cpp/io/basic streambuf/sgetn

@1@ Calls of the most derived class.

@2@ Reads characters from the input sequence and stores them into a character array pointed to by. The characters are read as if by repeated calls to sbumpc. That is, if less than characters are immediately available, the function calls uflow to provide more until  is returned. @@Classes derived from are permitted to provide more efficient implementations of this function.

Return value
The number of characters successfully read. If it is less than the input sequence has reached the end.