c/string/multibyte/mblen

Determines the size, in bytes, of the multibyte character whose first byte is pointed to by.

If is a null pointer,  determined whether shift sequences are used.

This function is equivalent to the call, except that conversion state of mbtowc is unaffected.

Return value
If is not a null pointer, returns the number of bytes that are contained in the multibyte character or  if the first bytes pointed to by  do not form a valid multibyte character or  if  is pointing at the null charcter.

If is a null pointer,  returns  if the current multibyte encoding is not state-dependent (does not use shift sequences) or a non-zero value if the current multibyte encoding is state-dependent (uses shift sequences).