Index: prelude.rb
===================================================================
--- prelude.rb	(revision 31612)
+++ prelude.rb	(working copy)
@@ -29,3 +29,90 @@
     }
   end
 end
+
+
+class IO
+#
+#  call-seq:
+#     IO.write(name, string, [offset] )   => fixnum
+#     IO.write(name, string, [offset], open_args )   => fixnum
+#
+#  Opens the file, optionally seeks to the given <i>offset</i>, writes
+#  <i>string</i>, then returns the length written.
+#  <code>write</code> ensures the file is closed before returning.
+#  If <i>offset</i> is not given, the file is truncated.  Otherwise,
+#  it is not truncated.
+#
+#  If the last argument (open_args) is a hash, it specifies options for the internal
+#  call to open().  The keys would be the following.  open_args: is exclusive
+#  to others, meaning that if you specify an element, it will use that instead
+#  of the default.
+#
+#   encoding: string or encoding
+#
+#    specifies encoding of the read string.  encoding will be ignored
+#    if length is specified.
+#
+#   mode: string
+#
+#    specifies mode argument for open().  it should start with "w" or "a" or "r+"
+#    otherwise it would cause error.
+#
+#   perm: fixnum
+#
+#    specifies perm argument for open().
+#
+#   open_args: array of strings
+#
+#    specifies arguments for open() as an array.
+#
+#     IO.write("testfile", "0123456789")      #=> "0123456789"
+#     IO.write("testfile", "0123456789", 20)  #=> "This is line one\nThi0123456789two\nThis is line three\nAnd so on...\n"
+#
+#   More examples:
+#     File.write(path, "foo", encoding: "UTF-8")
+#     File.write(path, "foo", 10, encoding: "UTF-8") # insert "foo" at offset 10
+#
+  def self.write name, string, offset = nil, open_args_hash = nil
+    # 3rd param is either offset or open_args
+    if !open_args_hash && offset.is_a?(Hash)
+      open_args_hash = offset
+      offset = nil
+    end
+    open_args_hash ||= {}
+    if offset && File.exist?(name)
+      open_args_hash[:mode] ||= "r+" # File::RDWR|File::CREAT
+    else
+      open_args_hash[:mode] ||= "w"  # File::WRONLY|File::TRUNC|File::CREAT
+    end
+    open(name, open_args_hash) do |f|
+      if offset
+        f.seek offset
+      end
+      f.write string
+    end
+  end
+  
+#  call-seq:
+#     IO.binwrite(name, string, [offset] )   => fixnum
+#
+#  Opens the file, optionally seeks to the given <i>offset</i>, write
+#  <i>string</i> then returns the length written.
+#  <code>binwrite</code> ensures the file is closed before returning.
+#  The open mode will be "wb:ASCII-8BIT".
+#  If <i>offset</i> is not given, the file is truncated.  Otherwise,
+#  it is not truncated.
+#
+#     IO.binwrite("testfile", "0123456789")      #=> "0123456789"
+#     IO.binwrite("testfile", "0123456789", 20)  #=> "This is line one\nThi0123456789two\nThis is line three\nAnd so on...\n"
+#
+  def self.binwrite name, string, offset = nil
+    open_args = offset ? (File::RDWR|File::CREAT|File::BINARY) : 'wb'
+    open(name, open_args) do |f|
+      if offset
+        f.seek offset
+      end
+      f.write string
+    end
+  end
+end
\ No newline at end of file
Index: test/ruby/test_io.rb
===================================================================
--- test/ruby/test_io.rb	(revision 31612)
+++ test/ruby/test_io.rb	(working copy)
@@ -1900,4 +1900,64 @@
     end
     rescue NotImplementedError
   end
+  
+  
+  def test_s_write
+    t = Tempfile.new("foo")
+    path = t.path
+    t.close(false)
+    File.write(path, "foo\nbar\nbaz")
+    assert_equal("foo\nbar\nbaz", File.read(path))
+    File.write(path, "FOO", 0)
+    assert_equal("FOO\nbar\nbaz", File.read(path))
+    File.write(path, "BAR")
+    assert_equal("BAR", File.read(path))
+    File.write(path, "\u{3042}", mode: "w", encoding: "EUC-JP")
+    assert_equal("\u{3042}".encode("EUC-JP"), File.read(path, encoding: "EUC-JP"))
+    File.delete t
+    assert_equal(6, File.write(path,'string',2))
+    File.delete t
+    assert_raise(Errno::EINVAL) { File.write('/tmp/nonexisting','string',-2) }
+    assert_equal(6, File.write(path, 'string'))
+    assert_equal(3, File.write(path, 'sub', 1))
+    assert_equal("ssubng", File.read(path))
+    File.delete t
+    assert_equal(3, File.write(path, "foo", encoding: "UTF-8"))
+    File.delete t
+    assert_equal(3, File.write(path, "foo", 0, encoding: "UTF-8"))
+    assert_equal("foo", File.read(path))
+    assert_equal(1, File.write(path, "f", 1, encoding: "UTF-8"))
+    assert_equal("ffo", File.read(path))
+    File.delete t
+    assert_equal(1, File.write(path, "f", 1, encoding: "UTF-8"))
+    assert_equal("\00f", File.read(path))
+    assert_equal(1, File.write(path, "f", 0, encoding: "UTF-8"))
+    assert_equal("ff", File.read(path))
+    t.unlink
+  end
+
+  def test_s_binwrite
+    t = Tempfile.new("foo")
+    path = t.path
+    t.close(false)
+    File.binwrite(path, "foo\nbar\nbaz")
+    assert_equal("foo\nbar\nbaz", File.read(path))
+    File.binwrite(path, "FOO", 0)
+    assert_equal("FOO\nbar\nbaz", File.read(path))
+    File.binwrite(path, "BAR")
+    assert_equal("BAR", File.read(path))
+    File.binwrite(path, "\u{3042}")
+    assert_equal("\u{3042}".force_encoding("ASCII-8BIT"), File.binread(path))
+    File.delete t
+    assert_equal(6, File.binwrite(path,'string',2))
+    File.delete t
+    assert_equal(6, File.binwrite(path, 'string'))
+    assert_equal(3, File.binwrite(path, 'sub', 1))
+    assert_equal("ssubng", File.binread(path))
+    assert_equal(6, File.size(path))
+    assert_raise(Errno::EINVAL) { File.binwrite('/tmp/nonexisting','string',-2) }
+    assert_raise(TypeError) { File.binwrite(path, "string", mode: "w", encoding: "EUC-JP") }
+    t.unlink
+  end
+
 end