C bindings: Implement _Validate(), and some pre-requisites

mojo/public/c/bindings/lib/util.h

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef MOJO_PUBLIC_C_BINDINGS_LIB_UTIL_H_
 #define MOJO_PUBLIC_C_BINDINGS_LIB_UTIL_H_
 
 #include "mojo/public/c/system/macros.h"
 
 // Rounds-up |num| to 8. The result is undefined if this results in an overflow.
 #define MOJOM_INTERNAL_ROUND_TO_8(num) (((num) + 7) & ~7)
 
 // Represents the memory layout of a mojom pointer; it is a |ptr| (pointer to
 // some memory) when unencoded, and refers to a relative |offset| when encoded.
 union MojomPointer {
   void* ptr;
   uint64_t offset;
 };
 MOJO_STATIC_ASSERT(sizeof(union MojomPointer) == 8,
                    "union MojomPointer must be 8 bytes");
 
+// This struct represents the state we need to keep when validating an encoded
+// mojom type.
+struct MojomValidationContext {
+  // |next_handle_index| is a counter that represents the next-available index
+  // into the handle array. Previous indices have already been used up. This
+  // counter is non-decreasing as it is used throughout validation.
+  uint32_t next_handle_index;
+
+  // As we validate, we keep track of the point where pointers can point to,
+  // since two pointers may not point to the same memory region.
+  char* next_pointer;
+
+  // TODO(vardhan): Include an error string? How big should it be?
+};
+
 #endif  // MOJO_PUBLIC_C_BINDINGS_LIB_UTIL_H_