diff --git a/ext/stringio/stringio.c b/ext/stringio/stringio.c
index 54d36c9..cab9d92 100644
--- a/ext/stringio/stringio.c
+++ b/ext/stringio/stringio.c
@@ -1209,7 +1209,7 @@ strio_putc(VALUE self, VALUE ch)
 
 /*
  * call-seq:
- *   strio.read([length [, buffer]])    -> string, buffer, or nil
+ *   strio.read([length [, outbuf]])    -> string, outbuf, or nil
  *
  * See IO#read.
  */
diff --git a/io.c b/io.c
index f0c8515..120d8ce 100644
--- a/io.c
+++ b/io.c
@@ -2281,6 +2281,8 @@ io_getpartial(int argc, VALUE *argv, VALUE io, int nonblock)
  *  It doesn't block if some data available.
  *  If the optional <i>outbuf</i> argument is present,
  *  it must reference a String, which will receive the data.
+ *  The <i>outbuf</i> is to contain only the received data after the method call
+ *  even though it is not empty at the beginning.
  *  It raises <code>EOFError</code> on end of file.
  *
  *  readpartial is designed for streams such as pipe, socket, tty, etc.
@@ -2351,6 +2353,8 @@ io_readpartial(int argc, VALUE *argv, VALUE io)
  *
  *  If the optional <i>outbuf</i> argument is present,
  *  it must reference a String, which will receive the data.
+ *  The <i>outbuf</i> is to contain only the received data after the method call
+ *  even though it is not empty at the beginning.
  *
  *  read_nonblock just calls the read(2) system call.
  *  It causes all errors the read(2) system call causes: Errno::EWOULDBLOCK, Errno::EINTR, etc.
@@ -2485,7 +2489,7 @@ rb_io_write_nonblock(VALUE io, VALUE str)
 
 /*
  *  call-seq:
- *     ios.read([length [, buffer]])    -> string, buffer, or nil
+ *     ios.read([length [, outbuf]])    -> string, outbuf, or nil
  *
  *  Reads <i>length</i> bytes from the I/O stream.
  *
@@ -2505,8 +2509,10 @@ rb_io_write_nonblock(VALUE io, VALUE str)
  *
  *  If <i>length</i> is zero, it returns <code>""</code>.
  *
- *  If the optional <i>buffer</i> argument is present, it must reference
+ *  If the optional <i>outbuf</i> argument is present, it must reference
  *  a String, which will receive the data.
+ *  The <i>outbuf</i> is to contain only the received data after the method call
+ *  even though it is not empty at the beginning.
  *
  *  At end of file, it returns <code>nil</code> or <code>""</code>
  *  depend on <i>length</i>.
@@ -4284,6 +4290,8 @@ rb_io_syswrite(VALUE io, VALUE str)
  *  that read from <em>ios</em> or you may get unpredictable results.
  *  If the optional <i>outbuf</i> argument is present, it must reference
  *  a String, which will receive the data.
+ *  The <i>outbuf</i> is to contain only the received data after the method call
+ *  even though it is not empty at the beginning.
  *  Raises <code>SystemCallError</code> on error and
  *  <code>EOFError</code> at end of file.
  *
@@ -10335,7 +10343,7 @@ argf_eof(VALUE argf)
 
 /*
  *  call-seq:
- *     ARGF.read([length [, buffer]])    -> string, buffer, or nil
+ *     ARGF.read([length [, outbuf]])    -> string, outbuf, or nil
  *
  *  Reads _length_ bytes from ARGF. The files named on the command line
  *  are concatenated and treated as a single file by this method, so when
@@ -10352,8 +10360,10 @@ argf_eof(VALUE argf)
  *
  *  If _length_ is zero, it returns _""_.
  *
- *  If the optional _buffer_ argument is present, it must reference a String,
+ *  If the optional _outbuf_ argument is present, it must reference a String,
  *  which will receive the data.
+ *  The _outbuf_ is to contain only the received data after the method call
+ *  even though it is not empty at the beginning.
  *
  * For example:
  *
@@ -10439,7 +10449,10 @@ static VALUE argf_getpartial(int argc, VALUE *argv, VALUE argf, int nonblock);
  *  Reads at most _maxlen_ bytes from the ARGF stream. It blocks only if
  *  +ARGF+ has no data immediately available. If the optional _outbuf_
  *  argument is present, it must reference a String, which will receive the
- *  data. It raises <code>EOFError</code> on end of file.
+ *  data.
+ *  The _outbuf_ is to contain only the received data after the method call
+ *  even though it is not empty at the beginning.
+ *  It raises <code>EOFError</code> on end of file.
  *
  *  +readpartial+ is designed for streams such as pipes, sockets, and ttys. It
  *  blocks only when no data is immediately available. This means that it
diff --git a/test/ruby/test_argf.rb b/test/ruby/test_argf.rb
index 2f1ddf5..284f342 100644
--- a/test/ruby/test_argf.rb
+++ b/test/ruby/test_argf.rb
@@ -446,6 +446,16 @@ class TestArgf < Test::Unit::TestCase
     end
   end
 
+  def test_read2_with_not_empty_buffer
+    ruby('-e', <<-SRC, @t1.path, @t2.path, @t3.path) do |f|
+      s = "0123456789"
+      ARGF.read(8, s)
+      p s
+    SRC
+      assert_equal("\"1\\n2\\n3\\n4\\n\"\n", f.read)
+    end
+  end
+
   def test_read3
     ruby('-e', <<-SRC, @t1.path, @t2.path, @t3.path) do |f|
       nil while ARGF.gets
@@ -463,6 +473,8 @@ class TestArgf < Test::Unit::TestCase
         loop do
           s << ARGF.readpartial(1)
           t = ""; ARGF.readpartial(1, t); s << t
+          # not empty buffer
+          u = "abcdef"; ARGF.readpartial(1, u); s << u
         end
       rescue EOFError
         puts s
diff --git a/test/ruby/test_io.rb b/test/ruby/test_io.rb
index 0f4d523..d984252 100644
--- a/test/ruby/test_io.rb
+++ b/test/ruby/test_io.rb
@@ -991,6 +991,16 @@ class TestIO < Test::Unit::TestCase
     }
   end
 
+  def test_readpartial_with_not_empty_buffer
+    pipe(proc do |w|
+      w.write "foob"
+      w.close
+    end, proc do |r|
+      r.readpartial(5, s = "01234567")
+      assert_equal("foob", s)
+    end)
+  end
+
   def test_readpartial_buffer_error
     with_pipe do |r, w|
       s = ""
@@ -1026,6 +1036,16 @@ class TestIO < Test::Unit::TestCase
     end
   end
 
+  def test_read_with_not_empty_buffer
+    pipe(proc do |w|
+      w.write "foob"
+      w.close
+    end, proc do |r|
+      r.read(nil, s = "01234567")
+      assert_equal("foob", s)
+    end)
+  end
+
   def test_read_buffer_error
     with_pipe do |r, w|
       s = ""
@@ -1047,6 +1067,17 @@ class TestIO < Test::Unit::TestCase
     end)
   end
 
+  def test_read_nonblock_with_not_empty_buffer
+    skip "IO#read_nonblock is not supported on file/pipe." if /mswin|bccwin|mingw/ =~ RUBY_PLATFORM
+    pipe(proc do |w|
+      w.write "foob"
+      w.close
+    end, proc do |r|
+      r.read_nonblock(5, s = "01234567")
+      assert_equal("foob", s)
+    end)
+  end
+
   def test_read_nonblock_error
     return if !have_nonblock?
     skip "IO#read_nonblock is not supported on file/pipe." if /mswin|bccwin|mingw/ =~ RUBY_PLATFORM
@@ -1417,6 +1448,16 @@ class TestIO < Test::Unit::TestCase
     end
   end
 
+  def test_sysread_with_not_empty_buffer
+    pipe(proc do |w|
+      w.write "foob"
+      w.close
+    end, proc do |r|
+      r.sysread( 5, s = "01234567" )
+      assert_equal( "foob", s )
+    end)
+  end
+
   def test_flag
     t = make_tempfile
 
diff --git a/test/stringio/test_stringio.rb b/test/stringio/test_stringio.rb
index 26fcc62..22cba38 100644
--- a/test/stringio/test_stringio.rb
+++ b/test/stringio/test_stringio.rb
@@ -426,6 +426,11 @@ class TestStringIO < Test::Unit::TestCase
     s = ""
     f.read(nil, s)
     assert_equal("\u3042\u3044", s, bug5207)
+    f.rewind
+    # not empty buffer
+    s = "0123456789"
+    f.read(nil, s)
+    assert_equal("\u3042\u3044", s)
   end
 
   def test_readpartial
@@ -435,6 +440,10 @@ class TestStringIO < Test::Unit::TestCase
     assert_equal("\u3042\u3044", f.readpartial)
     f.rewind
     assert_equal("\u3042\u3044".force_encoding(Encoding::ASCII_8BIT), f.readpartial(f.size))
+    f.rewind
+    # not empty buffer
+    s = '0123456789'
+    assert_equal("\u3042\u3044".force_encoding(Encoding::ASCII_8BIT), f.readpartial(f.size, s))
   end
 
   def test_read_nonblock
@@ -444,6 +453,10 @@ class TestStringIO < Test::Unit::TestCase
     assert_equal("\u3042\u3044", f.read_nonblock)
     f.rewind
     assert_equal("\u3042\u3044".force_encoding(Encoding::ASCII_8BIT), f.read_nonblock(f.size))
+    f.rewind
+    # not empty buffer
+    s = '0123456789'
+    assert_equal("\u3042\u3044".force_encoding(Encoding::ASCII_8BIT), f.read_nonblock(f.size, s))
   end
 
   def test_size