text: #include <stdlib.h>
  int mblen(const char *s, size_t n);
  If s is not a null pointer, mblen() shall determine the number of bytes constituting the character pointed to by s. Except that the shift state of mbtowc() is not affected, it shall be equivalent to:
  mbtowc((wchar_t *)0, s, n);
  The implementation shall behave as if no function defined in this volume of IEEE Std 1003.1-2001 calls mblen()..
  If s is a null pointer, mblen() shall return a non-zero or 0 value, if character encodings, respectively, do or do not have state-dependent encodings. If s is not a null pointer, mblen() shall either return 0 (if s points to the null byte), or return the number of bytes that constitute the character (if the next n or fewer bytes form a valid character), or return -1 (if they do not form a valid character) and may set errno to indicate the error. In no case shall the value returned be greater than n or the value of the {MB_CUR_MAX} macro.
  The mblen() function may fail if:
  The following sections are informative.
  None.
  None.
  None.
  None.
  mbtowc() , mbstowcs() , wctomb() , wcstombs() , the Base Definitions volume of IEEE Std 1003.1-2001, <stdlib.h>