Originální popis anglicky: 
MB_CUR_MAX - maximum length of a multibyte character in the current locale
Návod, kniha: Linux Programmer's Manual
#include <stdlib.h>
The 
MB_CUR_MAX macro defines an integer expression giving the maximum
  number of bytes needed to represent a single wide character in the current
  locale. It is locale dependent and therefore not a compile-time constant.
An integer >= 1 and <= MB_LEN_MAX. The value 1 denotes traditional 8-bit
  encoded characters.
ANSI C, POSIX.1
MB_LEN_MAX(3), 
mblen(3), 
mbstowcs(3), 
mbtowc(3),
  
wcstombs(3), 
wctomb(3)