C16RTOMB(3) | Library Functions Manual | C16RTOMB(3)

# NAME

`c16rtomb` —
Restartable UTF-16 to multibyte conversion

# LIBRARY

Standard C Library (libc, -lc)

# 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, https://www.unicode.org/versions/Unicode15.0.0/UnicodeStandard-15.0.pdf, The Unicode Consortium, September 2022, Version 15.0 — Core Specification.

P. Hoffman and F. Yergeau, UTF-16, an encoding of ISO 10646, Internet Engineering Task Force, RFC 2781, https://datatracker.ietf.org/doc/html/rfc2781, February 2000.

# STANDARDS

The `c16rtomb` function conforms to ISO/IEC 9899:2011 (“ISO C11”).

# 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.

August 14, 2024 | NetBSD 10.1