MB_CUR_MAX(3) Library Functions Manual MB_CUR_MAX(3)

MB_CUR_MAXmaximum number of bytes in a multibyte character

#include <stdlib.h>


#include <limits.h>

#define MB_LEN_MAX 4

MB_CUR_MAX is a macro that returns the maximum number of bytes needed to represent any multibyte character in the current character encoding. Usually, the character encoding is selected for the whole program using setlocale(3) with a category argument of LC_CTYPE, but it can be overridden on a per-thread basis using uselocale(3).

By default and in the "C" locale, MB_CUR_MAX returns 1. On OpenBSD, the only other possible return value is 4; it occurs when using a UTF-8 locale. On other systems, MB_CUR_MAX may return positive values other than 1 or 4.

MB_LEN_MAX is a constant specifying the maximum number of bytes needed to represent any multibyte character in any supported character encoding. On OpenBSD, it is always 4. On other systems, it may have a different value greater than or equal to 1.

On any system, MB_CUR_MAX returns an integral value in the range from 1 to MB_LEN_MAX, inclusive.

Size a buffer in a portable way to hold one single multibyte character:

char	 buf[MB_LEN_MAX];
wchar_t	 wchar;  /* input value */

if (wctomb(buf, wchar) == -1)
	/* error */

Switch between code handling the ascii(7) and UTF-8 character encodings in an OpenBSD-specific way (not portable):

if (MB_CUR_MAX == 1) {
	/* Code to handle ASCII-encoded single-byte strings. */
} else {
	/* Code to handle UTF-8-encoded multibyte strings. */

mblen(3), setlocale(3), uselocale(3), wctomb(3)

MB_CUR_MAX and MB_LEN_MAX conform to ANSI X3.159-1989 (“ANSI C89”).

MB_CUR_MAX has been non-constant and thread-dependent since OpenBSD 6.2.

Since MB_CUR_MAX is thread-dependent, calling it in a loop that processes individual bytes or characters is likely to slow down the loop considerably. If possible, consider calling it once before the loop and caching the return value in a local variable to improve performance. The value remains valid as long as the thread does not call setlocale(3) or uselocale(3).

August 25, 2023 OpenBSD 7.5