Re: [musl] [PATCH] arm: Use __WCHAR_TYPE__ for wchar_t if defined

[Rich Felker <dalias@libc.org>]

On Mon, Feb 06, 2023 at 05:15:08PM -0800, Peter Collingbourne wrote:
> On Sun, Feb 5, 2023 at 3:49 PM Rich Felker <dalias@libc.org> wrote:
> >
> > On Sun, Feb 05, 2023 at 09:00:03PM +0100, Markus Wichmann wrote:
> > > On Sat, Feb 04, 2023 at 08:08:36AM +0100, alice wrote:
> > > > On Sat Feb 4, 2023 at 7:30 AM CET, Peter Collingbourne wrote:
> > > > > When building with -fshort-wchar the definition of wchar_t is
> > > > > incorrect. Get the correct definition from the compiler if available.
> > > > >
> > > > > This is useful when reusing the freestanding parts of musl on a
> > > > > bare-metal target that uses -fshort-wchar.
> > > >
> > > > somebody talked about this in 2015, see
> > > > https://www.openwall.com/lists/musl/2015/02/18/2
> > > > for the previous discussion.
> > > >
> > > > i understand in this case it's proposed a little different-
> > > > "reusing freestanding parts" as opposed to building a whole libc.so, but in
> > > > that case you could most likely patch this in when reusing it standalone only?
> > > >
> > > > it doesn't seem a good idea for it to be there, in general.
> > >
> > > Seconded. A lot of code in musl depends on wchar_t being able to hold
> > > the current maximum Unicode codepoint of 0x10FFFF at least, so the type
> > > must be at least 21 bits.
> >
> > Absolutely. -fshort-wchar requests a different ABI that is
> > fundamentally incompatible with libc and with use of the libc headers,
> > and also fundamentally incompatible with Unicode and the requirements
> > of the C language (unless you only want to support the BMP) -- C does
> > not allow "multi-wchar_t characters".
> >
> > If you're targeting freestanding environment not using libc, you
> > should use -nostdinc and provide headers suitable to your environment
> > instead of the libc ones. But really you should fix the offending code
> > not to use wchar_t for UTF-16, and not use -fshort-wchar. Modern C has
> > a char16_t type for this purpose.
> 
> Thanks, I agree with this and the other replies that I got. It did
> seem at first that musl could be used unmodified in projects that
> build with -fshort-wchar, but given the implications of a UTF-16
> wchar_t for the code that implements <wchar.h>, it makes more sense
> for this flag to be unsupported by musl and for any utilizing projects
> to be fixed to not require -fshort-wchar.
> 
> Currently we accidentally "support" -fshort-wchar on architectures
> that happen to use __WCHAR_TYPE__ to define wchar_t. Would it make
> sense to add something like a static assert to alltypes.h that checks
> that sizeof(wchar_t) >= 4?

If you count target-specific options, GCC probably has hundreds of
options that produce incompatible/broken ABIs. We certainly don't have
the means to trap all or even most of them. In the case of most,
including -fshort-wchar, GCC documents this:

    "Warning: the -fshort-wchar switch causes GCC to generate code
    that is not binary compatible with code generated without that
    switch. Use it to conform to a non-default application binary
    interface."

so I don't really think any action is needed.

Rich