.\"	$NetBSD: mbrtoc16.3,v 1.10.2.2 2024/10/14 17:20:17 martin Exp $
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.Dd August 14, 2024
.Dt MBRTOC16 3
.Os
.\"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
.Sh NAME
.Nm mbrtoc16
.Nd Restartable multibyte to UTF-16 conversion
.\"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
.Sh LIBRARY
.Lb libc
.\"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
.Sh SYNOPSIS
.
.In uchar.h
.
.Ft size_t
.Fo mbrtoc16
.Fa "char16_t * restrict pc16"
.Fa "const char * restrict s"
.Fa "size_t n"
.Fa "mbstate_t * restrict ps"
.Fc
.\"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
.Sh DESCRIPTION
The
.Nm
function decodes multibyte characters in the current locale and
converts them to UTF-16, keeping state so it can restart after
incremental progress.
.Pp
Each call to
.Nm :
.Bl -enum -compact
.It
examines up to
.Fa n
bytes starting at
.Fa s ,
.It
yields a UTF-16 code unit if available by storing it at
.Li * Ns Fa pc16 ,
.It
saves state at
.Fa ps ,
and
.It
returns either the number of bytes consumed if any or a special return
value.
.El
.Pp
Specifically:
.Bl -bullet
.It
If the multibyte sequence at
.Fa s
is invalid after any previous input saved at
.Fa ps ,
or if an error occurs in decoding,
.Nm
returns
.Li (size_t)-1
and sets
.Xr errno 2
to indicate the error.
.It
If the multibyte sequence at
.Fa s
is still incomplete after
.Fa n
bytes, including any previous input saved in
.Fa ps ,
.Nm
saves its state in
.Fa ps
after all the input so far and returns
.Li "(size_t)-2".
.Sy All
.Fa n
bytes of input are consumed in this case.
.It
If
.Nm
had previously decoded a multibyte character but has not yet yielded
all the code units of its UTF-16 encoding, it stores the next UTF-16
code unit at
.Li * Ns Fa pc16
and returns
.Li "(size_t)-3" .
.Sy \&No
bytes of input are consumed in this case.
.It
If
.Nm
decodes the null multibyte character, then it stores zero at
.Li * Ns Fa pc16
and returns zero.
.It
Otherwise,
.Nm
decodes a single multibyte character, stores the first (and possibly
only) code unit in its UTF-16 encoding at
.Li * Ns Fa pc16 ,
and returns the number of bytes consumed to decode the first multibyte
character.
.El
.Pp
If
.Fa pc16
is a null pointer, nothing is stored, but the effects on
.Fa ps
and the return value are unchanged.
.Pp
If
.Fa s
is a null pointer, the
.Nm
call is equivalent to:
.Bd -ragged -offset indent
.Fo mbrtoc16
.Li NULL ,
.Li \*q\*q ,
.Li 1 ,
.Fa ps
.Fc
.Ed
.Pp
This always returns zero, and has the effect of resetting
.Fa ps
to the initial conversion state, without writing to
.Fa pc16 ,
even if it is nonnull.
.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
.Xr mbrtoc8 3 ,
.Xr mbrtoc32 3 ,
.Xr c8rtomb 3 ,
.Xr c16rtomb 3 ,
and
.Xr c32rtomb 3
.Pc ,
which is initialized at program startup to the initial conversion
state.
.\"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
.Sh IMPLEMENTATION NOTES
On well-formed input, the
.Nm
function yields either a Unicode scalar value in the Basic Multilingual
Plane (BMP), i.e., a 16-bit Unicode code point that is not a surrogate
code point, or, over two successive calls, yields the high and low
surrogate code points (in that order) of a Unicode scalar value outside
the BMP.
.\"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
.Sh RETURN VALUES
The
.Nm
function returns:
.Bl -tag -width Li
.It Li 0
.Bq null
if
.Nm
decoded a null multibyte character.
.It Ar i
.Bq code unit
where
.Li 1
\*(Le
.Ar i
\*(Le
.Fa n ,
if
.Nm
consumed
.Ar i
bytes of input to decode the next multibyte character, yielding a
UTF-16 code unit.
.It Li (size_t)-3
.Bq continuation
if
.Nm
consumed no new bytes of input but yielded a UTF-16 code unit that was
pending from previous input.
.It Li (size_t)-2
.Bq incomplete
if
.Nm
found only an incomplete multibyte sequence after all
.Fa n
bytes of input and any previous input, and saved its state to restart
in the next call with
.Fa ps .
.It Li (size_t)-1
.Bq error
if any encoding error was detected;
.Xr errno 2
is set to reflect the error.
.El
.\"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
.Sh EXAMPLES
Print the UTF-16 code units of a multibyte string in hexadecimal text:
.Bd -literal -offset indent
char *s = ...;
size_t n = ...;
mbstate_t mbs = {0};    /* initial conversion state */

while (n) {
        char16_t c16;
        size_t len;

        len = mbrtoc16(&c16, s, n, &mbs);
        switch (len) {
        case 0:         /* NUL terminator */
                assert(c16 == 0);
                goto out;
        default:        /* scalar value or high surrogate */
                printf("U+%04"PRIx16"\en", (uint16_t)c16);
                break;
        case (size_t)-3: /* low surrogate */
                printf("continue U+%04"PRIx16"\en", (uint16_t)c16);
                break;
        case (size_t)-2: /* incomplete */
                printf("incomplete\en");
                goto readmore;
        case (size_t)-1: /* error */
                printf("error: %d\en", errno);
                goto out;
        }
        s += len;
        n -= len;
}
.Ed
.\"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
.Sh ERRORS
.Bl -tag -width Bq
.It Bq Er EILSEQ
The multibyte sequence cannot be decoded in the current locale as a
Unicode scalar value.
.It Bq Er EIO
An error occurred in loading the locale's character conversions.
.El
.\"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
.Sh SEE ALSO
.Xr c16rtomb 3 ,
.Xr c32rtomb 3 ,
.Xr c8rtomb 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 .