diff --git a/io.c b/io.c
index 24f141c..4b03a45 100644
--- a/io.c
+++ b/io.c
@@ -2281,7 +2281,10 @@ io_getpartial(int argc, VALUE *argv, VALUE io, int nonblock)
             rb_sys_fail_path(fptr->pathv);
         }
     }
-    rb_str_set_len(str, n);
+    if (RSTRING_LEN(str) != n) {
+	rb_str_modify(str);
+	rb_str_set_len(str, n);
+    }
 
     if (n == 0)
         return Qnil;
@@ -2602,7 +2605,10 @@ io_read(int argc, VALUE *argv, VALUE io)
     previous_mode = set_binary_mode_with_seek_cur(fptr);
 #endif
     n = io_fread(str, 0, len, fptr);
-    rb_str_set_len(str, n);
+    if (RSTRING_LEN(str) != n) {
+	rb_str_modify(str);
+	rb_str_set_len(str, n);
+    }
 #if defined(RUBY_TEST_CRLF_ENVIRONMENT) || defined(_WIN32)
     if (previous_mode == O_TEXT) {
 	setmode(fptr->fd, O_TEXT);
@@ -4390,7 +4396,10 @@ rb_io_sysread(int argc, VALUE *argv, VALUE io)
     if (n == -1) {
 	rb_sys_fail_path(fptr->pathv);
     }
-    rb_str_set_len(str, n);
+    if (RSTRING_LEN(str) != n) {
+	rb_str_modify(str);
+	rb_str_set_len(str, n);
+    }
     if (n == 0 && ilen > 0) {
 	rb_eof_error();
     }
diff --git a/test/ruby/test_io.rb b/test/ruby/test_io.rb
index f165c64..91df2e6 100644
--- a/test/ruby/test_io.rb
+++ b/test/ruby/test_io.rb
@@ -2446,5 +2446,71 @@ End
       assert_raise(Errno::ESPIPE, Errno::EINVAL) { w.advise(:willneed) }
     end
   end
+
+  def test_read_buffer_not_raise_shared_string_error
+    bug6764 = '[ruby-core:46586]'
+    size = 28
+    t = Tempfile.new("test_io")
+    t.write("a" * size)
+    t.close
+    w = Tempfile.new("test_io")
+    assert_nothing_raised(RuntimeError, bug6764) do
+      File.open(t.path, "r") do |r|
+        buf = ''
+        while r.read(size, buf)
+          w << buf
+        end
+      end
+    end
+  ensure
+    t.close!
+    w.close!
+  end
+
+  def test_sysread_buffer_not_raise_shared_string_error
+    bug6764 = '[ruby-core:46586]'
+    size = 28
+    t = Tempfile.new("test_io")
+    t.write("a" * size)
+    t.close
+    w = Tempfile.new("test_io")
+    assert_nothing_raised(RuntimeError, bug6764) do
+      File.open(t.path, "r") do |r|
+        buf = ''
+        begin
+          while r.sysread(size, buf)
+            w << buf
+          end
+        rescue EOFError
+        end
+      end
+    end
+  ensure
+    t.close!
+    w.close!
+  end
+
+  def test_readpartial_buffer_not_raise_shared_string_error
+    bug6764 = '[ruby-core:46586]'
+    size = 28
+    t = Tempfile.new("test_io")
+    t.write("a" * size)
+    t.close
+    w = Tempfile.new("test_io")
+    assert_nothing_raised(RuntimeError, bug6764) do
+      File.open(t.path, "r") do |r|
+        buf = ''
+        begin
+          while r.readpartial(size, buf)
+            w << buf
+          end
+        rescue EOFError
+        end
+      end
+    end
+  ensure
+    t.close!
+    w.close!
+  end
 end