.\" $OpenBSD: mbtowc.3,v 1.6 2016/02/27 14:07:04 schwarze Exp $
.\" $NetBSD: mbtowc.3,v 1.5 2003/09/08 17:54:31 wiz Exp $
.\"
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.Dd $Mdocdate: February 27 2016 $
.Dt MBTOWC 3
.Os
.\" ----------------------------------------------------------------------
.Sh NAME
.Nm mbtowc
.Nd converts a multibyte character to a wide character
.\" ----------------------------------------------------------------------
.Sh SYNOPSIS
.In stdlib.h
.Ft int
.Fn mbtowc "wchar_t * restrict pwc" "const char * restrict s" "size_t n"
.Sh DESCRIPTION
The
.Fn mbtowc
function converts the multibyte character pointed to by
.Fa s
to a wide character, and stores it in the wchar_t object pointed to by
.Fa pwc .
This function may inspect at most
.Fa n
bytes of the array pointed to by
.Fa s .
.Pp
Unlike
.Xr mbrtowc 3 ,
the first
.Fa n
bytes pointed to by
.Fa s
need to form an entire multibyte character.
Otherwise, this function returns an error and the internal state will
be undefined.
.Pp
If a call to
.Fn mbtowc
resulted in an undefined internal state,
.Fn mbtowc
must be called with
.Ar s
set to
.Dv NULL
to reset the internal state before it can safely be used again.
.Pp
The behaviour of
.Fn mbtowc
is affected by the
.Dv LC_CTYPE
category of the current locale.
Calling any other functions in
.Em libc
never changes the internal
state of
.Fn mbtowc ,
except for calling
.Xr setlocale 3
with the
.Dv LC_CTYPE
category set to a different locale.
Such
.Xr setlocale 3
calls cause the internal state of this function to be undefined.
.Pp
In state-dependent encodings such as ISO/IEC 2022-JP,
.Fa s
may point to the special sequence of bytes to change the shift-state.
Because such sequence bytes do not correspond to any individual wide character,
.Fn mbtowc
treats them as if they were part of the subsequent multibyte character.
.Pp
The following special cases apply to the arguments:
.Bl -tag -width 012345678901
.It s == NULL
.Fn mbtowc
initializes its own internal state to the initial state, and
determines whether the current encoding is state-dependent.
.Fn mbtowc
returns 0 if the encoding is state-independent,
otherwise non-zero.
.Fa pwc
is ignored.
.It pwc == NULL
.Fn mbtowc
behaves just as if
.Fa pwc
was not
.Dv NULL ,
including modifications to internal state,
except that the result of the conversion is discarded.
This can be used to determine the size of the wide character
representation of a multibyte string.
Another use case is a check for illegal or incomplete multibyte sequences.
.It n == 0
In this case,
the first
.Fa n
bytes of the array pointed to by
.Fa s
never form a complete character and
.Fn mbtowc
always fails.
.El
.\" ----------------------------------------------------------------------
.Sh RETURN VALUES
Normally,
.Fn mbtowc
returns:
.Bl -tag -width 012345678901
.It 0
.Fa s
points to a null byte
.Pq Sq \e0 .
.It positive
Number of bytes for the valid multibyte character pointed to by
.Fa s .
There are no cases where the value returned is greater than
the value of the
.Dv MB_CUR_MAX
macro.
.It -1
.Fa s
points to an invalid or an incomplete multibyte character.
.Va errno
is set to indicate the error.
.El
.Pp
When
.Fa s
is
.Dv NULL ,
.Fn mbtowc
returns:
.Bl -tag -width 0123456789
.It 0
The current encoding is state-independent.
.It non-zero
The current encoding is state-dependent.
.El
.\" ----------------------------------------------------------------------
.Sh ERRORS
.Fn mbtowc
will set
.Va errno
in the following cases:
.Bl -tag -width Er
.It Bq Er EILSEQ
.Fa s
points to an invalid or incomplete multibyte character.
.El
.\" ----------------------------------------------------------------------
.Sh SEE ALSO
.Xr mblen 3 ,
.Xr mbrtowc 3 ,
.Xr setlocale 3
.\" ----------------------------------------------------------------------
.Sh STANDARDS
The
.Fn mbtowc
function conforms to
.St -ansiC .
The restrict qualifier is added at
.St -isoC-99 .
Setting
.Va errno
is an
.St -p1003.1-2008
extension.
.Sh CAVEATS
On error, callers of
.Fn mbtowc
cannot tell whether the multibyte character was invalid or incomplete.
To treat incomplete data differently from invalid data the
.Xr mbrtowc 3
function can be used instead.