How to create a QByteArray to send in QSerialPort

[Dooham]

Hello,
I have the following trouble: I want to send a message to a device that I have connected in a serial port. I can read the message that this device send me, but I can write a good QByteArray to send.

void MySerialPort::CodificarMensaje()
{
    mavlink_msg_heartbeat_pack(2,2,&msgSend,3,2,1,40,0);
    dataSend[0]=static_cast<char>(msgSend.magic);
    dataSend.append(static_cast<char>(msgSend.len));
    dataSend.append(static_cast<char>(msgSend.seq));
    dataSend.append(static_cast<char>(msgSend.sysid));
    dataSend.append(static_cast<char>(msgSend.compid));
    dataSend.append(static_cast<char>(msgSend.msgid));

    for (int i=0;i<msgSend.len;i++) {
        dataSend.append(static_cast<char>(msgSend.payload64[i]));
    }
    dataSend.append(static_cast<char>(msgSend.checksum>>8));
    dataSend.append(static_cast<char>(msgSend.checksum));
    qDebug()<<dataSend;
    qDebug()<<msgSend.len;


}

The first function creates a structure like this:

typedef struct __mavlink_message {
	uint16_t checksum; ///< sent at end of packet
	uint8_t magic;   ///< protocol magic marker
	uint8_t len;     ///< Length of payload
	uint8_t seq;     ///< Sequence of packet
	uint8_t sysid;   ///< ID of message sender system/aircraft
	uint8_t compid;  ///< ID of the message sender component
	uint8_t msgid;   ///< ID of message in payload
    uint8_t payload64[255];
    
}) mavlink_message_t;

This function is from a library (MavLink Library v1.0), and I think this function works well. However, my troubles starts when I try to write the QByteArray to send. I tried with the function append, and I got this QByteArray:

This result is incorrect, because it has a few troubles:
1-As you can see, in the posicion 1 (the second byte) has to be the len data (msgsend.len), however I got the result \t instead of \x09 that is the correct number (in the structure msgSend is correct).
2-The checksum is a uint16_t and I the solution to pass from uint16 to 2 uint8 is not correct, so I dont get the correct result.

Thanks you for your help

[jsulm]

@Dooham Do you mean you want to put data from __mavlink_message into QByteArray?
If so then take a look at https://doc.qt.io/qt-5/qdatastream.html and overload

&QDataStream::operator<<(const &mavlink_message_t);

then you can do

QByteArray array;
QDataStream stream(array);
mavlink_message_t message;
stream<<message;

[Dooham]

@jsulm That is what i want, but with the DataStream I'm still having the same issue:

The lenght is still a \t instead of a \x09.

QDataStream stream(&dataSend, QIODevice::ReadWrite);
    stream<<msgSend.magic;
    stream<<msgSend.len;
    stream<<msgSend.seq;
    stream<<msgSend.sysid;
    stream<<msgSend.compid;
    stream<<msgSend.msgid;
    for (int i=0;i<msgSend.len;i++) {
        stream<<msgSend.payload64[i];
    }


    qDebug()<<dataSend;

I didnt include the checksum.

[JonB]

@Dooham said in How to create a QByteArray to send in QSerialPort:

The lenght is still a \t instead of a \x09.

No, people keep stating this! Bytes are bytes in a QByteArray, period. And '\t' == '\x09'. The problem is that people keep looking at how qDebug() << QByteArray (or debugger) happens to display values, which it tries to display "as though" it were text to "look nice" to you humans :). Use https://doc.qt.io/Qt-5/qbytearray.html#toHex (for qDebug()) instead if you find that clearer....

[Dooham]

@jsulm Sorry, you were right, with this method I havent any problem, thanks you

[Dooham]

@JonB Yes, I just realized that. Thanks you.

[JonB]

@Dooham You're OK, you are not the first (or the last) here to ask/state this! :)