manual: Various fixes to the mbstouwcs example, and mbrtowc update

The example did not work because the null byte was not converted, and
mbrtowc was called with a zero-length input string.  This results in a
(size_t) -2 return value, so the function always returns NULL.

The size computation for the heap allocation of the result was
incorrect because it did not deal with integer overflow.

Error checking was missing, and the allocated memory was not freed on
error paths.  All error returns now set errno.  (Note that there is an
assumption that free does not clobber errno.)

The slightly unportable comparision against (size_t) -2 to catch both
(size_t) -1 and (size_t) -2 return values is gone as well.

A null wide character needs to be stored in the result explicitly, to
terminate it.

The description in the manual is updated to deal with these finer
points.  The (size_t) -2 behavior (consuming the input bytes) matches
what is specified in ISO C11.

2 files changed, 77 insertions, 43 deletions

diff --git a/manual/charset.texi b/manual/charset.texi
index 6831ebec27..a63d67045f 100644
--- a/manual/charset.texi
+++ b/manual/charset.texi
@@ -643,8 +643,8 @@ and they also do not require it to be in the initial state.
 @cindex stateful
 The @code{mbrtowc} function (``multibyte restartable to wide
 character'') converts the next multibyte character in the string pointed
-to by @var{s} into a wide character and stores it in the wide character
-string pointed to by @var{pwc}.  The conversion is performed according
+to by @var{s} into a wide character and stores it in the location
+pointed to by @var{pwc}.  The conversion is performed according
 to the locale currently selected for the @code{LC_CTYPE} category.  If
 the conversion for the character set used in the locale requires a state,
 the multibyte string is interpreted in the state represented by the
@@ -652,7 +652,7 @@ object pointed to by @var{ps}.  If @var{ps} is a null pointer, a static,
 internal state variable used only by the @code{mbrtowc} function is
 used.
 
-If the next multibyte character corresponds to the NUL wide character,
+If the next multibyte character corresponds to the null wide character,
 the return value of the function is @math{0} and the state object is
 afterwards in the initial state.  If the next @var{n} or fewer bytes
 form a correct multibyte character, the return value is the number of
@@ -665,50 +665,59 @@ by @var{pwc} if @var{pwc} is not null.
 If the first @var{n} bytes of the multibyte string possibly form a valid
 multibyte character but there are more than @var{n} bytes needed to
 complete it, the return value of the function is @code{(size_t) -2} and
-no value is stored.  Please note that this can happen even if @var{n}
-has a value greater than or equal to @code{MB_CUR_MAX} since the input
-might contain redundant shift sequences.
+no value is stored in @code{*@var{pwc}}.  The conversion state is
+updated and all @var{n} input bytes are consumed and should not be
+submitted again.  Please note that this can happen even if @var{n} has a
+value greater than or equal to @code{MB_CUR_MAX} since the input might
+contain redundant shift sequences.
 
 If the first @code{n} bytes of the multibyte string cannot possibly form
 a valid multibyte character, no value is stored, the global variable
 @code{errno} is set to the value @code{EILSEQ}, and the function returns
 @code{(size_t) -1}.  The conversion state is afterwards undefined.
 
+As specified, the @code{mbrtowc} function could deal with multibyte
+sequences which contain embedded null bytes (which happens in Unicode
+encodings such as UTF-16), but @theglibc{} does not support such
+multibyte encodings.  When encountering a null input byte, the function
+will either return zero, or return @code{(size_t) -1)} and report a
+@code{EILSEQ} error.  The @code{iconv} function can be used for
+converting between arbitrary encodings.  @xref{Generic Conversion
+Interface}.
+
 @pindex wchar.h
 @code{mbrtowc} was introduced in @w{Amendment 1} to @w{ISO C90} and
 is declared in @file{wchar.h}.
 @end deftypefun
 
-Use of @code{mbrtowc} is straightforward.  A function that copies a
-multibyte string into a wide character string while at the same time
-converting all lowercase characters into uppercase could look like this
-(this is not the final version, just an example; it has no error
-checking, and sometimes leaks memory):
+A function that copies a multibyte string into a wide character string
+while at the same time converting all lowercase characters into
+uppercase could look like this:
 
 @smallexample
 @include mbstouwcs.c.texi
 @end smallexample
 
-The use of @code{mbrtowc} should be clear.  A single wide character is
-stored in @code{@var{tmp}[0]}, and the number of consumed bytes is stored
-in the variable @var{nbytes}.  If the conversion is successful, the
-uppercase variant of the wide character is stored in the @var{result}
-array and the pointer to the input string and the number of available
-bytes is adjusted.
-
-The only non-obvious thing about @code{mbrtowc} might be the way memory
-is allocated for the result.  The above code uses the fact that there
-can never be more wide characters in the converted result than there are
-bytes in the multibyte input string.  This method yields a pessimistic
-guess about the size of the result, and if many wide character strings
-have to be constructed this way or if the strings are long, the extra
-memory required to be allocated because the input string contains
-multibyte characters might be significant.  The allocated memory block can
-be resized to the correct size before returning it, but a better solution
-might be to allocate just the right amount of space for the result right
-away.  Unfortunately there is no function to compute the length of the wide
-character string directly from the multibyte string.  There is, however, a
-function that does part of the work.
+In the inner loop, a single wide character is stored in @code{wc}, and
+the number of consumed bytes is stored in the variable @code{nbytes}.
+If the conversion is successful, the uppercase variant of the wide
+character is stored in the code{result} array and the pointer to the
+input string and the number of available bytes is adjusted.  If the
+@code{mbrtowc} function returns zero, the null input byte has not been
+converted, so it must be stored explicitly in the result.
+
+The above code uses the fact that there can never be more wide
+characters in the converted result than there are bytes in the multibyte
+input string.  This method yields a pessimistic guess about the size of
+the result, and if many wide character strings have to be constructed
+this way or if the strings are long, the extra memory required to be
+allocated because the input string contains multibyte characters might
+be significant.  The allocated memory block can be resized to the
+correct size before returning it, but a better solution might be to
+allocate just the right amount of space for the result right away.
+Unfortunately there is no function to compute the length of the wide
+character string directly from the multibyte string.  There is, however,
+a function that does part of the work.
 
 @deftypefun size_t mbrlen (const char *restrict @var{s}, size_t @var{n}, mbstate_t *@var{ps})
 @standards{ISO, wchar.h}
diff --git a/manual/examples/mbstouwcs.c b/manual/examples/mbstouwcs.c
index 5d223da2ae..c94e1fa790 100644
--- a/manual/examples/mbstouwcs.c
+++ b/manual/examples/mbstouwcs.c
@@ -1,3 +1,4 @@
+#include <stdbool.h>
 #include <stdlib.h>
 #include <string.h>
 #include <wchar.h>
@@ -7,22 +8,46 @@
 wchar_t *
 mbstouwcs (const char *s)
 {
-  size_t len = strlen (s);
-  wchar_t *result = malloc ((len + 1) * sizeof (wchar_t));
+  /* Include the null terminator in the conversion.  */
+  size_t len = strlen (s) + 1;
+  wchar_t *result = reallocarray (NULL, len, sizeof (wchar_t));
+  if (result == NULL)
+    return NULL;
+
   wchar_t *wcp = result;
-  wchar_t tmp[1];
   mbstate_t state;
-  size_t nbytes;
-
   memset (&state, '\0', sizeof (state));
-  while ((nbytes = mbrtowc (tmp, s, len, &state)) > 0)
+
+  while (true)
     {
-      if (nbytes >= (size_t) -2)
-        /* Invalid input string.  */
-        return NULL;
-      *wcp++ = towupper (tmp[0]);
-      len -= nbytes;
-      s += nbytes;
+      wchar_t wc;
+      size_t nbytes = mbrtowc (&wc, s, len, &state);
+      if (nbytes == 0)
+        {
+          /* Terminate the result string.  */
+          *wcp = L'\0';
+          break;
+        }
+      else if (nbytes == (size_t) -2)
+        {
+          /* Truncated input string.  */
+          errno = EILSEQ;
+          free (result);
+          return NULL;
+        }
+      else if (nbytes == (size_t) -1)
+        {
+          /* Some other error (including EILSEQ).  */
+          free (result);
+          return NULL;
+        }
+      else
+        {
+          /* A character was converted.  */
+          *wcp++ = towupper (wc);
+          len -= nbytes;
+          s += nbytes;
+        }
     }
   return result;
 }