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C8RTOMB(3)                 Library Functions Manual                 C8RTOMB(3)

NAME
     c8rtomb - Restartable UTF-8 to multibyte conversion

LIBRARY
     Standard C Library (libc, -lc)

SYNOPSIS
     #include <uchar.h>

     size_t
     c8rtomb(char * restrict s, char8_t c8, mbstate_t * restrict ps);

DESCRIPTION
     The c8rtomb function decodes UTF-8 and converts it to multibyte
     characters in the current locale, keeping state to remember incremental
     progress if restarted.

     Each call to c8rtomb updates the conversion state ps with a UTF-8 code
     unit c8, writes up to MB_CUR_MAX bytes (possibly none) to s, 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 c8 is zero, c8rtomb discards any pending incomplete UTF-8 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, c8rtomb 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 mbrtoc8(3)), which is
     initialized at program startup to the initial conversion state.

RETURN VALUES
     The c8rtomb 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-8 code unit sequence to a multibyte string, NUL-terminate
     it (with any shift sequence needed to restore the initial state), and
     print it:

           char8_t c8[] = { 0xf0, 0x9f, 0x92, 0xa9 };
           char buf[(__arraycount(c8) + 1)*MB_LEN_MAX], *s = buf;
           size_t i;
           mbstate_t mbs = {0};    /* initial conversion state */

           for (i = 0; i < __arraycount(c8); i++) {
                   size_t len;

                   len = c8rtomb(s, c8[i], &mbs);
                   if (len == (size_t)-1)
                           err(1, "c8rtomb");
                   assert(len < sizeof(buf) - (s - buf));
                   s += len;
           }
           len = c8rtomb(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]      c8 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
     c16rtomb(3), c32rtomb(3), mbrtoc8(3), mbrtoc16(3), mbrtoc32(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.

     F. Yergeau, UTF-8, a transformation format of ISO 10646, Internet
     Engineering Task Force, RFC 3629,
     https://datatracker.ietf.org/doc/html/rfc3629, November 2003.

HISTORY
     The c8rtomb function first appeared in NetBSD 11.0.

CAVEATS
     The standard requires that passing zero as c8 unconditionally reset the
     conversion state and output a NUL byte:

           If c8 is a null 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 glibc 2.36 ignore this clause and,
     if the zero was preceded by a nonempty incomplete UTF-8 code unit
     sequence, fail with EILSEQ instead.

NetBSD 11.99                    August 15, 2024                   NetBSD 11.99