.\"	$NetBSD: c16rtomb.3,v 1.11.2.2 2024/10/14 17:20:18 martin Exp $
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.Dd August 14, 2024
.Dt C16RTOMB 3
.Os
.\"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
.Sh NAME
.Nm c16rtomb
.Nd Restartable UTF-16 to multibyte conversion
.\"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
.Sh LIBRARY
.Lb libc
.\"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
.Sh SYNOPSIS
.
.In uchar.h
.
.Ft size_t
.Fo c16rtomb
.Fa "char * restrict s"
.Fa "char16_t c16"
.Fa "mbstate_t * restrict ps"
.Fc
.\"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
.Sh DESCRIPTION
The
.Nm
function decodes UTF-16 and converts it to multibyte characters in the
current locale, keeping state to remember incremental progress if
restarted.
.Pp
Each call to
.Nm
updates the conversion state
.Fa ps
with a UTF-16 code unit
.Fa c16 ,
writes up to
.Dv MB_CUR_MAX
bytes to
.Fa s
(possibly none), and returns either the number of bytes written to
.Fa s
or
.Li (size_t)-1
to denote error.
.Pp
If
.Fa s
is a null pointer,
no output is produced and
.Fa ps
is reset to the initial conversion state, as if the call had been
.Fo c8rtomb
.Va buf ,
.Li 0 ,
.Fa ps
.Fc
for some internal buffer
.Va buf .
.Pp
If
.Fa c16
is zero,
.Nm
discards any pending incomplete UTF-16 code unit sequence in
.Fa ps ,
outputs a (possibly empty) shift sequence to restore the initial state
followed by a NUL byte, and resets
.Fa ps
to the initial conversion state.
.Pp
If
.Fa ps
is a null pointer,
.Nm
uses an internal
.Vt mbstate_t
object with static storage duration, distinct from all other
.Vt mbstate_t
objects
.Po
including those used by other functions such as
.Xr mbrtoc16 3
.Pc ,
which is initialized at program startup to the initial conversion
state.
.\"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
.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 (with any shift sequence needed to restore the initial
state), and print it:
.Bd -literal -offset indent
char16_t c16[] = { 0xd83d, 0xdca9 };
char buf[(__arraycount(c16) + 1)*MB_LEN_MAX], *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;
}
len = c16rtomb(s, 0, &mbs);             /* NUL-terminate */
if (len == (size_t)-1)
        err(1, "c16rtomb");
assert(len <= sizeof(buf) - (s - buf));
printf("%s\en", buf);
.Ed
.Pp
To avoid a variable-length array, this code uses
.Dv MB_LEN_MAX ,
which is a constant upper bound on the locale-dependent
.Dv MB_CUR_MAX .
.\"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
.Sh ERRORS
.Bl -tag -width Bq
.It Bq Er EILSEQ
.Fa c16
is invalid as the next code unit in the conversion state
.Fa ps .
.It Bq Er EILSEQ
The input cannot be encoded as a multibyte sequence in the current
locale.
.It Bq Er EIO
An error occurred in loading the locale's character conversions.
.El
.\"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
.Sh SEE ALSO
.Xr c32rtomb 3 ,
.Xr c8rtomb 3 ,
.Xr mbrtoc16 3 ,
.Xr mbrtoc32 3 ,
.Xr mbrtoc8 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
The standard requires that passing zero as
.Fa c16
unconditionally reset the conversion state and output a NUL byte:
.Bd -filled -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
.Pp
However, some implementations such as
.Fx 14.0 ,
.Ox 7.4 ,
and glibc 2.36 ignore this clause and, if the zero was preceded by an
incomplete UTF-16 code unit sequence, fail with
.Er EILSEQ
instead.