btowc: Fix behaviour in the C locale.

* lib/btowc.c: Include <string.h>
(btowc): Use mbrtowc instead of mbtowc when possible.
* m4/btowc.m4 (gl_FUNC_BTOWC): Test for the mingw bug in the C locale.
Invoke gl_MBRTOWC_C_LOCALE. If mbrtowc is buggy in the C locale,
override also btowc.
(gl_PREREQ_BTOWC): Test whether mbrtowc exists.
* modules/btowc (Files): Add m4/mbrtowc.m4.
(Depends-on): Add mbrtowc.
* tests/test-btowc.c (main): Add a test of the C locale, based on
tests/test-mbrtowc.c.
* tests/test-btowc3.sh: New file, based on tests/test-mbrtowc5.sh.
* modules/btowc-tests (Files): Add it.
(Makefile.am): Test it.
* doc/posix-functions/btowc.texi: Mention the two C locale behaviour
bugs and that they are worked around.

1 files changed, 8 insertions, 4 deletions

diff --git a/doc/posix-functions/btowc.texi b/doc/posix-functions/btowc.texi
index fa1ea9b503..f4ca5d450a 100644
--- a/doc/posix-functions/btowc.texi
+++ b/doc/posix-functions/btowc.texi
@@ -17,6 +17,14 @@ Cygwin 1.7.2.
 @item
 This function does not return WEOF for an EOF argument on some platforms:
 IRIX 6.5.
+@item
+In the C or POSIX locales, this function is not consistent with
+Gnulib's @code{mbrtowc} and can return @code{WEOF}:
+glibc 2.35, MirOS BSD #10.
+@item
+In the C or POSIX locales, this function is not consistent with @code{mbrtowc}
+on some platforms:
+mingw.
 @end itemize
 
 Portability problems not fixed by Gnulib:
@@ -27,8 +35,4 @@ therefore cannot accommodate all Unicode characters.
 However, the Gnulib function @code{btoc32}, provided by Gnulib module
 @code{btoc32}, operates on 32-bit wide characters and therefore does not have
 this limitation.
-@item
-In the C or POSIX locales, this function is not consistent with
-Gnulib's @code{mbrtowc} and can return @code{WEOF}:
-glibc 2.23, MirOS BSD #10.
 @end itemize