.\" $OpenBSD: mbrtowc.3,v 1.4 2013/06/05 03:39:22 tedu Exp $
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.Dd $Mdocdate: June 5 2013 $
.Dt MBRTOWC 3
.Os
.Sh NAME
.Nm mbrtowc
.Nd converts a multibyte character to a wide character (restartable)
.Sh SYNOPSIS
.In wchar.h
.Ft size_t
.Fn mbrtowc "wchar_t * restrict wc" "const char * restrict s" "size_t n" \
"mbstate_t * restrict mbs"
.Sh DESCRIPTION
The
.Fn mbrtowc
function examines at most
.Fa n
bytes of the multibyte character byte string pointed to by
.Fa s ,
converts those bytes to a wide character, and stores the wide character
in the wchar_t object pointed to by
.Fa wc
if
.Fa wc
is not
.Dv NULL
and
.Fa s
points to a valid character.
.Pp
Conversion happens in accordance with the conversion state described
by the mbstate_t object pointed to by
.Fa mbs .
The mbstate_t object must be initialized to zero before the application's
first call to
.Fn mbrtowc .
If the previous call to
.Fn mbrtowc
did not return (size_t)-1, the mbstate_t object can safely be reused
without reinitialization.
.Pp
The behaviour of
.Fn mbrtowc
is affected by the
.Dv LC_CTYPE
category of the current locale.
If the locale is changed without reinitialization of the mbstate_t object
pointed to by
.Fa mbs ,
the behaviour of
.Fn mbrtowc
is undefined.
.Pp
Unlike
.Xr mbtowc 3 ,
.Fn mbrtowc
will accept an incomplete byte sequence pointed to by
.Fa s
which does not form a complete character but is potentially part of
a valid character.
In this case,
.Fn mbrtowc
consumes all such bytes.
The conversion state saved in the mbstate_t object pointed to by
.Fa mbs
will be used to restart the suspended conversion during the next
call to
.Fn mbrtowc .
.Pp
In state-dependent encodings,
.Fa s
may point to a special sequence of bytes called a
.Dq shift sequence .
Shift sequences switch between character code sets available within an
encoding scheme.
One encoding scheme using shift sequences is ISO/IEC 2022-JP, which
can switch e.g. from ASCII (which uses one byte per character) to
JIS X 0208 (which uses two bytes per character).
Shift sequence bytes correspond to no individual wide character, so
.Fn mbrtowc
treats them as if they were part of the subsequent multibyte character.
Therefore they do contribute to the number of bytes in the multibyte character.
.Pp
Special cases in interpretation of arguments are as follows:
.Bl -tag -width 012345678901
.It "wc == NULL "
The conversion from a multibyte character to a wide character is performed
and the conversion state may be affected, but the resulting wide character
is discarded.
.Pp
This can be used to find out how many bytes are contained in the
multibyte character pointed to by
.Fa s .
.It "s == NULL "
.Fn mbrtowc
ignores
.Fa wc
and
.Fa n ,
and behaves equivalent to
.Bd -literal -offset indent
mbrtowc(NULL, "", 1, mbs);
.Ed
.Pp
which attempts to use the mbstate_t object pointed to by
.Fa mbs
to start or continue conversion using the empty string as input,
and discards the conversion result.
.Pp
If conversion succeeds, this call always returns zero.
Unlike
.Xr mbtowc 3 ,
the value returned does not indicate whether the current encoding of
the locale is state-dependent, i.e. uses shift sequences.
.It "mbs == NULL "
.Fn mbrtowc
uses its own internal state object to keep the conversion state,
instead of an mbstate_t object pointed to by
.Fa mbs .
This internal conversion state is initialized once at program startup.
It is not safe to call
.Fn mbrtowc
again with a
.Dv NULL
.Fa mbs
argument if
.Fn mbrtowc
returned (size_t)-1 because at this point the internal conversion state
is undefined.
.Pp
Calling any other functions in
.Em libc
never changes the internal
conversion state object of
.Fn mbrtowc .
.El
.Sh RETURN VALUES
.Bl -tag -width 012345678901
.It 0
The bytes pointed to by
.Fa s
form a terminating NUL character.
If
.Fa wc
is not
.Dv NULL ,
a NUL wide character has been stored in the wchar_t object pointed to by
.Fa wc .
.It positive
.Fa s
points to a valid character, and the value returned is the number of
bytes completing the character.
If
.Fa wc
is not
.Dv NULL ,
the corresponding wide character has been stored in the wchar_t object
pointed to by
.Fa wc .
.It (size_t)-1
.Fa s
points to an illegal byte sequence which does not form a valid multibyte
character in the current locale.
.Fn mbrtowc
sets
.Va errno
to EILSEQ.
The conversion state object pointed to by
.Fa mbs
is left in an undefined state and must be reinitialized before being
used again.
.Pp
Because applications using
.Fn mbrtowc
are shielded from the specifics of the multibyte character encoding scheme,
it is impossible to repair byte sequences containing encoding errors.
Such byte sequences must be treated as invalid and potentially malicious input.
Applications must stop processing the byte string pointed to by
.Fa s
and either discard any wide characters already converted, or cope with
truncated input.
.It (size_t)-2
.Fa s
points to an incomplete byte sequence of length
.Fa n
which has been consumed and contains part of a valid multibyte character.
.Fn mbrtowc
sets
.Va errno
to EILSEQ.
The character may be completed by calling
.Fn mbrtowc
again with
.Fa s
pointing to one or more subsequent bytes of the multibyte character and
.Fa mbs
pointing to the conversion state object used during conversion of the
incomplete byte sequence.
.El
.Sh ERRORS
The
.Fn mbrtowc
function may cause an error in the following cases:
.Bl -tag -width Er
.It Bq Er EILSEQ
.Fa s
points to an invalid or incomplete multibyte character.
.It Bq Er EINVAL
.Fa mbs
points to an invalid or uninitialized mbstate_t object.
.El
.Sh SEE ALSO
.Xr mbrlen 3 ,
.Xr mbtowc 3 ,
.Xr setlocale 3
.Sh STANDARDS
The
.Fn mbrtowc
function conforms to
.\" .St -isoC-amd1 .
ISO/IEC 9899/AMD1:1995
.Pq Dq ISO C90, Amendment 1 .
The restrict qualifier is added at
.\" .St -isoC99 .
ISO/IEC 9899:1999
.Pq Dq ISO C99 .
.Sh CAVEATS
.Fn mbrtowc
is not suitable for programs that care about internals of the character
encoding scheme used by the byte string pointed to by
.Fa s .
.Pp
It is possible that
.Fn mbrtowc
fails because of locale configuration errors.
An
.Dq invalid
character sequence may simply be encoded in a different encoding than that
of the current locale.
.Pp
The special cases for
.Fa s
== NULL and
.Fa mbs
== NULL do not make any sense.
Instead of passing
.Dv NULL
for
.Fa mbs ,
.Xr mbtowc 3
can be used.
.Pp
Earlier versions of this man page implied that calling
.Fn mbrtowc
with a
.Dv NULL
.Fa s
argument would always set
.Fa mbs
to the initial conversion state.
But this is true only if the previous call to
.Fn mbrtowc
using
.Fa mbs
did not return (size_t)-1 or (size_t)-2.
It is recommended to zero the mbstate_t object instead.