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.\" Copyright 2020 Robert Mustacchi
.\"
.Dd April 23, 2020
.Dt C16RTOMB 3C
.Os
.Sh NAME
.Nm c16rtomb ,
.Nm c32rtomb ,
.Nm wcrtomb ,
.Nm wcrtomb_l
.Nd convert wide-characters to character sequences
.Sh SYNOPSIS
.In uchar.h
.Ft size_t
.Fo c16rtomb
.Fa "char *restrict str"
.Fa "char16_t c16"
.Fa "mbstate_t *restrict ps"
.Fc
.Ft size_t
.Fo c32rtomb
.Fa "char *restrict str"
.Fa "char32_t c32"
.Fa "mbstate_t *restrict ps"
.Fc
.In stdio.h
.Ft size_t
.Fo wcrtomb
.Fa "char *restrict str"
.Fa "wchar_t wc"
.Fa "mbstate_t *restrict ps"
.Fc
.In stdio.h
.In xlocale.h
.Ft size_t
.Fo wcrtomb_l
.Fa "char *restrict str"
.Fa "wchar_t wc"
.Fa "mbstate_t *restrict ps"
.Fa "locale_t loc"
.Fc
.Sh DESCRIPTION
The
.Fn c16rtomb ,
.Fn c32rtomb ,
.Fn wcrtomb ,
and
.Fn wcrtomb_l
functions convert wide-character sequences into a series of multi-byte
characters.
The functions work in the following formats:
.Bl -tag -width wcrtomb_l
.It Fn c16rtomb
A UTF-16 code sequence, where every code point is represented by one or
two
.Vt char16_t .
The UTF-16 encoding will encode certain Unicode code points as a pair of
two 16-bit code sequences, commonly referred to as a surrogate pair.
.It Fn c32rtomb
A UTF-32 code sequence, where every code point is represented by a
single
.Vt char32_t .
It is illegal to pass reserved Unicode code points.
.It Fn wcrtomb , Fn wcrtomb_l
Wide characters, being a 32-bit value where every code point is
represented by a single
.Vt wchar_t .
While the
.Vt wchar_t
and
.Vt char32_t
are different types, in this implementation, they are similar encodings.
.El
.Pp
The functions all work by looking at the passed in wide-character
.Po
.Fa c16 ,
.Fa c32 ,
.Fa wc
.Pc
and appending it to the current conversion state,
.Fa ps .
Once a valid code point, based on the current locale, is found, then it
will be converted into a series of characters that are stored in
.Fa str .
Up to
.Dv MB_CUR_MAX
bytes will be stored in
.Fa str .
It is the caller's responsibility to ensure that there is sufficient
space in
.Fa str .
.Pp
The functions are all influenced by the
.Dv LC_CTYPE
category of the current locale for determining what is considered a
valid character.
For example, in the
.Sy C
locale,
only ASCII characters are recognized, while in a
.Sy UTF-8
based locale like
.Sy en_us.UTF-8 ,
all valid Unicode code points are recognized and will be converted into
the corresponding multi-byte sequence.
The
.Fn wcrtomb_l
function uses the locale passed in
.Fa loc
rather than the locale of the current thread.
.Pp
The
.Fa ps
argument represents a multi-byte conversion state which can be used
across multiple calls to a given function
.Pq but not mixed between functions .
These allow for characters to be consumed from subsequent buffers, e.g.
different values of
.Fa str .
The functions may be called from multiple threads as long as they use
unique values for
.Fa ps .
If
.Fa ps
is
.Dv NULL ,
then a function-specific buffer will be used for the conversion state;
however, this is stored between all threads and its use is not
recommended.
.Pp
The functions all have a special behavior when
.Dv NULL
is passed for
.Fa str .
They instead will treat it as though a the NULL wide-character was
passed in
.Fa c16 ,
.Fa c32 ,
or
.Fa wc
and an internal buffer
.Pq buf
will be used to write out the results of the
converstion.
In other words, the functions would be called as:
.Bd -literal -offset indent
c16rtomb(buf, L'\\0', ps)
c32rtomb(buf, L'\\0', ps)
wcrtomb(buf, L'\\0', ps)
wcrtomb_l(buf, L'\\0', ps, loc)
.Ed
.Ss Locale Details
Not all locales in the system are Unicode based locales.
For example, ISO 8859 family locales have code points with values that
do not match their counterparts in Unicode.
When using these functions with non-Unicode based locales, the code
points returned will be those determined by the locale.
They will not be converted from the corresponding Unicode code point.
For example, if using the Euro sign in ISO 8859-15, these functions
will not encode the Unicode value 0x20ac into the ISO 8859-15 value
0xa4.
.Pp
Regardless of the locale, the characters returned will be encoded as
though the code point were the corresponding value in Unicode.
This means that when using UTF-16, if the corresponding code point were
in the range for surorgate pairs, then the
.Fn c16rtomb
function will expect to receive that code point in that fashion.
.Pp
This behavior of the
.Fn c16rtomb
and
.Fn c32rtomb
functions should not be relied upon, is not portable, and subject to
change for non-Unicode locales.
.Sh RETURN VALUES
Upon successful completion, the
.Fn c16rtomb ,
.Fn c32rtomb ,
.Fn wcrtomb ,
and
.Fn wcrtomb_l
functions return the number of bytes stored in
.Fa str .
Otherwise,
.Sy (size_t)-1
is returned to indicate an encoding error and
.Va errno
is set.
.Sh EXAMPLES
.Sy Example 1
Converting a UTF-32 character into a multi-byte character sequence.
.Bd -literal
#include <locale.h>
#include <stdlib.h>
#include <string.h>
#include <err.h>
#include <stdio.h>
#include <uchar.h>

int
main(void)
{
        mbstate_t mbs;
        size_t ret;
        char buf[MB_CUR_MAX];
        char32_t val = 0x5149;
        const char *uchar_exp = "\exe5\ex85\ex89";

        (void) memset(&mbs, 0, sizeof (mbs));
        (void) setlocale(LC_CTYPE, "en_US.UTF-8");
        ret = c32rtomb(buf, val, &mbs);
        if (ret != strlen(uchar_exp)) {
                errx(EXIT_FAILURE, "failed to convert string, got %zd",
                    ret);
        }

        if (strncmp(buf, uchar_exp, ret) != 0) {
                errx(EXIT_FAILURE, "converted char32_t does not match "
                    "expected value");
        }

        return (0);
}
.Ed
.Sh ERRORS
The
.Fn c16rtomb ,
.Fn c32rtomb ,
.Fn wcrtomb ,
and
.Fn wcrtomb_l
functions will fail if:
.Bl -tag -width Er
.It Er EINVAL
The conversion state in
.Fa ps
is invalid.
.It Er EILSEQ
An invalid character sequence has been detected.
.El
.Sh MT-LEVEL
The
.Fn c16rtomb ,
.Fn c32rtomb ,
.Fn wcrtomb ,
and
.Fn wcrtomb_l
functions are
.Sy MT-Safe
as long as different
.Vt mbstate_t
structures are passed in
.Fa ps .
If
.Fa ps
is
.Dv NULL
or different threads use the same value for
.Fa ps ,
then the functions are
.Sy Unsafe .
.Sh INTERFACE STABILITY
.Sy Committed
.Sh SEE ALSO
.Xr mbrtoc16 3C ,
.Xr mbrtoc32 3C ,
.Xr mbrtowc 3C ,
.Xr newlocale 3C ,
.Xr setlocale 3C ,
.Xr uselocale 3C ,
.Xr uchar.h 3HEAD ,
.Xr environ 7