diff --git a/ChangeLog b/ChangeLog
index 1f7f41fb26..5f17e97421 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Wed Sep 15 09:12:03 2010  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+
+	* io.c (rb_io_puts): fix for wide char encoding strings.
+	  [ruby-dev:42212]
+
 Wed Sep 15 07:27:52 2010  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* string.c (rb_str_times): mentioned about Hash argument.  a patch
diff --git a/io.c b/io.c
index 7a4be8a447..2a5a6452f7 100644
--- a/io.c
+++ b/io.c
@@ -6074,6 +6074,22 @@ rb_f_putc(VALUE recv, VALUE ch)
     return rb_funcall2(rb_stdout, rb_intern("putc"), 1, &ch);
 }
 
+
+static int
+str_end_with_asciichar(VALUE str, int c)
+{
+    long len = RSTRING_LEN(str);
+    const char *ptr = RSTRING_PTR(str);
+    rb_encoding *enc = rb_enc_from_index(ENCODING_GET(str));
+    int n;
+
+    if (len == 0) return 0;
+    if ((n = rb_enc_mbminlen(enc)) == 1) {
+	return ptr[len - 1] == c;
+    }
+    return rb_enc_ascget(ptr + ((len - 1) / n) * n, ptr + len, &n, enc) == c;
+}
+
 static VALUE
 io_puts_ary(VALUE ary, VALUE out, int recur)
 {
@@ -6137,7 +6153,7 @@ rb_io_puts(int argc, VALUE *argv, VALUE out)
       string:
 	rb_io_write(out, line);
 	if (RSTRING_LEN(line) == 0 ||
-            RSTRING_PTR(line)[RSTRING_LEN(line)-1] != '\n') {
+            !str_end_with_asciichar(line, '\n')) {
 	    rb_io_write(out, rb_default_rs);
 	}
     }
diff --git a/test/ruby/test_io_m17n.rb b/test/ruby/test_io_m17n.rb
index 21b7941cbe..3d3b64c6cc 100644
--- a/test/ruby/test_io_m17n.rb
+++ b/test/ruby/test_io_m17n.rb
@@ -1833,5 +1833,23 @@ EOT
       assert_equal((bug3534[1]+"\n").encode(e), r.gets(), bug3534[1])
     end
   end
+
+  def test_puts_widechar
+    bug = '[ruby-dev:42212]'
+    r, w = IO.pipe(Encoding::ASCII_8BIT)
+    r.binmode
+    w.binmode
+    w.puts(0x010a.chr(Encoding::UTF_32BE))
+    w.puts(0x010a.chr(Encoding::UTF_16BE))
+    w.puts(0x0a010000.chr(Encoding::UTF_32LE))
+    w.puts(0x0a01.chr(Encoding::UTF_16LE))
+    w.close
+    assert_equal("\x00\x00\x01\x0a\n", r.read(5), bug)
+    assert_equal("\x01\x0a\n", r.read(3), bug)
+    assert_equal("\x00\x00\x01\x0a\n", r.read(5), bug)
+    assert_equal("\x01\x0a\n", r.read(3), bug)
+    assert_equal("", r.read, bug)
+    r.close
+  end
 end