Fix FFI::Pointer#write_string to terminate with a NUL byte if not given a length

* Fixes #805

2 files changed, 12 insertions, 6 deletions

diff --git a/lib/ffi/pointer.rb b/lib/ffi/pointer.rb
index a5ee655..ec6b1b1 100644
--- a/lib/ffi/pointer.rb
+++ b/lib/ffi/pointer.rb
@@ -97,11 +97,15 @@ module FFI
     # @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+.
+    # +len+ is not +nil+. A final \0 byte is written if +len+ is not given.
     def write_string(str, len=nil)
-      len = str.bytesize unless len
-      # Write the string data without NUL termination
-      put_bytes(0, str, 0, len)
+      if len
+        # No NUL termination
+        put_bytes(0, str, 0, len)
+      else
+        # With NUL termination
+        put_string(0, str)
+      end
     end unless method_defined?(:write_string)
 
     # @param [Type] type type of data to read from pointer's contents
diff --git a/spec/ffi/string_spec.rb b/spec/ffi/string_spec.rb
index 6b080e4..53739a1 100644
--- a/spec/ffi/string_spec.rb
+++ b/spec/ffi/string_spec.rb
@@ -98,11 +98,13 @@ describe "String tests" do
   end
 
   describe "#write_string" do
-    it "does not write a final \\0 when given no length" do
+    # https://github.com/ffi/ffi/issues/805
+    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\xFF".b)
+      expect(ptr.read_bytes(4)).to eq("abc\x00")
+      expect(ptr.read_string).to eq("abc")
     end
 
     it "does not write a final \\0 when given a length" do