MBLEN(3) | Library Functions Manual | MBLEN(3)

# NAME

`mblen` — get number of bytes in a multibyte character

# SYNOPSIS

`#include <stdlib.h>`

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

# DESCRIPTION

The `mblen`() function returns the number of bytes in the first multibyte character of the multibyte string s. It examines at most the first n bytes of s.

In state-dependent encodings, 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 `mblen`() function, and are treated 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.

`mblen`() 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 function in libc never changes the internal state of `mblen`(), except for calling setlocale(3) with an `LC_CTYPE` that differs from the current locale. Such setlocale(3) calls cause the internal state of this function to become indeterminate.

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

There are special cases:

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

# RETURN VALUES

The `mblen`() function returns:

0
  s points to a NUL byte (‘\0’).
positive
  The value returned is the number of bytes in the valid multibyte character pointed to by s. There are no cases when this value is greater than n or 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`, `mblen`() returns:

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

# ERRORS

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

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

# SEE ALSO

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

# STANDARDS

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

June 7, 2025 | Debian