[ruby/fiddle] Add a helper method for reading/writing memory

(https://github.com/ruby/fiddle/pull/123)

This commit adds two new methods, `Fiddle::Pointer.read` and
`Fiddle::Pointer.write`. Both methods take an address, and will read or
write bytes at that address respectively.

For example we can read from an address without making a Pointer object:

```ruby
Fiddle::Pointer.read(address, 5) # read 5 bytes
```

We can also write to an address without allocating a Pointer object:

```ruby
Fiddle::Pointer.write(address, "bytes") # write 5 bytes
```

This allows us to read / write memory at arbitrary addresses without
instantiating a new `Fiddle::Pointer` object.

Examples where this API would be useful
[1](https://github.com/tenderlove/tenderjit/blob/f03481d28bff4d248746e596929b0841de65f181/lib/tenderjit/fiddle_hacks.rb#L26-L28)
[2](https://github.com/tenderlove/ruby/blob/77c8daa2d40dd58eeb3785ce17dea2ee38f308d1/lib/ruby_vm/rjit/c_pointer.rb#L193)
[3](https://github.com/tenderlove/ruby/blob/77c8daa2d40dd58eeb3785ce17dea2ee38f308d1/lib/ruby_vm/rjit/c_pointer.rb#L284)

I also added a writer method for the same reasons as the reader.

---------

https://github.com/ruby/fiddle/commit/04238cefed

Co-authored-by: Sutou Kouhei <kou@clear-code.com>

1 files changed, 16 insertions, 0 deletions

diff --git a/test/fiddle/test_pointer.rb b/test/fiddle/test_pointer.rb
index d6cba04bbe..f2c1d285ad 100644
--- a/test/fiddle/test_pointer.rb
+++ b/test/fiddle/test_pointer.rb
@@ -10,6 +10,22 @@ module Fiddle
       Fiddle.dlwrap arg
     end
 
+    def test_can_read_write_memory
+      # Allocate some memory
+      address = Fiddle.malloc(Fiddle::SIZEOF_VOIDP)
+
+      bytes_to_write = Fiddle::SIZEOF_VOIDP.times.to_a.pack("C*")
+
+      # Write to the memory
+      Fiddle::Pointer.write(address, bytes_to_write)
+
+      # Read the bytes out again
+      bytes = Fiddle::Pointer.read(address, Fiddle::SIZEOF_VOIDP)
+      assert_equal bytes_to_write, bytes
+    ensure
+      Fiddle.free address
+    end
+
     def test_cptr_to_int
       null = Fiddle::NULL
       assert_equal(null.to_i, null.to_int)