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

Alejandro Colomar <[email protected]> Thu, 28 May 2026 14:11:35 +0200

Newsgroups	org.kernel.vger.linux-man
Message-ID	<ahgw1XTxn7iJ6B_3@devuan>

Hi Kang-Che,

On 2026-05-21T23:08:20+0800, Kang-Che Sung wrote:
> 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.

Thanks!  I've opened this bug report:

<https://www.austingroupbugs.net/view.php?id=1982>


Have a lovely day!
Alex

-- 
<https://www.alejandro-colomar.es>

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