diff --git a/ChangeLog b/ChangeLog
index b1bd6e67cb..be3548be62 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Wed Apr 14 17:56:06 2010  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+
+	* io.c (io_shift_cbuf): should terminate.
+	  http://golf.shinh.org/p.rb?DECimal+number+to+BINary#Ruby
+
 Wed Apr 14 16:29:02 2010  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* gc.c (GET_STACK_BOUNDS): fixed macro argument names.  a patch from
diff --git a/io.c b/io.c
index 2656b2bae8..3afa413183 100644
--- a/io.c
+++ b/io.c
@@ -1698,11 +1698,8 @@ io_shift_cbuf(rb_io_t *fptr, int len, VALUE *strp)
         *strp = str = rb_str_new(fptr->cbuf+fptr->cbuf_off, len);
     }
     else {
-        size_t slen;
         str = *strp;
-        slen = RSTRING_LEN(str);
-        rb_str_resize(str, RSTRING_LEN(str) + len);
-        memcpy(RSTRING_PTR(str)+slen, fptr->cbuf+fptr->cbuf_off, len);
+        rb_str_cat(str, fptr->cbuf+fptr->cbuf_off, len);
     }
     fptr->cbuf_off += len;
     fptr->cbuf_len -= len;