Get some characters.
  Gets a series of characters from in input sequence and stores them in the array pointed by s until n characters have been copied or the input stream has no more characters.
  This is a virtual member function that can be overridden in derived classes.

Parameters.

s

Pointer to a block of memory where the character sequence will be stored.

n

Number of character to be gotten. This is an object of type streamsize.

Return Value.
  The number of characters read.

Basic template member declaration ( basic_streambuf<charT,traits> ):

typedef charT char_type; streamsize xsgetn (char_type * s, streamsize n);

See also.
  sgetn
  streambuf class