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.

1 files changed, 11 insertions, 0 deletions

diff --git a/ChangeLog b/ChangeLog
index 9b30d0ce3b..c36b56a8e9 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,16 @@
 2018-04-05  Florian Weimer  <fweimer@redhat.com>
 
+	* manual/examples/mbstouwcs.c (mbstouwcs): Fix loop termination,
+	integer overflow, memory leak on error, and indeterminate errno
+	value.  Add a null wide character to terminate the result string.
+	* manual/charset.texi (Converting a Character): Mention embedded
+	null bytes in the mbrtowc input string.  Explain what happens in
+	the -2 result case.  Do not claim that mbrtowc is simple or
+	obvious to use.  Adjust the description of the code example.  Use
+	@code, not @var, for concrete variables.
+
+2018-04-05  Florian Weimer  <fweimer@redhat.com>
+
 	* manual/examples/mbstouwcs.c: New file.
 	* manual/charset.texi (Converting a Character): Include it.