.\" $OpenBSD: mblen.3,v 1.6 2025/06/07 09:45:07 schwarze Exp $
.\" $NetBSD: mblen.3,v 1.4 2003/04/16 13:34:40 wiz Exp $
.\"
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.Dd $Mdocdate: June 7 2025 $
.Dt MBLEN 3
.Os
.\" ----------------------------------------------------------------------
.Sh NAME
.Nm mblen
.Nd get number of bytes in a multibyte character
.\" ----------------------------------------------------------------------
.Sh SYNOPSIS
.In stdlib.h
.Ft int
.Fn mblen "const char *s" "size_t n"
.\" ----------------------------------------------------------------------
.Sh DESCRIPTION
The
.Fn mblen
function returns the number of bytes
in the first multibyte character of the multibyte string
.Fa s .
It examines at most the first
.Fa n
bytes of
.Fa s .
.Pp
In state-dependent encodings,
.Fa s
may point to special sequence bytes changing the shift state.
Although such sequence bytes correspond to no wide character,
they affect the internal conversion state of the
.Fn mblen
function, and are treated
as if they were part of the subsequent multibyte character.
.Pp
Unlike
.Xr mbrlen 3 ,
the first
.Fa n
bytes pointed to by
.Fa s
need to form an entire multibyte character.
Otherwise, this function causes an error.
.Pp
.Fn mblen
is equivalent to the following call, except the internal state of the
.Xr mbtowc 3
function is not affected:
.Bd -literal -offset indent
mbtowc(NULL, s, n);
.Ed
.Pp
Calling any other function in
.Em libc
never changes the internal state of
.Fn mblen ,
except for calling
.Xr setlocale 3
with an
.Dv LC_CTYPE
that differs from the current locale.
Such
.Xr setlocale 3
calls cause the internal state of this function to become indeterminate.
.Pp
The behaviour of
.Fn mblen
is affected by the
.Dv LC_CTYPE
category of the current locale.
.Pp
There are special cases:
.Bl -tag -width 0123456789
.It "s == NULL"
.Fn mblen
initializes its own internal state to an initial state, and
determines whether the current encoding is state-dependent.
This function returns 0 if the encoding is state-independent,
otherwise non-zero.
.It "n == 0"
In this case,
the first
.Fa n
bytes of the array pointed to by
.Fa s
never form a complete character.
Thus,
.Fn mblen
always fails.
.El
.\" ----------------------------------------------------------------------
.Sh RETURN VALUES
The
.Fn mblen
function returns:
.Bl -tag -width 0123456789
.It "0"
.Fa s
points to a NUL byte
.Pq Sq \e0 .
.It "positive"
The value returned is
the number of bytes in the valid multibyte character pointed to by
.Fa s .
There are no cases when this value is greater than
.Fa n
or the value of the
.Dv MB_CUR_MAX
macro.
.It "-1"
.Fa s
points to an invalid or incomplete multibyte character.
The
.Fn mblen
function also sets
.Va errno
to indicate the error.
.El
.Pp
When
.Fa s
is equal to
.Dv NULL ,
.Fn mblen
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
The
.Fn mblen
function may cause an error in the following case:
.Bl -tag -width Er
.It Bq Er EILSEQ
.Fa s
points to an invalid or incomplete multibyte character.
.El
.\" ----------------------------------------------------------------------
.Sh SEE ALSO
.Xr mbrlen 3 ,
.Xr mbtowc 3 ,
.Xr setlocale 3
.\" ----------------------------------------------------------------------
.Sh STANDARDS
The
.Fn mblen
function conforms to
.St -ansiC .