std::basic_streambuf::sgetn

std::streamsize sgetn( char_type* s, std::streamsize count );

protected:
virtual std::streamsize xsgetn( char_type* s, std::streamsize count );

1) Calls xsgetn(s, count) of the most derived class.

2) Reads count characters from the input sequence and stores them into a character array pointed to by s. The characters are read as if by repeated calls to sbumpc(). That is, if less than count characters are immediately available, the function calls uflow() to provide more until traits::eof() is returned.