QByte Array for 16 Bytes

nanamo

Hello
I am reading in a 16 Byte stream from a device.
The first byte is always 0x18
The second byte is always 0xFF
And the last byte is always 0x00

So I only need a QByteArray of that size (16 Byte)
Does this look correct or is there a cleaner method of setting up my QByte Array?

const unsigned char str[] = { 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00 };
QByteArray oxy_data(reinterpret_cast<const char*>(&str[0]), std::extent<decltype(str)>::value);

	oxy_data = serPort->readAll();
	qDebug("myVariable : %u ", oxy_data);

JonB

@nanamo
Stop concentrating on demanding that some QByteArray (you create) be 16 bytes long and instead concentrate on receiving the actual 16 bytes.

Furthermore, you should not rely on any individual call to readAll() returning exactly 16 bytes. The way Qt/asynchronous operation works is that each call to readAll() could return either the first/next n bytes less than 16, or even that (theoretically) it could return more than the 16 bytes you want if (somehow) a further "message" was sent after the 16 bytes one. This is true even if you know the device sends 16 bytes: Qt does not guarantee that all 16 bytes will be received in a call to readAll().

There are a couple of algorithms possible for this. You could buffer the incoming bytes until they reach 16, and then process the first 16, leaving any remainder for future processing. For your purposes I think you could use the following simpler code:

signals:
void  signal16received(const QByteArray &ba);

if (serPort->bytesAvailable() >= 16)
{
    QByteArray ba = serPort->read(16);
    Q_ASSERT(ba.length() == 16);
    emit signal16received(ba);
}

and attach slot to this signal to process the 16 bytes.

NOTE
@Bonnie's post has crossed with mine. I do not agree with his code [on this occasion, I usually do!], which relies on read(16) finding all 16 bytes in one call. It will not work correctly if a "packet" of less than 16 bytes arrives, which Qt does not guarantee will not happen.

J.Hilk

@nanamo QIODevice::readAll returns a QByteArray .... so why are you doing this ridiculous setup beforehand ?

nanamo

@J-Hilk Because I only want to store one response i.e. One 16 byte value in the array so I was setting it up of size - 16 Bytes

JonB

@nanamo
We don't know what you mean. Your oxy_data content is discarded. readAll() returns a brand new QByteArray.

J.Hilk

@nanamo well ok, but this oxy_data = serPort->readAll(); completely overrides your previous QByteArray.

I wouldn't even be surprised if the compiler optimised these two lines away

const unsigned char str[] = { 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00 };
QByteArray oxy_data(reinterpret_cast<const char*>(&str[0]), std::extent<decltype(str)>::value);

JonB

@nanamo
BTW, you could initialse a QByteArray to 16 bytes of 0 via QByteArray(16, '\0'). But that is not relevant to your current code per the explanation earlier.

nanamo

@JonB Yes I am just creating an array and initializing it to all zeroes.
Then it will be overwritten from reading from the serial port, that is fine.

Your method here looks better to create a QByteArray of 16 bytes.
Once it is full I just want to read it and process.
Then it can be overwritten again and I can reread.
Was just wondering the best way to create a 16 byte QByteArray

J.Hilk

@nanamo
great please mark the topic as solved, thank you.

Can't wait for your next topic in 2 weeks:
"My QByteArray is longer/shorter than 16 Byte and I don't know why"

nanamo

@J-Hilk Fantastic help, thanks a lot

JonB

@nanamo said in QByte Array for 16 Bytes:

Your method here looks better to create a QByteArray of 16 bytes.
Once it is full I just want to read it and process.
Then it can be overwritten again and I can reread.

You do not seem to have understood that your QByteArray is not being reused or overwritten. In Qt that rarely happens, instead a new QByteArray is returned.

nanamo

@JonB Oh OK thanks Jon. I understand now. Is there a way to ensure that the QByteArray that is read from the serial port is 16 Bytes long only?

As I need to be able to extract individual bytes from the message.

For example byte 15 is the checksum (8 bit 2s compliment of the sum of bytes 1-14)
So I would like a 16 byte message to work with

Bonnie

@nanamo

QByteArray oxy_data(16, 0x00);
qint64 bytesRead = serPort->read(oxy_data.data(), 16);

or

QByteArray oxy_data =  serPort->read(16);
if(oxy_data.size() < 16)
    oxy_data.append(16 - oxy_data.size(), 0x00);

JonB

@nanamo
Stop concentrating on demanding that some QByteArray (you create) be 16 bytes long and instead concentrate on receiving the actual 16 bytes.

Furthermore, you should not rely on any individual call to readAll() returning exactly 16 bytes. The way Qt/asynchronous operation works is that each call to readAll() could return either the first/next n bytes less than 16, or even that (theoretically) it could return more than the 16 bytes you want if (somehow) a further "message" was sent after the 16 bytes one. This is true even if you know the device sends 16 bytes: Qt does not guarantee that all 16 bytes will be received in a call to readAll().

There are a couple of algorithms possible for this. You could buffer the incoming bytes until they reach 16, and then process the first 16, leaving any remainder for future processing. For your purposes I think you could use the following simpler code:

signals:
void  signal16received(const QByteArray &ba);

if (serPort->bytesAvailable() >= 16)
{
    QByteArray ba = serPort->read(16);
    Q_ASSERT(ba.length() == 16);
    emit signal16received(ba);
}

and attach slot to this signal to process the 16 bytes.

NOTE
@Bonnie's post has crossed with mine. I do not agree with his code [on this occasion, I usually do!], which relies on read(16) finding all 16 bytes in one call. It will not work correctly if a "packet" of less than 16 bytes arrives, which Qt does not guarantee will not happen.

nanamo

@JonB OK thanks a lot Jon for your input. I will try

Bonnie

@JonB said in QByte Array for 16 Bytes:

@Bonnie's post has crossed with mine. I do not agree with his code, which relies on read(16) finding all 16 bytes in one call. It will not work correctly if a "packet" of less than 16 bytes arrives, which Qt does not guarantee will not happen.

Sure, I haven't considered that much. :)
What I want is just tell OP how to use QByteArray and read().

nanamo

@JonB Thanks Jon, this works well. You're a genius