(udp) convert one QByteArray to multiple structs ?

Natalya

Hi eveyone,

I'm learning c++ and I try to make my own telemetry application for F1 2018 as the game can send its data via UDP.
The reception works fine, but I don't know how I can convert the QByteArray that readDatagram() gives me.

The data is sent using multiple structs, some are inside others, here's the doc : link to CodeMaster's forum thread. For example, the speed of the car is in the CarTelemetryData struct that is in the PacketCarTelemetryData struct, I know I can do PacketCarTelemetryData->CarTelemetryData->m_speed to get it but when I do it, the app crashes with no error. The variables types are all the same both in the .h and .cpp.
(I just tried to put all the structures in one, and use reinterpret_cast to this new one but still the same problem ...)

My question is : is there any way I can "cast" (or whatever) my datagram to my multiple structures ?

Thanks a lot !

ps: tell me if you don't understand, also my structs file is 260 lines long so I assume it's better to copy to something like pastebin ?

mpergand

Hi,

First of all, look in the debugger to see how the data are arranged, if it looks correct or messy.

There are several possible issues in your case.

how the data structures are packed, also called Data structure alignment
You may need to specified it with the structs def in the .h file
see: http://www.cplusplus.com/forum/general/14659/
you may have to care about the endianness and possibly do some conversion regarding the host architecture.

Looking at the link your provide above, i don't see any information about the data alignment and the endianess, unfortunatly.

Maybe i'm out of the subject, i don't know ;)

J.Hilk

Hi @Natalya
Have you thought about using QDataStream ?
If you provide >> overloads for QDataStream and your structs, you should be able to „stream“ the data from your QByteArray into your structs.

What @mpergand said still applies, so keep that in mind.

Usefull links:
http://doc.qt.io/qt-5/qdatastream.html
https://www.qtcentre.org/threads/15168-QDataStream-class-struct-amp-stream-operators

Natalya

Hi @mpergand and @J-Hilk

I took a look in the debugger, I have 'SIGSEGV Segmentation Fault' error on an empty line after commented lines what ever i do ... I've read that it have something to do with the pointers ?

And sorry, but I don't understand how to use QDataStream :/

But thanks to you guys.

mpergand

@Natalya said in (udp) convert one QByteArray to multiple structs ?:

I have 'SIGSEGV Segmentation Fault' error on an empty line after commented lines what ever i do

Yes, you and i already know that :)
The point is why.
If you put a breakpoint just after the struct pointer affectation, then in the debugger you can look at the struct field values.

The first value you can check is the m_packetFormat field, it must be 2018 (0x7E2)
Any other value means something wrong occured.

J.Hilk

@Natalya said in (udp) convert one QByteArray to multiple structs ?:

And sorry, but I don't understand how to use QDataStream :/

here,

to pratice with:

//PacketHeader.h
#ifndef PACKETHEADER_H
#define PACKETHEADER_H

#include <cstdint>
#include <QDataStream>

struct PacketHeader
{
    uint16_t    m_packetFormat;         // 2018
    uint8_t     m_packetVersion;        // Version of this packet type, all start from 1
    uint8_t     m_packetId;             // Identifier for the packet type, see below
    uint64_t    m_sessionUID;           // Unique identifier for the session
    float       m_sessionTime;          // Session timestamp
    uint16_t    m_frameIdentifier;      // Identifier for the frame the data was retrieved on
    uint8_t     m_playerCarIndex;       // Index of player's car in the array


    friend QDataStream& operator<<(QDataStream &out, const PacketHeader &p);

    friend QDataStream& operator>>(QDataStream &in, PacketHeader &p);
};

#endif // PACKETHEADER_H

//PacketHeader.cpp
#include "packetheader.h"

QDataStream &operator<<(QDataStream &out, const PacketHeader &p)
{
    out << p.m_packetFormat         // 2018
        << p.m_packetVersion        // Version of this packet type, all start from 1
        << p.m_packetId             // Identifier for the packet type, see below
        << p.m_sessionUID           // Unique identifier for the session
        << p.m_sessionTime          // Session timestamp
        << p.m_frameIdentifier      // Identifier for the frame the data was retrieved on
        << p.m_playerCarIndex;       // Index of player's car in the array
    return out;
}

QDataStream &operator>>(QDataStream &in, PacketHeader &p)
{
    in >> p.m_packetFormat         // 2018
        >> p.m_packetVersion        // Version of this packet type, all start from 1
        >> p.m_packetId             // Identifier for the packet type, see below
        >> p.m_sessionUID           // Unique identifier for the session
        >> p.m_sessionTime          // Session timestamp
        >> p.m_frameIdentifier      // Identifier for the frame the data was retrieved on
        >> p.m_playerCarIndex;       // Index of player's car in the array
    return in;
}

//main
int main(int argc, char *argv[])
{
    QApplication a(argc, argv);

    PacketHeader p;
    p.m_playerCarIndex = 5;

    qDebug() << p.m_playerCarIndex;

    QByteArray ba;

    QDataStream in(&ba, QIODevice::WriteOnly);

    in << p;
    qDebug() << ba;


    PacketHeader p2;
    QDataStream Out(ba);
    Out >> p2;

    qDebug() << p2.m_playerCarIndex;


    return a.exec();
}
//Console output to see that its written and read
5
"4m\xC0s\x00\xB5\xFD\xE0\x00\x00\x00\x01""Fx\x07U\xA0\x00\x00\x00]\x04\x05"
5

Natalya

@J-Hilk Thanks, I'll look at that

@mpergand the error keeps happening, I've made a new struct containing all the others (not the ones like telemetry that is only in PacketTelemetry of course) and I displayed that value, I've got '0xffffffe2' for the entire structure and so for the entire datagram ...
I think that my part where I receive is wrong, like if it was cutting the data. I have a datagram.resize() to what looks like to be the received datagram's size, and it works on my 2017 version but not here...

I'm quite lost to be honest

mpergand

looking at the faqs, i see:

What encoding format is used?
All values are encoded using Little Endian format.

Is the data packed?
Yes, all data is packed.

Some code trying to simulate the stuation:

struct PacketHeader
{
    uint16_t    m_packetFormat;         // 2018
    uint8_t     m_packetVersion;        // Version of this packet type, all start from 1
    uint8_t     m_packetId;             // Identifier for the packet type, see below
    uint64_t    m_sessionUID;           // Unique identifier for the session
    float     m_sessionTime;          // Session timestamp
    uint      m_frameIdentifier;      // Identifier for the frame the data was retrieved on
    uint8_t     m_playerCarIndex;       // Index of player's car in the array
};

struct Packet
{
    struct PacketHeader p;
    uint8_t m;
};


int main(int argc, char *argv[])
{

    QByteArray dat;
    dat.append(226);    // 2018 intel format
    dat.append(7);

    dat.append(1);      // version
    dat.append(2);      // id

    Packet* pa=(Packet*)dat.data();

qDebug()<<dat;
qDebug()<<pa->p.m_packetFormat;

What i see in the debugger:

	dat	@0x7fff5fbffb88	QByteArray
		d	(hex) 0	QByteArray::Data
			size	4	int
			alloc	8	int
	pa	@0x7fff5fbffb70	Packet
		m	0	uint8_t
		p	@0x10b804228	PacketHeader
			m_frameIdentifier	4026531840	uint
			m_packetFormat	2018	uint16_t
			m_packetId	2	uint8_t
			m_packetVersion	1	uint8_t
			m_playerCarIndex	88	uint8_t
			m_sessionTime	0.0	float
			m_sessionUID	4487907200	uint64_t

and with qDebug message:

qDebug()<<dat;
qDebug()<<pa->p.m_packetFormat;

results:
"\xE2\x07\x01\x02"
2018