Newlib: nano-formatted-io and libstdc++ compatibility

[Mark Goncharov <mark.goncharov@syntacore.com>]

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Hello.
My name is Mark.
My problem has appeared on RISC-V target, but seems to be the same for every target.

To reproduce the bug:

#include <iostream>
int main() { std::cout << "Hello world"; }

Compile: riscv64-unknown-elf-g++ --specs=nano.specs a.cpp

Have different outputs with gcc-13.2 that was built with newlib-nano and --enable-newlib-nano-formatted-io option as in https://github.com/riscv-collab/riscv-gnu-toolchain do:

1) newlib-4.3
Works good

2) newlib-4.4 without patch - https://cygwin.com/git/?p=newlib-cygwin.git;a=commit;h=61ccd3f94f92bcfc0940f0595ea5b3b72bce3c6d
Also works good

3) newlib-4.4
Fail with error
libc_nano.a(libc_a-svfwprintf.o): in function `_svfwprintf_r':
(.text._svfwprintf_r+0x7a): undefined reference to `__ssputws_r'

4) newlib-4.4 + fix regression for arm https://cygwin.com/git/?p=newlib-cygwin.git;a=commitdiff;h=3b97a5ec67a5a52c130158bb143949cd842de305
Fail with error
in function `__gnu_cxx::stdio_sync_filebuf<wchar_t, std::char_traits<wchar_t> >::uflow()':
(.text._ZN9__gnu_cxx18stdio_sync_filebufIwSt11char_traitsIwEE5uflowEv[_ZN9__gnu_cxx18stdio_sync_filebufIwSt11char_traitsIwEE5uflowEv]+0xa): undefined reference to `getwc
... // same for putwc, ungetwc and swprintf

The first patch changes newlib/libc/stdio/vfwprintf.c file and defines __ssputws_r instead of __ssputs_r. Of course, it fails, because nano-formatted-io doesn't implement this function (see newlib/libc/stdio/Makefile.inc for NEWLIB_NANO_FORMATTED_IO case)

The second patch removes every wide-char usage for nano_formatted_io, so, the simplest "Hello world" can't be compiled with newlib-nano, because libstdc++  links even with unused symbols.

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Finally, I have a question.

In newlib/README:
--enable-newlib-nano-formatted-io
  This option does not affect wide-char formatted I/O functions

But current implementation affect wide-char formatted I/O. We really want to broke compatibility with nano-formatted-io and libstdc++?

I propose, we need to get back wide-char support for nano-formatted-io.

With best regards,
Mark