我在 Go 中讀取一些數(shù)據(jù)包，其中的字段是 C++ 數(shù)據(jù)類型。我嘗試解析數(shù)據(jù)，但我正在讀取垃圾值。這是一個小例子 - 特定數(shù)據(jù)類型的數(shù)據(jù)規(guī)格表在 C++ 中如下所示，struct CarTelemetryData{ uint16 m_speed; uint8 m_throttle; int8 m_steer; uint8 m_brake; uint8 m_clutch; int8 m_gear; uint16 m_engineRPM; uint8 m_drs; uint8 m_revLightsPercent; uint16 m_brakesTemperature[4]; uint16 m_tyresSurfaceTemperature[4]; uint16 m_tyresInnerTemperature[4]; uint16 m_engineTemperature; float m_tyresPressure[4]; };以下是我在 Go 中定義的內(nèi)容type CarTelemetryData struct { Speed uint16 Throttle uint8 Steer int8 Brake uint8 Clutch uint8 Gear int8 EngineRPM uint16 DRS uint8 RevLightsPercent uint8 BrakesTemperature [4]uint16 TyresSurfaceTemperature [4]uint16 TyresInnerTemperature [4]uint16 EngineTemperature uint16 TyresPressure [4]float32}對于實際的解組，我正在這樣做 -func decodePayload(dataStruct interface{}, payload []byte) { dataReader := bytes.NewReader(payload[:]) binary.Read(dataReader, binary.LittleEndian, dataStruct)}payload := make([]byte, 2048)s.conn.ReadFromUDP(payload[:])telemetryData := &data.CarTelemetryData{}s.PacketsRcvd += 1decodePayload(telemetryData, payload)我懷疑這是因為數(shù)據(jù)類型不等效，并且在將字節(jié)讀入 Go 數(shù)據(jù)類型時存在一些轉(zhuǎn)換問題，而它們最初是作為 C++ 打包的。我該如何處理？注意：我無法控制發(fā)送的數(shù)據(jù)，這是由第三方服務(wù)發(fā)送的。
1 回答
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TA貢獻1815條經(jīng)驗 獲得超10個贊
您面臨的問題與結(jié)構(gòu)成員的對齊方式有關(guān)。您可以在此處閱讀有關(guān)它的更多信息，但簡而言之，C++ 編譯器有時會添加填充字節(jié)以保持架構(gòu)所期望的自然對齊。如果不使用該對齊方式，可能會導(dǎo)致性能下降甚至訪問沖突。
例如，對于 x86/x64，大多數(shù)類型的對齊方式通常（但不一定保證）與大小相同。我們可以看到
#include <cstdint>
#include <type_traits>
std::size_t offsets[] = {
std::alignment_of_v<std::uint8_t>,
std::alignment_of_v<std::uint16_t>,
std::alignment_of_v<std::uint32_t>,
std::alignment_of_v<std::uint64_t>,
std::alignment_of_v<__uint128_t>,
std::alignment_of_v<std::int8_t>,
std::alignment_of_v<std::int16_t>,
std::alignment_of_v<std::int32_t>,
std::alignment_of_v<std::int64_t>,
std::alignment_of_v<__int128_t>,
std::alignment_of_v<float>,
std::alignment_of_v<double>,
std::alignment_of_v<long double>,
std::alignment_of_v<void*>,
};
編譯為
offsets:
.quad 1
.quad 2
.quad 4
.quad 8
.quad 16
.quad 1
.quad 2
.quad 4
.quad 8
.quad 16
.quad 4
.quad 8
.quad 16
.quad 8
由于這些（和其他）實現(xiàn)細節(jié)，建議不要依賴內(nèi)部表示。但是，在某些情況下，其他方法可能不夠快（例如逐字段序列化），或者您可能無法更改 C++ 代碼，例如 OP。
binary.Read需要打包數(shù)據(jù)，但 C++ 將使用填充。我們需要使用依賴于編譯器的指令，例如#pragma pack(1)或添加填充 Go 結(jié)構(gòu)。第一個不是 OP 的選項，所以我們將使用第二個。
我們可以使用offsetof宏來確定結(jié)構(gòu)成員相對于結(jié)構(gòu)本身的偏移量。我們可以做類似的事情
#include <array>
#include <cstddef>
#include <cstdint>
using int8 = std::int8_t;
using uint8 = std::uint8_t;
using uint16 = std::uint16_t;
struct CarTelemetryData {
uint16 m_speed;
uint8 m_throttle;
int8 m_steer;
uint8 m_brake;
uint8 m_clutch;
int8 m_gear;
uint16 m_engineRPM;
uint8 m_drs;
uint8 m_revLightsPercent;
uint16 m_brakesTemperature[4];
uint16 m_tyresSurfaceTemperature[4];
uint16 m_tyresInnerTemperature[4];
uint16 m_engineTemperature;
float m_tyresPressure[4];
};
// C++ has no reflection (yet) so we need to list every member
constexpr auto offsets = std::array{
offsetof(CarTelemetryData, m_speed),
offsetof(CarTelemetryData, m_throttle),
offsetof(CarTelemetryData, m_steer),
offsetof(CarTelemetryData, m_brake),
offsetof(CarTelemetryData, m_clutch),
offsetof(CarTelemetryData, m_gear),
offsetof(CarTelemetryData, m_engineRPM),
offsetof(CarTelemetryData, m_drs),
offsetof(CarTelemetryData, m_revLightsPercent),
offsetof(CarTelemetryData, m_brakesTemperature),
offsetof(CarTelemetryData, m_tyresSurfaceTemperature),
offsetof(CarTelemetryData, m_tyresInnerTemperature),
offsetof(CarTelemetryData, m_engineTemperature),
offsetof(CarTelemetryData, m_tyresPressure),
};
constexpr auto sizes = std::array{
sizeof(CarTelemetryData::m_speed),
sizeof(CarTelemetryData::m_throttle),
sizeof(CarTelemetryData::m_steer),
sizeof(CarTelemetryData::m_brake),
sizeof(CarTelemetryData::m_clutch),
sizeof(CarTelemetryData::m_gear),
sizeof(CarTelemetryData::m_engineRPM),
sizeof(CarTelemetryData::m_drs),
sizeof(CarTelemetryData::m_revLightsPercent),
sizeof(CarTelemetryData::m_brakesTemperature),
sizeof(CarTelemetryData::m_tyresSurfaceTemperature),
sizeof(CarTelemetryData::m_tyresInnerTemperature),
sizeof(CarTelemetryData::m_engineTemperature),
sizeof(CarTelemetryData::m_tyresPressure),
};
constexpr auto computePadding() {
std::array<std::size_t, offsets.size()> result;
std::size_t expectedOffset = 0;
for (std::size_t i = 0; i < offsets.size(); i++) {
result.at(i) = offsets.at(i) - expectedOffset;
expectedOffset = offsets.at(i) + sizes.at(i);
}
return result;
}
auto padding = computePadding();
編譯為（constexprFTW）
padding:
.quad 0
.quad 0
.quad 0
.quad 0
.quad 0
.quad 0
.quad 1
.quad 0
.quad 0
.quad 0
.quad 0
.quad 0
.quad 0
.quad 2
所以，在 x86 上，我們需要一個字節(jié)之前EngineRPM和兩個字節(jié)之前TyresPressure。
所以，讓我們檢查一下它是否有效。
C++：
#include <cstddef>
#include <cstdint>
#include <iomanip>
#include <iostream>
#include <span>
using int8 = std::int8_t;
using uint8 = std::uint8_t;
using uint16 = std::uint16_t;
struct CarTelemetryData {
uint16 m_speed;
uint8 m_throttle;
int8 m_steer;
uint8 m_brake;
uint8 m_clutch;
int8 m_gear;
uint16 m_engineRPM;
uint8 m_drs;
uint8 m_revLightsPercent;
uint16 m_brakesTemperature[4];
uint16 m_tyresSurfaceTemperature[4];
uint16 m_tyresInnerTemperature[4];
uint16 m_engineTemperature;
float m_tyresPressure[4];
};
int main() {
CarTelemetryData data = {
.m_speed = 1,
.m_throttle = 2,
.m_steer = 3,
.m_brake = 4,
.m_clutch = 5,
.m_gear = 6,
.m_engineRPM = 7,
.m_drs = 8,
.m_revLightsPercent = 9,
.m_brakesTemperature = {10, 11, 12, 13},
.m_tyresSurfaceTemperature = {14, 15, 16, 17},
.m_tyresInnerTemperature = {18, 19, 20, 21},
.m_engineTemperature = 22,
.m_tyresPressure = {23, 24, 25, 26},
};
std::cout << "b := []byte{" << std::hex << std::setfill('0');
for (auto byte : std::as_bytes(std::span(&data, 1))) {
std::cout << "0x" << std::setw(2) << static_cast<unsigned>(byte)
<< ", ";
}
std::cout << "}";
}
結(jié)果是
b := []byte{0x01, 0x00, 0x02, 0x03, 0x04, 0x05, 0x06, 0x00, 0x07, 0x00, 0x08, 0x09, 0x0a, 0x00, 0x0b, 0x00, 0x0c, 0x00, 0x0d, 0x00, 0x0e, 0x00, 0x0f, 0x00, 0x10, 0x00, 0x11, 0x00, 0x12, 0x00, 0x13, 0x00, 0x14, 0x00, 0x15, 0x00, 0x16, 0x00, 0x00, 0x00, 0x00, 0x00, 0xb8, 0x41, 0x00, 0x00, 0xc0, 0x41, 0x00, 0x00, 0xc8, 0x41, 0x00, 0x00, 0xd0, 0x41, }
讓我們在 Go 中使用它：
// Type your code here, or load an example.
// Your function name should start with a capital letter.
package main
import (
"bytes"
"encoding/binary"
"fmt"
)
type CarTelemetryData struct {
Speed uint16
Throttle uint8
Steer int8
Brake uint8
Clutch uint8
Gear int8
_ uint8
EngineRPM uint16
DRS uint8
RevLightsPercent uint8
BrakesTemperature [4]uint16
TyresSurfaceTemperature [4]uint16
TyresInnerTemperature [4]uint16
EngineTemperature uint16
_ uint16
TyresPressure [4]float32
}
func main() {
b := []byte{0x01, 0x00, 0x02, 0x03, 0x04, 0x05, 0x06, 0x00, 0x07, 0x00, 0x08, 0x09, 0x0a, 0x00, 0x0b, 0x00, 0x0c, 0x00, 0x0d, 0x00, 0x0e, 0x00, 0x0f, 0x00, 0x10, 0x00, 0x11, 0x00, 0x12, 0x00, 0x13, 0x00, 0x14, 0x00, 0x15, 0x00, 0x16, 0x00, 0x00, 0x00, 0x00, 0x00, 0xb8, 0x41, 0x00, 0x00, 0xc0, 0x41, 0x00, 0x00, 0xc8, 0x41, 0x00, 0x00, 0xd0, 0x41}
var dataStruct CarTelemetryData
dataReader := bytes.NewReader(b[:])
binary.Read(dataReader, binary.LittleEndian, &dataStruct)
fmt.Printf("%+v", dataStruct)
}
哪個打印
{Speed:1 Throttle:2 Steer:3 Brake:4 Clutch:5 Gear:6 _:0 EngineRPM:7 DRS:8 RevLightsPercent:9 BrakesTemperature:[10 11 12 13] TyresSurfaceTemperature:[14 15 16 17] TyresInnerTemperature:[18 19 20 21] EngineTemperature:22 _:0 TyresPressure:[23 24 25 26]}
取出填充字節(jié)，它會失敗。
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