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Message-ID: <CADDzAfO4_fcrefSR5OVHipoB0QCx7t+jMffd5m9Zotxqbyxp-w@mail.gmail.com>
Date: Tue, 29 Apr 2025 17:14:54 +0800
From: Kang-Che Sung <explorer09@...il.com>
To: musl@...ts.openwall.com
Cc: Alejandro Colomar <alx@...nel.org>
Subject: mbsnrtowcs(3) behavior not compatible with POSIX.1-2024
Hi, musl libc developers,
I just tested the mbsnrtowcs function in musl libc and discovered there is one
behavior that is not compatible with the new POSIX.1-2024 standard.
It's this thing: POSIX.1-2017 stated
"If the input buffer ends with an incomplete character, it is unspecified
whether conversion stops at the end of the previous character (if any), or at
the end of the input buffer.
[...] A future version may require that when the input buffer ends with an
incomplete character, conversion stops at the end of the input buffer."
(Reference: https://pubs.opengroup.org/onlinepubs/9699919799/functions/mbsrtowcs.html)
POSIX.1-2024 now requires the conversion stop at the end of the input buffer in
that case.
(https://pubs.opengroup.org/onlinepubs/9799919799/functions/mbsrtowcs.html)
(https://www.austingroupbugs.net/view.php?id=616)
Test code
```c
#include <locale.h>
#include <stdio.h>
#include <string.h>
#include <wchar.h>
wchar_t wcs[100];
char mbs[100];
int main()
{
mbstate_t state; const char *s;
setlocale(LC_CTYPE, "en_US.UTF-8");
memset(&state, 0, sizeof(state));
// U+754C U+7DDA
memcpy(mbs, "\xe7\x95\x8c\xe7\xb7\x9a", 7);
s = mbs;
printf("%zu, ", mbsnrtowcs(wcs, &s, 5, 100, &state));
printf("%td\n", s - mbs);
// Expected output: "1, 5". Actual output in musl: "1, 3".
memset(&state, 0, sizeof(state));
memcpy(mbs, "\xe7\x95\x8c\xe7\xb7", 6);
s = mbs;
printf("%zu, ", mbsnrtowcs(wcs, &s, 6, 100, &state));
printf("%td\n", s - mbs);
// Expected output: "18446744073709551615, 3"
}
```
By the way, I Cc'd the Linux man pages' maintainer as I plan to suggest a patch
to the mbsnrtowcs(3) man page. And it would be good to see the behaviors of
mbsnrtowcs consistent between glibc and musl libc.
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