MBLEN(3) Library Functions Manual MBLEN(3)

mblen — get number of bytes in a multibyte character

Standard C Library (libc, -lc)

#include <stdlib.h>

int
mblen(const char *s, size_t n);

The () function usually determines the number of bytes in a multibyte character pointed to by s and returns it. This function examines at most n bytes of the array beginning from s.

In state-dependent encodings, s may point to the special sequence bytes to change the shift-state. Although such sequence bytes corresponds to no individual wide-character code, the () function changes its own state by them and treats them as if they were part of the subsequent multibyte character.

Unlike mbrlen(3), the first n bytes pointed to by s need to form an entire multibyte character. Otherwise, this function causes an error.

() is equivalent to the following call, except the internal state of the mbtowc(3) function is not affected:

mbtowc(NULL, s, n);

Calling any other functions in Standard C Library (libc, -lc) never changes the internal state of (), except for calling setlocale(3) with the LC_CTYPE category changed to that of the current locale. Such setlocale(3) calls cause the internal state of this function to be indeterminate.

The behaviour of () is affected by the LC_CTYPE category of the current locale.

These are the special cases:

s == NULL
() 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.
n == 0
The first n bytes of the array pointed to by s never form a complete character. Thus, mblen() always fails.

Normally, mblen() returns:

0
s points to a nul byte (‘\0’).
positive
The value returned is the number of bytes for the valid multibyte character pointed to by s. The value returned is at most n, and at most the value of the MB_CUR_MAX macro.
-1
s points to an invalid or incomplete multibyte character. The mblen() function also sets errno to indicate the error.

When s is equal to NULL, the mblen() function returns:

0
The current encoding is state-independent.
non-zero
The current encoding is state-dependent.

mblen() may cause an error in the following case:

[]
s points to an invalid or incomplete multibyte character.

mbrlen(3), mbtowc(3), setlocale(3)

The mblen() function conforms to ANSI X3.159-1989 (“ANSI C89”).

February 3, 2024 NetBSD 11.0