C16RTOMB(3)

NAME
c16rtomb - Restartable UTF-16 to multibyte conversion

LIBRARY
libc

SYNOPSIS
#include <uchar.h>
size_t
c16rtomb(char * restrict s, char16_t c16, mbstate_t * restrict ps);

DESCRIPTION
The c16rtomb function decodes UTF-16 and converts it to multibyte characters in the current locale, keeping state to remember incremental progress if restarted.

Each call to c16rtomb updates the conversion state ps with a UTF-16 code unit c16, writes up to MB_CUR_MAX bytes to s (possibly none), and returns either the number of bytes written to s or (size_t)-1 to denote error.

If s is a null pointer, no output is produced and ps is reset to the initial conversion state, as if the call had been c8rtomb( buf, 0,ps) for some internal buffer buf.

If c16 is zero, c16rtomb discards any pending incomplete UTF-16 code unit sequence in ps, outputs a (possibly empty) shift sequence to restore the initial state followed by a NUL byte, and resets ps to the initial conversion state.

If ps is a null pointer, c16rtomb uses an internal mbstate_t object with static storage duration, distinct from all other mbstate_t objects ( including those used by other functions such as mbrtoc16(3)), which is initialized at program startup to the initial conversion state.

RETURN VALUES
The c16rtomb function returns the number of bytes written to s on success, or sets errno(2) and returns (size_t)-1 on failure.

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:

    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\n", buf);

To avoid a variable-length array, this code uses MB_LEN_MAX, which is a constant upper bound on the locale-dependent MB_CUR_MAX.

ERRORS

[EILSEQ]
    c16 is invalid as the next code unit in the conversion state ps.

[EILSEQ]
    The input cannot be encoded as a multibyte sequence in the current locale.

[EIO]
    An error occurred in loading the locale's character conversions.

SEE ALSO
c32rtomb(3), c8rtomb(3), mbrtoc16(3), mbrtoc32(3), mbrtoc8(3), uchar(3) The Unicode Standard, Version 15.0 — Core Specification, The Unicode Consortium, September 2022, https://www.unicode.org/versions/Unicode15.0.0/UnicodeStandard-15.0.pdf. P. Hoffman, F. Yergeau, UTF-16, an encoding of ISO 10646, RFC 2781, February 2000, Internet Engineering Task Force, https://datatracker.ietf.org/doc/html/rfc2781.

STANDARDS
The c16rtomb function conforms to isoC-2011.

HISTORY
The c16rtomb function first appeared in NetBSD 11.0.

BUGS
The standard requires that passing zero as c16 unconditionally reset the conversion state and output a NUL byte:

If 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.

However, some implementations such as FreeBSD 14.0, OpenBSD 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 EILSEQ instead.