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* Copyright (C) 2016 The Android Open Source Project
* 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
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* See the License for the specific language governing permissions and
* limitations under the License.
#include <cstdint>
#include <memory>
#include <tuple>
#include "android-base/macros.h"
#include "wifilogd/local_utils.h"
namespace android {
namespace wifilogd {
// A fixed-size buffer, which provides FIFO access to read and write
// a sequence of messages.
class MessageBuffer {
// A wrapper which guarantees that a MessageBuffer will be rewound,
// when the program exits the wrapper's scope. The user must ensure that
// |buffer| does not expire before the ScopedRewinder.
class ScopedRewinder {
explicit ScopedRewinder(NONNULL MessageBuffer* buffer) : buffer_(buffer) {}
~ScopedRewinder() { buffer_->Rewind(); }
MessageBuffer* const buffer_;
// Constructs the buffer. |size| must be greater than GetHeaderSize().
explicit MessageBuffer(size_t size);
// Appends a single message to the buffer. |data_len| must be >=1. Returns
// true if the message was added to the buffer.
bool Append(NONNULL const uint8_t* data, uint16_t data_len);
// Returns true if the buffer is large enough to hold |length| bytes of user
// data, when the buffer is empty.
bool CanFitEver(uint16_t length) const;
// Returns true if the buffer currently has enough free space to hold |length|
// bytes of user data.
bool CanFitNow(uint16_t length) const;
// Clears the buffer. An immediately following read operation will return an
// empty message. An immediately following write operation will write to the
// head of the buffer. Clearing may be lazy (i.e., underlying storage is not
// necessarily zeroed).
void Clear();
// Returns the first unread message in the buffer. If there is no such
// message, returns {nullptr, 0}. MessageBuffer retains ownership of the
// message's storage.
std::tuple<const uint8_t*, size_t> ConsumeNextMessage();
// Returns the size of MessageBuffer's per-message header.
static constexpr size_t GetHeaderSize() { return sizeof(LengthHeader); }
// Returns the total available free space in the buffer. This may be
// larger than the usable space, due to overheads.
size_t GetFreeSize() const { return capacity_ - write_pos_; }
// Resets the read pointer to the start of the buffer. An immediately
// following read will return the first message in the buffer. An immediately
// following write, however, will be placed at the same position as if
// Rewind() had not been called.
void Rewind();
struct LengthHeader {
uint16_t payload_len;
// Prepares a header, and writes that header into the buffer.
void AppendHeader(uint16_t message_len);
// Writes arbitrary data into the buffer.
void AppendRawBytes(NONNULL const void* data_start, size_t data_len);
// Returns the total number of bytes available for reading. This number
// includes headers.
size_t GetReadableSize() const { return write_pos_ - read_pos_; }
const std::unique_ptr<uint8_t[]> data_;
const size_t capacity_;
size_t read_pos_;
size_t write_pos_;
// MessageBuffer is a value type, so it would be semantically reasonable to
// support copy and assign. Performance-wise, though, we should avoid
// copies. Remove the copy constructor and the assignment operator, to
// ensure that we don't accidentally make copies.
} // namespace wifilogd
} // namespace android