Add a helper method for reading/writing memory (#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.

---------

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

ext/fiddle/pointer.c

@@ -799,6 +799,33 @@ rb_fiddle_ptr_s_to_ptr(VALUE self, VALUE val)
     return ptr;
 }
 
+/*
+ * call-seq:
+ *    Fiddle::Pointer.read(address, len)     => string
+ *
+ * Or read the memory at address +address+ with length +len+ and return a
+ * string with that memory
+ */
+
+static VALUE
+rb_fiddle_ptr_read_mem(VALUE klass, VALUE address, VALUE len)
+{
+    return rb_str_new((char *)NUM2PTR(address), NUM2ULONG(len));
+}
+
+/*
+ * call-seq:
+ *    Fiddle::Pointer.write(address, str)
+ *
+ * Write bytes in +str+ to the location pointed to by +address+.
+ */
+static VALUE
+rb_fiddle_ptr_write_mem(VALUE klass, VALUE addr, VALUE str)
+{
+    memcpy(NUM2PTR(addr), StringValuePtr(str), RSTRING_LEN(str));
+    return str;
+}
+
 void
 Init_fiddle_pointer(void)
 {
@@ -815,6 +842,8 @@ Init_fiddle_pointer(void)
     rb_define_singleton_method(rb_cPointer, "malloc", rb_fiddle_ptr_s_malloc, -1);
     rb_define_singleton_method(rb_cPointer, "to_ptr", rb_fiddle_ptr_s_to_ptr, 1);
     rb_define_singleton_method(rb_cPointer, "[]", rb_fiddle_ptr_s_to_ptr, 1);
+    rb_define_singleton_method(rb_cPointer, "read", rb_fiddle_ptr_read_mem, 2);
+    rb_define_singleton_method(rb_cPointer, "write", rb_fiddle_ptr_write_mem, 2);
     rb_define_method(rb_cPointer, "initialize", rb_fiddle_ptr_initialize, -1);
     rb_define_method(rb_cPointer, "free=", rb_fiddle_ptr_free_set, 1);
     rb_define_method(rb_cPointer, "free",  rb_fiddle_ptr_free_get, 0);

test/fiddle/test_pointer.rb

@@ -10,6 +10,22 @@ def dlwrap arg
       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)