Always write a final null termination in write_string

When len is given the null character is appended after the truncated string.
Due to discussion in https://github.com/ffi/ffi/pull/806#pullrequestreview-451056095

Also add some Unicode characters to point out we're counting bytes, not characters.

2 files changed, 13 insertions, 13 deletions

diff --git a/lib/ffi/pointer.rb b/lib/ffi/pointer.rb
index ec6b1b1..778f2b5 100644
--- a/lib/ffi/pointer.rb
+++ b/lib/ffi/pointer.rb
@@ -96,14 +96,14 @@ module FFI
     # @param [String] str string to write
     # @param [Numeric] len length of string to return
     # @return [self]
-    # Write +str+ in pointer's contents, or first +len+ bytes if 
-    # +len+ is not +nil+. A final \0 byte is written if +len+ is not given.
+    # Write +str+ in pointer's contents.
+    # If +len+ is given, write the first +len+ bytes of +str+
+    # and append a final \0 byte after the string.
     def write_string(str, len=nil)
       if len
-        # No NUL termination
         put_bytes(0, str, 0, len)
+        put_bytes(len, "\0")
       else
-        # With NUL termination
         put_string(0, str)
       end
     end unless method_defined?(:write_string)
diff --git a/spec/ffi/string_spec.rb b/spec/ffi/string_spec.rb
index 53739a1..dea4e93 100644
--- a/spec/ffi/string_spec.rb
+++ b/spec/ffi/string_spec.rb
@@ -102,16 +102,16 @@ describe "String tests" do
     it "writes a final \\0 when given no length" do
       ptr = FFI::MemoryPointer.new(8)
       ptr.write_int64(-1)
-      ptr.write_string("abc")
-      expect(ptr.read_bytes(4)).to eq("abc\x00")
-      expect(ptr.read_string).to eq("abc")
+      ptr.write_string("äbc")
+      expect(ptr.read_bytes(5)).to eq("äbc\x00".b)
+      expect(ptr.read_string).to eq("äbc".b)
     end
 
-    it "does not write a final \\0 when given a length" do
+    it "writes a final \\0 when given a length" do
       ptr = FFI::MemoryPointer.new(8)
       ptr.write_int64(-1)
-      ptr.write_string("abc", 3)
-      expect(ptr.read_bytes(4)).to eq("abc\xFF".b)
+      ptr.write_string("äbcd", 3)
+      expect(ptr.read_bytes(5)).to eq("äb\x00\xFF".b)
     end
   end
 
@@ -119,9 +119,9 @@ describe "String tests" do
     it "writes a final \\0" do
       ptr = FFI::MemoryPointer.new(8)
       ptr.write_int64(-1)
-      ptr.put_string(0, "abc")
-      expect(ptr.read_bytes(4)).to eq("abc\x00")
-      expect(ptr.read_string).to eq("abc")
+      ptr.put_string(0, "äbc")
+      expect(ptr.read_bytes(5)).to eq("äbc\x00".b)
+      expect(ptr.read_string).to eq("äbc".b)
     end
   end
 end