1/*
2 * Copyright 2021 Google Inc. All rights reserved.
3 *
4 * Licensed under the Apache License, Version 2.0 (the "License");
5 * you may not use this file except in compliance with the License.
6 * You may obtain a copy of the License at
7 *
8 * http://www.apache.org/licenses/LICENSE-2.0
9 *
10 * Unless required by applicable law or agreed to in writing, software
11 * distributed under the License is distributed on an "AS IS" BASIS,
12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13 * See the License for the specific language governing permissions and
14 * limitations under the License.
15 */
16
17#ifndef FLATBUFFERS_BUFFER_H_
18#define FLATBUFFERS_BUFFER_H_
19
20#include "flatbuffers/base.h"
21
22namespace flatbuffers {
23
24// Wrapper for uoffset_t to allow safe template specialization.
25// Value is allowed to be 0 to indicate a null object (see e.g. AddOffset).
26template<typename T> struct Offset {
27 uoffset_t o;
28 Offset() : o(0) {}
29 Offset(uoffset_t _o) : o(_o) {}
30 Offset<void> Union() const { return Offset<void>(o); }
31 bool IsNull() const { return !o; }
32};
33
34inline void EndianCheck() {
35 int endiantest = 1;
36 // If this fails, see FLATBUFFERS_LITTLEENDIAN above.
37 FLATBUFFERS_ASSERT(*reinterpret_cast<char *>(&endiantest) ==
38 FLATBUFFERS_LITTLEENDIAN);
39 (void)endiantest;
40}
41
42template<typename T> FLATBUFFERS_CONSTEXPR size_t AlignOf() {
43 // clang-format off
44 #ifdef _MSC_VER
45 return __alignof(T);
46 #else
47 #ifndef alignof
48 return __alignof__(T);
49 #else
50 return alignof(T);
51 #endif
52 #endif
53 // clang-format on
54}
55
56// Lexicographically compare two strings (possibly containing nulls), and
57// return true if the first is less than the second.
58static inline bool StringLessThan(const char *a_data, uoffset_t a_size,
59 const char *b_data, uoffset_t b_size) {
60 const auto cmp = memcmp(a_data, b_data, (std::min)(a_size, b_size));
61 return cmp == 0 ? a_size < b_size : cmp < 0;
62}
63
64// When we read serialized data from memory, in the case of most scalars,
65// we want to just read T, but in the case of Offset, we want to actually
66// perform the indirection and return a pointer.
67// The template specialization below does just that.
68// It is wrapped in a struct since function templates can't overload on the
69// return type like this.
70// The typedef is for the convenience of callers of this function
71// (avoiding the need for a trailing return decltype)
72template<typename T> struct IndirectHelper {
73 typedef T return_type;
74 typedef T mutable_return_type;
75 static const size_t element_stride = sizeof(T);
76 static return_type Read(const uint8_t *p, uoffset_t i) {
77 return EndianScalar((reinterpret_cast<const T *>(p))[i]);
78 }
79};
80template<typename T> struct IndirectHelper<Offset<T>> {
81 typedef const T *return_type;
82 typedef T *mutable_return_type;
83 static const size_t element_stride = sizeof(uoffset_t);
84 static return_type Read(const uint8_t *p, uoffset_t i) {
85 p += i * sizeof(uoffset_t);
86 return reinterpret_cast<return_type>(p + ReadScalar<uoffset_t>(p));
87 }
88};
89template<typename T> struct IndirectHelper<const T *> {
90 typedef const T *return_type;
91 typedef T *mutable_return_type;
92 static const size_t element_stride = sizeof(T);
93 static return_type Read(const uint8_t *p, uoffset_t i) {
94 return reinterpret_cast<const T *>(p + i * sizeof(T));
95 }
96};
97
98/// @brief Get a pointer to the the file_identifier section of the buffer.
99/// @return Returns a const char pointer to the start of the file_identifier
100/// characters in the buffer. The returned char * has length
101/// 'flatbuffers::FlatBufferBuilder::kFileIdentifierLength'.
102/// This function is UNDEFINED for FlatBuffers whose schema does not include
103/// a file_identifier (likely points at padding or the start of a the root
104/// vtable).
105inline const char *GetBufferIdentifier(const void *buf,
106 bool size_prefixed = false) {
107 return reinterpret_cast<const char *>(buf) +
108 ((size_prefixed) ? 2 * sizeof(uoffset_t) : sizeof(uoffset_t));
109}
110
111// Helper to see if the identifier in a buffer has the expected value.
112inline bool BufferHasIdentifier(const void *buf, const char *identifier,
113 bool size_prefixed = false) {
114 return strncmp(GetBufferIdentifier(buf, size_prefixed), identifier,
115 flatbuffers::kFileIdentifierLength) == 0;
116}
117
118/// @cond FLATBUFFERS_INTERNAL
119// Helpers to get a typed pointer to the root object contained in the buffer.
120template<typename T> T *GetMutableRoot(void *buf) {
121 EndianCheck();
122 return reinterpret_cast<T *>(
123 reinterpret_cast<uint8_t *>(buf) +
124 EndianScalar(*reinterpret_cast<uoffset_t *>(buf)));
125}
126
127template<typename T> T *GetMutableSizePrefixedRoot(void *buf) {
128 return GetMutableRoot<T>(reinterpret_cast<uint8_t *>(buf) +
129 sizeof(uoffset_t));
130}
131
132template<typename T> const T *GetRoot(const void *buf) {
133 return GetMutableRoot<T>(const_cast<void *>(buf));
134}
135
136template<typename T> const T *GetSizePrefixedRoot(const void *buf) {
137 return GetRoot<T>(reinterpret_cast<const uint8_t *>(buf) + sizeof(uoffset_t));
138}
139
140} // namespace flatbuffers
141
142#endif // FLATBUFFERS_BUFFER_H_