mbrtowc(3) state after an invalid sequence "undefined" or "unspecified"?

[Kang-Che Sung]

Hi, Alejandro (or anyone else interested),

There's a discrepancy in the wording of the mbrtowc(3) function (and
similarly, mbsrtowcs(3) function) between in POSIX and ISO C. It could be
reported as an issue to POSIX (the Austin Group), and I am not sure if you
can do that.

In ISO C (I checked in both C99 and C23, in particular the N3220 draft),
there's a statement that if mbrtowc() returns a (size_t)(-1) as an encoding
error occurs, "the conversion state is unspecified".

POSIX (see <
https://pubs.opengroup.org/onlinepubs/9799919799/functions/mbrtowc.html>),
for the same part it says "the conversion state is undefined".

This wording difference matters when the "unspecified behavior" and
"undefined behavior" are technically different. An example is how the
mbstate_t object can be reused after an invalid sequence is encountered.
When the state is said to be "undefined" it's implied to be not usable
again (unless it is reset, e.g., by an `mbrtowc(NULL, "", 1, ps)` call).
When it's "unspecified" then implementations can allow the state to be
reused for certain encodings (possible for UTF-8, for example).

This is something I discovered accidentally when researching the multibyte
functions in the C standard library and how they work with an encoding like
UTF-8.
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