% svn diff --diff-cmd diff -x '-u -p'
Index: io.c
===================================================================
--- io.c	(revision 36217)
+++ io.c	(working copy)
@@ -140,6 +140,7 @@ static VALUE argf;
 static ID id_write, id_read, id_getc, id_flush, id_readpartial, id_set_encoding;
 static VALUE sym_mode, sym_perm, sym_extenc, sym_intenc, sym_encoding, sym_open_args;
 static VALUE sym_textmode, sym_binmode, sym_autoclose;
+static VALUE sym_set, sym_cur, sym_end;
 
 struct argf {
     VALUE filename, current_file;
@@ -1443,6 +1444,18 @@ rb_io_seek(VALUE io, VALUE offset, int w
     return INT2FIX(0);
 }
 
+static int
+interpret_seek_whence(VALUE vwhence)
+{
+    if (vwhence == sym_set)
+        return SEEK_SET;
+    if (vwhence == sym_cur)
+        return SEEK_CUR;
+    if (vwhence == sym_end)
+        return SEEK_END;
+    return NUM2INT(vwhence);
+}
+
 /*
  *  call-seq:
  *     ios.seek(amount, whence=IO::SEEK_SET)  ->  0
@@ -1450,12 +1463,12 @@ rb_io_seek(VALUE io, VALUE offset, int w
  *  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_
+ *    :cur or IO::SEEK_CUR  | Seeks to _amount_ plus current position
+ *    ----------------------+--------------------------------------------------
+ *    :end or IO::SEEK_END  | Seeks to _amount_ plus end of stream (you
+ *                          | probably want a negative value for _amount_)
+ *    ----------------------+--------------------------------------------------
+ *    :set or IO::SEEK_SET  | Seeks to the absolute location given by _amount_
  *
  *  Example:
  *
@@ -1471,7 +1484,7 @@ rb_io_seek_m(int argc, VALUE *argv, VALU
     int whence = SEEK_SET;
 
     if (rb_scan_args(argc, argv, "11", &offset, &ptrname) == 2) {
-	whence = NUM2INT(ptrname);
+	whence = interpret_seek_whence(ptrname);
     }
 
     return rb_io_seek(io, offset, whence);
@@ -4233,7 +4246,7 @@ rb_io_sysseek(int argc, VALUE *argv, VAL
     off_t pos;
 
     if (rb_scan_args(argc, argv, "11", &offset, &ptrname) == 2) {
-	whence = NUM2INT(ptrname);
+	whence = interpret_seek_whence(ptrname);
     }
     pos = NUM2OFFT(offset);
     GetOpenFile(io, fptr);
@@ -11623,4 +11636,7 @@ Init_IO(void)
     sym_willneed = ID2SYM(rb_intern("willneed"));
     sym_dontneed = ID2SYM(rb_intern("dontneed"));
     sym_noreuse = ID2SYM(rb_intern("noreuse"));
+    sym_set = ID2SYM(rb_intern("set"));
+    sym_cur = ID2SYM(rb_intern("cur"));
+    sym_end = ID2SYM(rb_intern("end"));
 }
Index: test/ruby/test_io.rb
===================================================================
--- test/ruby/test_io.rb	(revision 36217)
+++ test/ruby/test_io.rb	(working copy)
@@ -1477,6 +1477,26 @@ class TestIO < Test::Unit::TestCase
     }
   end
 
+  def test_seek_symwhence
+    make_tempfile {|t|
+      open(t.path) { |f|
+        f.seek(9, :set)
+        assert_equal("az\n", f.read)
+      }
+
+      open(t.path) { |f|
+        f.seek(-4, :end)
+        assert_equal("baz\n", f.read)
+      }
+
+      open(t.path) { |f|
+        assert_equal("foo\n", f.gets)
+        f.seek(2, :cur)
+        assert_equal("r\nbaz\n", f.read)
+      }
+    }
+  end
+
   def test_sysseek
     t = make_tempfile