basic_streambuf* setbuf(char_type* s, streamsize n);
pos_type seekoff(off_type off, ios_base::seekdir way,
ios_base::openmode which
= ios_base::in | ios_base::out);
pos_type seekpos(pos_type sp,
ios_base::openmode which
= ios_base::in | ios_base::out);
int sync();