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.

8 files changed, 123 insertions, 6 deletions

diff --git a/ChangeLog b/ChangeLog
index a118bfc950..a4af96ed5e 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,24 @@
 2023-03-30  Bruno Haible  <bruno@clisp.org>
 
+	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.
+
+2023-03-30  Bruno Haible  <bruno@clisp.org>
+
 	mbrtowc tests: Add comment.
 	* tests/test-mbrtowc.c: Add comment.
 	* tests/test-mbrtowc5.sh: Use symmetric coding style.
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
diff --git a/lib/btowc.c b/lib/btowc.c
index caadbd7608..4defbdda72 100644
--- a/lib/btowc.c
+++ b/lib/btowc.c
@@ -22,6 +22,7 @@
 
 #include <stdio.h>
 #include <stdlib.h>
+#include <string.h>
 
 wint_t
 btowc (int c)
@@ -32,7 +33,14 @@ btowc (int c)
       wchar_t wc;
 
       buf[0] = c;
+#if HAVE_MBRTOWC
+      mbstate_t state;
+      memset (&state, 0, sizeof (mbstate_t));
+      size_t ret = mbrtowc (&wc, buf, 1, &state);
+      if (!(ret == (size_t)(-1) || ret == (size_t)(-2)))
+#else
       if (mbtowc (&wc, buf, 1) >= 0)
+#endif
         return wc;
     }
   return WEOF;
diff --git a/m4/btowc.m4 b/m4/btowc.m4
index 77218a7d1c..1cd100a2d7 100644
--- a/m4/btowc.m4
+++ b/m4/btowc.m4
@@ -1,4 +1,4 @@
-# btowc.m4 serial 12
+# btowc.m4 serial 13
 dnl Copyright (C) 2008-2023 Free Software Foundation, Inc.
 dnl This file is free software; the Free Software Foundation
 dnl gives unlimited permission to copy and/or distribute it,
@@ -88,6 +88,49 @@ int main ()
         fi
       ])
 
+    dnl On mingw, in the C locale, btowc is inconsistent with mbrtowc:
+    dnl mbrtowc avoids calling MultiByteToWideChar when MB_CUR_MAX is 1 and
+    dnl ___lc_codepage_func() is 0, but btowc is lacking this special case.
+    AC_CHECK_FUNCS_ONCE([mbrtowc])
+    AC_CACHE_CHECK([whether btowc is consistent with mbrtowc in the C locale],
+      [gl_cv_func_btowc_consistent],
+      [
+        AC_RUN_IFELSE(
+          [AC_LANG_SOURCE([[
+#include <stdlib.h>
+#include <string.h>
+#include <wchar.h>
+int main ()
+{
+#if HAVE_MBRTOWC
+  wint_t wc1 = btowc (0x80);
+  wchar_t wc2 = (wchar_t) 0xbadface;
+  char buf[1] = { 0x80 };
+  mbstate_t state;
+  memset (&state, 0, sizeof (mbstate_t));
+  if (mbrtowc (&wc2, buf, 1, &state) != 1 || wc1 != wc2)
+    return 1;
+#endif
+  return 0;
+}]])],
+          [gl_cv_func_btowc_consistent=yes],
+          [gl_cv_func_btowc_consistent=no],
+          [case "$host_os" in
+                     # Guess no on mingw.
+             mingw*) AC_EGREP_CPP([Problem], [
+#ifdef __MINGW32__
+ Problem
+#endif
+                       ],
+                       [gl_cv_func_btowc_consistent="guessing no"],
+                       [gl_cv_func_btowc_consistent="guessing yes"])
+                     ;;
+                     # Guess yes otherwise.
+             *)      gl_cv_func_btowc_consistent="guessing yes" ;;
+           esac
+          ])
+      ])
+
     case "$gl_cv_func_btowc_nul" in
       *yes) ;;
       *) REPLACE_BTOWC=1 ;;
@@ -96,10 +139,22 @@ int main ()
       *yes) ;;
       *) REPLACE_BTOWC=1 ;;
     esac
+    case "$gl_cv_func_btowc_consistent" in
+      *yes) ;;
+      *) REPLACE_BTOWC=1 ;;
+    esac
+    if test $REPLACE_BTOWC = 0; then
+      gl_MBRTOWC_C_LOCALE
+      case "$gl_cv_func_mbrtowc_C_locale_sans_EILSEQ" in
+        *yes) ;;
+        *) REPLACE_BTOWC=1 ;;
+      esac
+    fi
   fi
 ])
 
 # Prerequisites of lib/btowc.c.
 AC_DEFUN([gl_PREREQ_BTOWC], [
   :
+  AC_CHECK_FUNCS_ONCE([mbrtowc])
 ])
diff --git a/modules/btowc b/modules/btowc
index 80d786cfa6..4788b3ec13 100644
--- a/modules/btowc
+++ b/modules/btowc
@@ -4,11 +4,13 @@ btowc() function: convert unibyte character to wide character.
 Files:
 lib/btowc.c
 m4/btowc.m4
+m4/mbrtowc.m4
 m4/locale-fr.m4
 
 Depends-on:
 wchar
 mbtowc          [test $HAVE_BTOWC = 0 || test $REPLACE_BTOWC = 1]
+mbrtowc         [test $HAVE_BTOWC = 0 || test $REPLACE_BTOWC = 1]
 
 configure.ac:
 gl_FUNC_BTOWC
diff --git a/modules/btowc-tests b/modules/btowc-tests
index 6bd3258520..59d33eb00b 100644
--- a/modules/btowc-tests
+++ b/modules/btowc-tests
@@ -1,6 +1,7 @@
 Files:
 tests/test-btowc1.sh
 tests/test-btowc2.sh
+tests/test-btowc3.sh
 tests/test-btowc.c
 tests/signature.h
 tests/macros.h
@@ -15,7 +16,7 @@ gt_LOCALE_FR
 gt_LOCALE_FR_UTF8
 
 Makefile.am:
-TESTS += test-btowc1.sh test-btowc2.sh
+TESTS += test-btowc1.sh test-btowc2.sh test-btowc3.sh
 TESTS_ENVIRONMENT += LOCALE_FR='@LOCALE_FR@' LOCALE_FR_UTF8='@LOCALE_FR_UTF8@'
 check_PROGRAMS += test-btowc
 test_btowc_LDADD = $(LDADD) $(SETLOCALE_LIB)
diff --git a/tests/test-btowc.c b/tests/test-btowc.c
index 849db470f4..e5918c36e9 100644
--- a/tests/test-btowc.c
+++ b/tests/test-btowc.c
@@ -57,6 +57,25 @@ main (int argc, char *argv[])
         for (c = 0x80; c < 0x100; c++)
           ASSERT (btowc (c) == WEOF);
         return 0;
+
+      case '3':
+        /* C or POSIX locale.  */
+        for (c = 0; c < 0x100; c++)
+          if (c != 0)
+            {
+              /* We are testing all nonnull bytes.  */
+              wint_t wc = btowc (c);
+              /* POSIX:2018 says: "In the POSIX locale, btowc() shall not return
+                 WEOF if c has a value in the range 0 to 255 inclusive."  */
+              if (c < 0x80)
+                /* c is an ASCII character.  */
+                ASSERT (wc == c);
+              else
+                /* On most platforms, the bytes 0x80..0xFF map to U+0080..U+00FF.
+                   But on musl libc, the bytes 0x80..0xFF map to U+DF80..U+DFFF.  */
+                ASSERT (wc == c || wc == 0xDF00 + c);
+            }
+        return 0;
       }
 
   return 1;
diff --git a/tests/test-btowc3.sh b/tests/test-btowc3.sh
new file mode 100755
index 0000000000..ee9e143c1c
--- /dev/null
+++ b/tests/test-btowc3.sh
@@ -0,0 +1,9 @@
+#!/bin/sh
+
+# Test whether the POSIX locale has encoding errors.
+LC_ALL=C \
+${CHECKER} ./test-btowc${EXEEXT} 3 || exit 1
+LC_ALL=POSIX \
+${CHECKER} ./test-btowc${EXEEXT} 3 || exit 1
+
+exit 0