Re: [musl] swprintf %lc directive does not work for some wide characters

[Rich Felker <dalias@libc.org>]

On Mon, Jun 12, 2023 at 04:44:44PM +0200, Bruno Haible wrote:
> According to ISO C 11 § 7.29.2.1, in the *wprintf family of functions, the
> %lc directive works like this:
>   "[If an l length modifier is present,] the wint_t argument is converted to
>    wchar_t and written."
> 
> Likewise in ISO C 17 § 7.29.2 and ISO C 23 § 7.31.2.1 and in POSIX:2018
> <https://pubs.opengroup.org/onlinepubs/9699919799/functions/fwprintf.html>.
> 
> In musl libc 1.2.4 (as part of Alpine Linux 3.18.0) this does not work for
> some characters.
> 
> How to reproduce:
> =================================== foo.c ===================================
> #include <stdio.h>
> #include <wchar.h>
> int main ()
> {
>   static wint_t L_invalid = (wchar_t) 0x76543210;
>   wchar_t buf[3];
>   int ret = swprintf (buf, 3, L"%lc", L_invalid);
>   if (ret >= 0)
>     fprintf (stderr, "OK, %d characters\n", ret);
>   else
>     perror ("swprintf failed");
> }
> =============================================================================
> $ gcc -Wall foo.c
> $ ./a.out
> 
> Expected output:
> OK, 1 characters
> 
> Actual output:
> swprintf failed: Illegal byte sequence
> 
> 

Per my reading of the specification, this is not a bug but is the
expected behavior.

    In addition, all forms of fwprintf() shall fail if:

    [EILSEQ]
            A wide-character code that does not correspond to a valid
            character has been detected.

Since the language "has been detected" is used here, this seems to
allow for an implementation not to make it an error if the condition
is not "detected". We make it an error because all wide stdio takes
place through a byte-oriented buffer, and the conversions back and
forth inherently "detect" the condition and have no way to pass the
invalid wchar_t thru. There is no concept of directly writing wide
characters.

Note that the error here is happening not as part of the conversion
specifier, but the output operation.

Rich