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.Dd August 15, 2024 .Dt C8RTOMB 3 .Os """"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
.Sh NAME .Nm c8rtomb .Nd Restartable UTF-8 to multibyte conversion """"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
.Sh LIBRARY .Lb libc """"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
.Sh SYNOPSIS . n uchar.h . .Ft size_t .Fo c8rtomb .Fa "char * restrict s" .Fa "char8_t c8" .Fa "mbstate_t * restrict ps" .Fc """"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
.Sh DESCRIPTION The .Nm function decodes UTF-8 and converts it to multibyte characters in the current locale, keeping state to remember incremental progress if restarted.
p Each call to .Nm updates the conversion state .Fa ps with a UTF-8 code unit .Fa c8 , writes up to .Dv MB_CUR_MAX bytes (possibly none) to .Fa s , and returns either the number of bytes written to .Fa s or .Li (size_t)-1 to denote error.
p If .Fa s is a null pointer, no output is produced and .Fa ps is reset to the initial conversion state, as if the call had been .Fo c8rtomb .Va buf , .Li 0 , .Fa ps .Fc for some internal buffer .Va buf .
p If .Fa c8 is zero, .Nm discards any pending incomplete UTF-8 code unit sequence in .Fa ps , outputs a (possibly empty) shift sequence to restore the initial state followed by a NUL byte, and resets .Fa ps to the initial conversion state.
p 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
o including those used by other functions such as .Xr mbrtoc8 3
c ,
which is initialized at program startup to the initial conversion
state.
""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
.Sh RETURN VALUES
The
.Nm
function returns the number of bytes written to
.Fa s
on success, or sets
.Xr errno 2
and returns
.Li "(size_t)-1"
on failure.
""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
.Sh 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:
d -literal -offset indent 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\en", buf);
.Ed
p
To avoid a variable-length array, this code uses
.Dv MB_LEN_MAX ,
which is a constant upper bound on the locale-dependent
.Dv MB_CUR_MAX .
""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
.Sh ERRORS
l -tag -width Bq t Bq Er EILSEQ .Fa c8
is invalid as the next code unit in the conversion state
.Fa ps .
t Bq Er EILSEQ The input cannot be encoded as a multibyte sequence in the current
locale.
t Bq Er EIO An error occurred in loading the locale's character conversions.
.El
""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
.Sh SEE ALSO
.Xr c16rtomb 3 ,
.Xr c32rtomb 3 ,
.Xr mbrtoc8 3 ,
.Xr mbrtoc16 3 ,
.Xr mbrtoc32 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 F. Yergeau
.%T UTF-8, a transformation format of ISO 10646
.%R RFC 3629
.%D November 2003
.%I Internet Engineering Task Force
.%U https://datatracker.ietf.org/doc/html/rfc3629
.Re
""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
.Sh STANDARDS
The
.Nm
function conforms to
.St -isoC-2023 .
.\" XXX PR misc/58600: man pages lack C17, C23, C++98, C++03, C++11, C++17, C++20, C++23 citation syntax
""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
.Sh HISTORY
The
.Nm
function first appeared in
.Nx 11.0 .
""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
.Sh CAVEATS
The standard requires that passing zero as
.Fa c8
unconditionally reset the conversion state and output a NUL byte:
d -filled -offset indent If
.Fa 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.
.Ed
p 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 .Er EILSEQ instead.
Indexes created Mon Sep 29 21:09:56 GMT 2025