ios.seek(amount, whence=SEEK_SET) → 0

Seeks to a given offset anInteger in the stream according to the value of whence:

IO::SEEK_CUR  | Seeks to _amount_ plus current position
--------------+----------------------------------------------------
IO::SEEK_END  | Seeks to _amount_ plus end of stream (you probably
              | want a negative value for _amount_)
--------------+----------------------------------------------------
IO::SEEK_SET  | Seeks to the absolute location given by _amount_

Example:

f = File.new("testfile")
f.seek(-13, IO::SEEK_END)   #=> 0
f.readline                  #=> "And so on...\n"

Source Code

/*
*  call-seq:
*     ios.seek(amount, whence=SEEK_SET) -> 0
*  
*  Seeks to a given offset <i>anInteger</i> in the stream according to
*  the value of <i>whence</i>:
*
*    IO::SEEK_CUR  | Seeks to _amount_ plus current position
*    --------------+----------------------------------------------------
*    IO::SEEK_END  | Seeks to _amount_ plus end of stream (you probably 
*                  | want a negative value for _amount_)
*    --------------+----------------------------------------------------
*    IO::SEEK_SET  | Seeks to the absolute location given by _amount_
*
*  Example:
*     
*     f = File.new("testfile")
*     f.seek(-13, IO::SEEK_END)   #=> 0
*     f.readline                  #=> "And so on...\n"
*/

static VALUE
rb_io_seek_m(argc, argv, io)
   int argc;
   VALUE *argv;
   VALUE io;
{
   VALUE offset, ptrname;
   int whence = SEEK_SET;

   if (rb_scan_args(argc, argv, "11", &offset, &ptrname) == 2) {
       whence = NUM2INT(ptrname);
   }

   return rb_io_seek(io, offset, whence);
}