QByteArray qint32 and little endianness

Tessellate

Hello,

I'm re-writing a small C program in QT. I have not used QT before and have very limited experience in C++. My program connects to a small device over serial and retrieves some values from it. I'm having a bit of a difficult time trying to build my binary packet, and wondered if there is a better or proper way doing it with QT and the included types. The packet consists of a byte or two bytes, and the format is in little endian. The data length is 4 bytes. But split into two 2Bytes. Below is an example of the packet.

The packet consists of the following [Command - Byte] [Subcommand - Byte] [DataLength1 - 2Bytes] [Datalength2 - 2Bytes] [Data - nBytes]

I'm using QByteArray for the packet and defined Command as an qint8, Subcommand as qint16, and Datalength as qint32. I then use QDataStream to assign the data as follows:

    QDataStream data(&_packet, QIODevice::WriteOnly);
    data << _command;
    data << _param1;
    data << _dataSize;

This gives me the "correct" data with the correct final length and all that. The problem now is that I need to split dataSize into two Words, and assign them in little endian format. So the data ;

CMD     PARAM   Length      DATA
0x65    0x02    0xAABBCCDD  0X00

Should be as below:

CMD     PARAM   Len1    len2        DATA
0x65    0x02    0xBBAA  0xDDCC      0X00

The way I have been doing it in my small C program is using a typedef like below:

typedef union _WORD_VAL {
	WORD Word;BYTE Bytes[2];
} WORD_VAL;
/////////
WORD_VAL wParam;
wParam = 10;
byte[0] = wParam.Bytes[0];
byte[1] = wParam.Bytes[1];

So my question is; what is the best way to do this in QT or C++. Or is it easier to just define my own type like I did before.

Any help would be greatly appreciated.

hskoglund

Hi, you can try setting the byte order directly in your QDataStream, like this:

QDataStream data(&_packet, QIODevice::WriteOnly);
data.setByteOrder(QDataStream::LittleEndian);    
data << _command;
data << _param1;
data << _dataSize;

Tessellate

Hi Hskoglund,

Thank you very much for the reply and the solution. This is working perfectly!

Much appreciated!