C bindings pt3: Type table definitions and barebones files to get generated code to compile.

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

Index: mojo/public/c/bindings/lib/type_descriptor.h
diff --git a/mojo/public/c/bindings/lib/type_descriptor.h b/mojo/public/c/bindings/lib/type_descriptor.h
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+// 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.
+
+// This file contains structs used for constructing type descriptors for
+// generated mojom types. A type descriptor for a mojom struct is a table
+// describing the byte-offsets of all the pointers and handles in the struct,
+// and has references to the type descriptorss that further describe the
+// pointers. The table is used for doing all computations for the struct --
+// determining serialized size, encoding and decoding recursively, etc. A type
+// descriptor is generated for each struct, union, array and map. Note that
+// mojom maps are just mojom structs with two mojom arrays, so there is no
+// separate descriptor for it.
+//
+// The user is not expected to construct type descriptors -- a bindings
+// generator will do this when it generates bindings for a mojom file.
+//
+// A type descriptor for a mojom struct is a |MojomTypeDescriptorStruct|
+// containing an array of entries of types of |MojomTypeDescriptorStructEntry|.
+// Similarly, unions are described with |MojomTypeDescriptorUnion| with entries
+// of |MojomTypeDescriptorUnionEntry|. Arrays are described with
+// |MojomTypeDescriptorArray|.
+
+#ifndef MOJO_PUBLIC_C_BINDINGS_LIB_TYPE_DESCRIPTOR_H_
+#define MOJO_PUBLIC_C_BINDINGS_LIB_TYPE_DESCRIPTOR_H_
+
+#include <stdbool.h>
+#include <stdint.h>
+
+#include "mojo/public/c/system/macros.h"
+
+MOJO_BEGIN_EXTERN_C
+
+// This enum is used in a type descriptor entry for the |elem_type| field, and
+// indicates which type the accompanying |elem_table| is describing (if it is
+// describing a reference type), or to indicate that it's a handle type.
+// Values that correspond to an |elem_table|'s pointer type:
+// - If MOJOM_TYPE_DESCRIPTOR_TYPE_STRUCT, then |elem_table| points to a
+//   |MojomTypeDescriptorStruct|.
+// - If MOJOM_TYPE_DESCRIPTOR_TYPE_UNION, then |elem_table| points to a
+//   |MojomTypeDescriptorUnion|.
+// - If MOJOM_TYPE_DESCRIPTOR_TYPE_ARRAY, then |elem_table| points to a
+//   |MojomTypeDescriptorArray|.
+// - For any other value, |elem_table| is NULL.
+enum MojomTypeDescriptorType {
+  // Note: A map is a mojom struct with 2 mojom arrays, so we don't have a
+  // separate descriptor type for it.
+  MOJOM_TYPE_DESCRIPTOR_TYPE_STRUCT = 0,
+  MOJOM_TYPE_DESCRIPTOR_TYPE_ARRAY = 1,
+  MOJOM_TYPE_DESCRIPTOR_TYPE_UNION = 2,
+  MOJOM_TYPE_DESCRIPTOR_TYPE_HANDLE = 3,
+  MOJOM_TYPE_DESCRIPTOR_TYPE_INTERFACE = 4,
+  // This is only used in an array descriptor, and serves as a way to terminate
+  // a chain of array descriptors; the last entry in the chain always contains a
+  // plain-old-data type.
+  MOJOM_TYPE_DESCRIPTOR_TYPE_POD = 5,
+};
+
+// Mojom structs are described using this struct.
+struct MojomTypeDescriptorStruct {
+  size_t num_entries;
+  // |entries| is an array of |num_entries|, each describing a field of
+  // reference or handle type.
+  const struct MojomTypeDescriptorStructEntry* entries;
+};
+
+// This struct is used to describe each entry in a mojom struct. Each entry
+// indicates whether it is describing a reference type, or a handle type through
+// the field |elem_type|. |elem_table| points to the type descriptor describing
+// the field, if the type has one (handles don't have type descriptors).
+// |offset| indicates the starting byte offset of the element within the struct.
+struct MojomTypeDescriptorStructEntry {
+  // See comments for |MojomTypeDescriptorType| on possible values and
+  // corresponding behaviour with |elem_table|.
+  enum MojomTypeDescriptorType elem_type;
+  // Use |elem_type| to decide which type |elem_table| should be casted to.
+  const void* elem_table;

[viettrungluu, 2016/06/22 15:33:02]

Probably you should call it elem_descriptor instead of elem_table (also in the
various comments, obv.).

[vardhan, 2016/06/22 15:44:27]

Done.

+  // |offset| does not account for the struct header. Offset 0 always refers to
+  // the first element.
+  uint32_t offset;
+  // Corresponds to the '[MinVersion]' attribute in mojom IDL. This determines
+  // if this field should be ignored if its min_version < version of the struct
+  // we are dealing with.
+  uint32_t min_version;
+  // Is this field nullable?
+  bool nullable;
+};
+
+// Mojom unions are described using this struct.
+struct MojomTypeDescriptorUnion {
+  size_t num_entries;
+  const struct MojomTypeDescriptorUnionEntry* entries;
+};
+
+// Like |MojomTypeDescriptorStructEntry|, this variant is used to construct a
+// type descriptor for a union. Instead of an offset, it describes a union field
+// by its |tag|.
+struct MojomTypeDescriptorUnionEntry {
+  // See comments for |MojomTypeDescriptorType| on possible values and
+  // corresponding behaviour with |elem_table|.
+  enum MojomTypeDescriptorType elem_type;
+  const void* elem_table;

[viettrungluu, 2016/06/22 15:33:02]

"

[vardhan, 2016/06/22 15:44:27]

Done.

+  // The tag of the union field.
+  uint32_t tag;
+  bool nullable;
+};
+
+// Describes a mojom array. To describe an array, we don't need a table of
+// entries, since arrays can only describe 1 type. However, that one type can
+// recursively be an array (e.g, array<array<int>>), in which case a chain of
+// array entries are constructed.
+struct MojomTypeDescriptorArray {
+  enum MojomTypeDescriptorType elem_type;
+  const void* elem_table;

[viettrungluu, 2016/06/22 15:33:02]

"

[vardhan, 2016/06/22 15:44:27]

Done.

+  // How many elements is this array expected to hold?
+  // 0 means unspecified.
+  uint32_t num_elements;
+  bool nullable;
+};
+
+// This describes a mojom string.
+// A mojom string is a mojom array of chars without a fixed-sized.
+extern const struct MojomTypeDescriptorArray g_mojom_string_type_description;
+
+MOJO_END_EXTERN_C
+
+#endif  // MOJO_PUBLIC_C_BINDINGS_LIB_TYPE_DESCRIPTOR_H_