[ruby/fiddle] Add "offsetof" to Struct classes (https://github.com/ruby/fiddle/pull/83)

* Add "offsetof" to Struct classes

I need to get the offset of a member inside a struct without allocating
the struct.  This patch adds an "offsetof" class method to structs that
are generated.

The usage is like this:

```ruby
MyStruct = struct [
  "int64_t i",
  "char c",
]

MyStruct.offsetof("i") # => 0
MyStruct.offsetof("c") # => 8
```

* Update test/fiddle/test_c_struct_builder.rb

Co-authored-by: Sutou Kouhei <kou@cozmixng.org>

https://github.com/ruby/fiddle/commit/4e3b60c5b6

Co-authored-by: Sutou Kouhei <kou@cozmixng.org>

1 files changed, 36 insertions, 0 deletions

diff --git a/test/fiddle/test_c_struct_builder.rb b/test/fiddle/test_c_struct_builder.rb
new file mode 100644
index 0000000000..187424c92d
--- /dev/null
+++ b/test/fiddle/test_c_struct_builder.rb
@@ -0,0 +1,36 @@
+# frozen_string_literal: true
+begin
+  require_relative 'helper'
+  require 'fiddle/struct'
+  require 'fiddle/cparser'
+rescue LoadError
+end
+
+module Fiddle
+  class TestCStructBuilder < TestCase
+    include Fiddle::CParser
+
+    def test_offsetof
+      types, members = parse_struct_signature(['int64_t i','char c'])
+      my_struct = Fiddle::CStructBuilder.create(Fiddle::CStruct, types, members)
+      assert_equal 0, my_struct.offsetof("i")
+      assert_equal Fiddle::SIZEOF_INT64_T, my_struct.offsetof("c")
+    end
+
+    def test_offset_with_gap
+      types, members = parse_struct_signature(['void *p', 'char c', 'long x'])
+      my_struct = Fiddle::CStructBuilder.create(Fiddle::CStruct, types, members)
+
+      assert_equal PackInfo.align(0, ALIGN_VOIDP), my_struct.offsetof("p")
+      assert_equal PackInfo.align(SIZEOF_VOIDP, ALIGN_CHAR), my_struct.offsetof("c")
+      assert_equal SIZEOF_VOIDP + PackInfo.align(SIZEOF_CHAR, ALIGN_LONG), my_struct.offsetof("x")
+    end
+
+    def test_union_offsetof
+      types, members = parse_struct_signature(['int64_t i','char c'])
+      my_struct = Fiddle::CStructBuilder.create(Fiddle::CUnion, types, members)
+      assert_equal 0, my_struct.offsetof("i")
+      assert_equal 0, my_struct.offsetof("c")
+    end
+  end
+end if defined?(Fiddle)