diff --git a/ChangeLog b/ChangeLog
index 832e53d2d9..ba736b44a5 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,4 +1,7 @@
-Tue Feb 28 18:33:30 2012  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+Tue Feb 28 18:47:44 2012  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+
+	* io.c (io_binwrite, rb_io_syswrite): use shared frozen source
+	  strings.
 
 	* io.c (io_fread, io_getpartial, rb_io_sysread): set buffer size
 	  after check if readable, which can cause thread switch.
diff --git a/io.c b/io.c
index 15b714560e..5d49f7f9f0 100644
--- a/io.c
+++ b/io.c
@@ -1243,6 +1243,8 @@ io_write(VALUE io, VALUE str, int nosync)
     io = tmp;
     if (RSTRING_LEN(str) == 0) return INT2FIX(0);
 
+    str = rb_str_new_frozen(str);
+
     GetOpenFile(io, fptr);
     rb_io_check_writable(fptr);
 
@@ -4219,6 +4221,8 @@ rb_io_syswrite(VALUE io, VALUE str)
     GetOpenFile(io, fptr);
     rb_io_check_writable(fptr);
 
+    str = rb_str_new_frozen(str);
+
     if (fptr->wbuf.len) {
 	rb_warn("syswrite for buffered IO");
     }