16-bit wchar_t on Windows and Cygwin

Corinna Vinschen corinna-cygwin@cygwin.com
Mon Jan 31 20:49:00 GMT 2011

On Jan 31 09:58, Eric Blake wrote:
> >   2) Code that uses mbrtowc() or wcrtomb() is also likely to malfunction.
> >      On Cygwin >= 1.7 mbrtowc() and wcrtomb() is implemented in an intelligent
> >      but somewhat surprising way: wcrtomb() may return 0, that is, produce no
> >      output bytes when it consumes a wchar_t.
> >   Now with a chinese character outside the BMP:
> >   $ 	
> >         1       4
> >   $ printf 'a \xf0\xa1\x88\xb4 b\n' | wc -w -m
> >         3       6
> > 
> >   On Cygwin 1.7.5 (with LANG=C.UTF-8 and 'wc' from GNU coreutils 8.5):
> > 
> >   $ printf 'a\xf0\xa1\x88\xb4b\n' | wc -w -m
> >         1       5
> >   $ printf 'a \xf0\xa1\x88\xb4 b\n' | wc -w -m
> >         2       7
> >
> >   So both the number of characters and the number of words are counted
> >   wrong as soon as non-BMP characters occur.
> >
> Does this represent a bug in cygwin's mbrtowc routines that could be
> fixed by cygwin?
> Or, does this represent a bug in coreutils for using mbrtowc one
> character at a time instead of something like mbsrtowcs to do bulk
> conversions?

Just to clarify a bit.  This has been discussed on the cygwin-developer
mailing list back in 2009.  The original code which handled UTF-16
surrogates always wrote at least 1 byte to the destination UTF-8 string.
However, the problem is that Windows filenames may contain lone
surrogate pairs, even though the filename is usually interpreted as

So the current code returns 0 bytes for the first surrogate half and
only writes the full UTF-8 sequence after the second surrogate half has
been evaluated.  In the case where a lone high surrogate is still
pending, but the low surrogate is missing, we can just write out the
high surrogate in CESU-8 encoding.  This would not have been possible if
we had already written the first byte of the UTF-8 string.  Lone low
surrogates are written as CESU-8 sequence immediately so they are nothing
to worry about.

As for wctomb/wcrtomb returning 0:  Even if this looks like kind of a
stretch, this should not be a problem per POSIX.  A return value of 0
from wctomb/wcrtomb has no special meaning(*).  Even in the case where
the incoming wide char is L'\0', the resulting \0 is written and 1 is
returned.  Since 0 bytes have been written to the destination string,
returning 0 is perfectly valid.  If a calling function misinterprets the
return value of 0 as an error or EOF, it's not a bug in wctomb/wcrtomb.

For the original discussion, see


(*) http://pubs.opengroup.org/onlinepubs/9699919799/functions/wcrtomb.html

Corinna Vinschen                  Please, send mails regarding Cygwin to
Cygwin Project Co-Leader          cygwin AT cygwin DOT com
Red Hat

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