flag to treat
> certain locale properties (at least the problematic LC_NUMERIC ones)
> as if the current locale were "C". This is weaker than the uselocale
> API from POSIX, but doesn't have the problems with the possibility of
> failure (likely with no way to make forward progress) like it does,
> and more importantly, would avoid *breaking* m17n/i18n functionality
> by turning off other unrelated, non-problematic locale features.
> Application or library code could then just set/restore this flag
> around *printf/*scanf/strto*/etc calls, or could set it and leave it
> if they never want to see ',' again.

Interesting.

Would this be difficult to implement in musl? (I guess not)

Would you be willing to write this up?

Once we'd have that in musl (even before having it in C2x) it could be
easier for ourselves to convice us to have full locale support.

Thanks
Jens

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