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/*
* Copyright 2021 Google Inc. All rights reserved.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
#ifndef FLATBUFFERS_BUFFER_H_
#define FLATBUFFERS_BUFFER_H_
#include <algorithm>
#include "flatbuffers/base.h"
namespace flatbuffers {
// Wrapper for uoffset_t to allow safe template specialization.
// Value is allowed to be 0 to indicate a null object (see e.g. AddOffset).
template<typename T = void> struct Offset {
// The type of offset to use.
typedef uoffset_t offset_type;
offset_type o;
Offset() : o(0) {}
Offset(const offset_type _o) : o(_o) {}
Offset<> Union() const { return o; }
bool IsNull() const { return !o; }
};
// Wrapper for uoffset64_t Offsets.
template<typename T = void> struct Offset64 {
// The type of offset to use.
typedef uoffset64_t offset_type;
offset_type o;
Offset64() : o(0) {}
Offset64(const offset_type offset) : o(offset) {}
Offset64<> Union() const { return o; }
bool IsNull() const { return !o; }
};
// Litmus check for ensuring the Offsets are the expected size.
static_assert(sizeof(Offset<>) == 4, "Offset has wrong size");
static_assert(sizeof(Offset64<>) == 8, "Offset64 has wrong size");
inline void EndianCheck() {
int endiantest = 1;
// If this fails, see FLATBUFFERS_LITTLEENDIAN above.
FLATBUFFERS_ASSERT(*reinterpret_cast<char *>(&endiantest) ==
FLATBUFFERS_LITTLEENDIAN);
(void)endiantest;
}
template<typename T> FLATBUFFERS_CONSTEXPR size_t AlignOf() {
// clang-format off
#ifdef _MSC_VER
return __alignof(T);
#else
#ifndef alignof
return __alignof__(T);
#else
return alignof(T);
#endif
#endif
// clang-format on
}
// Lexicographically compare two strings (possibly containing nulls), and
// return true if the first is less than the second.
static inline bool StringLessThan(const char *a_data, uoffset_t a_size,
const char *b_data, uoffset_t b_size) {
const auto cmp = memcmp(a_data, b_data, (std::min)(a_size, b_size));
return cmp == 0 ? a_size < b_size : cmp < 0;
}
// When we read serialized data from memory, in the case of most scalars,
// we want to just read T, but in the case of Offset, we want to actually
// perform the indirection and return a pointer.
// The template specialization below does just that.
// It is wrapped in a struct since function templates can't overload on the
// return type like this.
// The typedef is for the convenience of callers of this function
// (avoiding the need for a trailing return decltype)
template<typename T> struct IndirectHelper {
typedef T return_type;
typedef T mutable_return_type;
static const size_t element_stride = sizeof(T);
static return_type Read(const uint8_t *p, const size_t i) {
return EndianScalar((reinterpret_cast<const T *>(p))[i]);
}
static mutable_return_type Read(uint8_t *p, const size_t i) {
return reinterpret_cast<mutable_return_type>(
Read(const_cast<const uint8_t *>(p), i));
}
};
// For vector of Offsets.
template<typename T, template<typename> class OffsetT>
struct IndirectHelper<OffsetT<T>> {
typedef const T *return_type;
typedef T *mutable_return_type;
typedef typename OffsetT<T>::offset_type offset_type;
static const offset_type element_stride = sizeof(offset_type);
static return_type Read(const uint8_t *const p, const offset_type i) {
// Offsets are relative to themselves, so first update the pointer to
// point to the offset location.
const uint8_t *const offset_location = p + i * element_stride;
// Then read the scalar value of the offset (which may be 32 or 64-bits) and
// then determine the relative location from the offset location.
return reinterpret_cast<return_type>(
offset_location + ReadScalar<offset_type>(offset_location));
}
static mutable_return_type Read(uint8_t *const p, const offset_type i) {
// Offsets are relative to themselves, so first update the pointer to
// point to the offset location.
uint8_t *const offset_location = p + i * element_stride;
// Then read the scalar value of the offset (which may be 32 or 64-bits) and
// then determine the relative location from the offset location.
return reinterpret_cast<mutable_return_type>(
offset_location + ReadScalar<offset_type>(offset_location));
}
};
// For vector of structs.
template<typename T> struct IndirectHelper<const T *> {
typedef const T *return_type;
typedef T *mutable_return_type;
static const size_t element_stride = sizeof(T);
static return_type Read(const uint8_t *const p, const size_t i) {
// Structs are stored inline, relative to the first struct pointer.
return reinterpret_cast<return_type>(p + i * element_stride);
}
static mutable_return_type Read(uint8_t *const p, const size_t i) {
// Structs are stored inline, relative to the first struct pointer.
return reinterpret_cast<mutable_return_type>(p + i * element_stride);
}
};
/// @brief Get a pointer to the file_identifier section of the buffer.
/// @return Returns a const char pointer to the start of the file_identifier
/// characters in the buffer. The returned char * has length
/// 'flatbuffers::FlatBufferBuilder::kFileIdentifierLength'.
/// This function is UNDEFINED for FlatBuffers whose schema does not include
/// a file_identifier (likely points at padding or the start of a the root
/// vtable).
inline const char *GetBufferIdentifier(const void *buf,
bool size_prefixed = false) {
return reinterpret_cast<const char *>(buf) +
((size_prefixed) ? 2 * sizeof(uoffset_t) : sizeof(uoffset_t));
}
// Helper to see if the identifier in a buffer has the expected value.
inline bool BufferHasIdentifier(const void *buf, const char *identifier,
bool size_prefixed = false) {
return strncmp(GetBufferIdentifier(buf, size_prefixed), identifier,
flatbuffers::kFileIdentifierLength) == 0;
}
/// @cond FLATBUFFERS_INTERNAL
// Helpers to get a typed pointer to the root object contained in the buffer.
template<typename T> T *GetMutableRoot(void *buf) {
if (!buf) return nullptr;
EndianCheck();
return reinterpret_cast<T *>(
reinterpret_cast<uint8_t *>(buf) +
EndianScalar(*reinterpret_cast<uoffset_t *>(buf)));
}
template<typename T, typename SizeT = uoffset_t>
T *GetMutableSizePrefixedRoot(void *buf) {
return GetMutableRoot<T>(reinterpret_cast<uint8_t *>(buf) + sizeof(SizeT));
}
template<typename T> const T *GetRoot(const void *buf) {
return GetMutableRoot<T>(const_cast<void *>(buf));
}
template<typename T, typename SizeT = uoffset_t>
const T *GetSizePrefixedRoot(const void *buf) {
return GetRoot<T>(reinterpret_cast<const uint8_t *>(buf) + sizeof(SizeT));
}
} // namespace flatbuffers
#endif // FLATBUFFERS_BUFFER_H_
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