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.\" $NetBSD: c16rtomb.3,v 1.11 2024/08/20 20:36:30 riastradh Exp $
.\"
.\" Copyright (c) 2024 The NetBSD Foundation, Inc.
.\" All rights reserved.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\" 1. Redistributions of source code must retain the above copyright
.\" notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\" notice, this list of conditions and the following disclaimer in the
.\" documentation and/or other materials provided with the distribution.
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS
.\" ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
.\" TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
.\" PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS
.\" BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
.\" CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
.\" SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
.\" INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
.\" CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
.\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
.\" POSSIBILITY OF SUCH DAMAGE.
.\"
.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.
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