diff --git a/ChangeLog b/ChangeLog
index dc7718fe3a..89962d2dea 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,7 @@
+Tue Dec  7 22:37:15 2010  Masaya Tarui  <tarui@ruby-lnag.org>
+
+	* io.c (io_read): duplicate string if shared. [ruby-dev:42719]
+
 Tue Dec  7 22:31:08 2010  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* lib/optparse.rb (OptionParser::Officious): separate completion
diff --git a/io.c b/io.c
index 26af3ce6ef..d5d1220294 100644
--- a/io.c
+++ b/io.c
@@ -2212,7 +2212,10 @@ io_read(int argc, VALUE *argv, VALUE io)
     rb_scan_args(argc, argv, "02", &length, &str);
 
     if (NIL_P(length)) {
-	if (!NIL_P(str)) StringValue(str);
+	if (!NIL_P(str)){
+	    StringValue(str);
+	    rb_str_modify(str);
+	}
 	GetOpenFile(io, fptr);
 	rb_io_check_char_readable(fptr);
 	return read_all(fptr, remain_size(fptr), str);