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.Dd August 14, 2024 .Dt C16RTOMB 3 .Os """"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
.Sh NAME .Nm c16rtomb .Nd Restartable UTF-16 code unit to multibyte conversion """"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
.Sh LIBRARY .Lb libc """"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
.Sh SYNOPSIS n uchar.h .Ft size_t .Fn c16rtomb "char * restrict s" \ "char16_t c16" \ "mbstate_t * restrict ps" """"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
.Sh DESCRIPTION The .Nm function attempts to encode Unicode input as a multibyte character sequence output at .Fa s in the current locale, writing anywhere between zero and .Dv MB_CUR_MAX bytes, inclusive, to .Fa s , depending on the inputs and conversion state .Fa ps .
p The input .Fa c16 is a UTF-16 code unit, which can be either: l -bullet t a Unicode scalar value in the Basic Multilingual Plane (BMP), that is, a 16-bit code unit outside the interval [0xd800,0xdfff]; or, t over the course of two consecutive calls to .Nm , the high and low surrogate code points of a Unicode scalar value outside the BMP. .El
p
If a low surrogate code point, that is, a value of
.Fa c16
in [0xdc00,0xdfff], is passed to
.Nm
without the preceding call to it with the same
.Fa ps
having been passed a high surrogate code point, that is, a value of
.Fa c16
in [0xd800,0xdbff], or if a high surrogate was passed in the previous
call and anything other than a low surrogate is passed, then
.Nm
will return
.Li (size_t)-1
to denote failure with
.Xr errno 2
set to
.Er EILSEQ .
""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
.Sh RETURN VALUES
The
.Nm
function returns the number of bytes written to
.Fa s
on success, or sets
.Xr errno 2
and returns
.Li "(size_t)-1"
on failure.
""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
.Sh EXAMPLES
Convert a UTF-16 code unit sequence to a multibyte string,
NUL-terminate it, and print it:
d -literal -offset indent char16_t c16[] = { 0xd83d, 0xdca9 };
char buf[__arraycount(c16)*MB_CUR_MAX + 1], *s = buf;
size_t i;
mbstate_t mbs = {0}; /* initial conversion state */
for (i = 0; i < __arraycount(c16); i++) {
size_t len;
len = c16rtomb(s, c16[i], &mbs);
if (len == (size_t)-1)
err(1, "c16rtomb");
assert(len < sizeof(buf) - (s - buf));
s += len;
}
*s = '\e0'; /* NUL-terminate */
printf("%s\en", buf);
.Ed
""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
.Sh ERRORS
l -tag -width ".Bq Er EILSEQ" t Bq Er EILSEQ A surrogate code point was passed as
.Fa c16
when it is inappropriate.
t Bq Er EILSEQ The Unicode scalar value requested cannot be encoded as a multibyte
sequence in the current locale.
t Bq Er EIO An error occurred in loading the locale's character conversions.
.El
""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
.Sh SEE ALSO
.Xr c32rtomb 3 ,
.Xr mbrtoc16 3 ,
.Xr mbrtoc32 3 ,
.Xr uchar 3
.Rs
.%B The Unicode Standard
.%O Version 15.0 \(em Core Specification
.%Q The Unicode Consortium
.%D September 2022
.%U https://www.unicode.org/versions/Unicode15.0.0/UnicodeStandard-15.0.pdf
.Re
.Rs
.%A P. Hoffman
.%A F. Yergeau
.%T UTF-16, an encoding of ISO 10646
.%R RFC 2781
.%D February 2000
.%I Internet Engineering Task Force
.%U https://datatracker.ietf.org/doc/html/rfc2781
.Re
""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
.Sh STANDARDS
The
.Nm
function conforms to
.St -isoC-2011 .
""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
.Sh HISTORY
The
.Nm
function first appeared in
.Nx 11.0 .
""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
.Sh BUGS
It is not clear from the standard how
.Nm
is supposed to behave when given a high surrogate code point followed
by a NUL:
d -literal -offset indent c16rtomb(s, 0xd800, ps);
c16rtomb(s, L'\e0', ps);
.Ed
p Currently this fails with .Er EILSEQ which matches other implementations, but this is at odds with language in the standard which suggests that passing .Li L'\e0' should unconditionally store a null byte and reset .Fa ps to the initial conversion state: d -offset indent If .Fa c16 is a null wide character, a null byte is stored, preceded by any shift sequence needed to restore the initial shift state; the resulting state described is the initial conversion state. .Ed
p However, it is unclear what else this should store besides a null byte. Should it discard the pending high surrogate, or convert it to something else and store that?
Indexes created Sun Oct 12 05:10:08 GMT 2025