Problem with a data from UDP

jenya7

I get some data

socket->readDatagram(udp_buffer.data(), udp_buffer.size(), &sender, &senderPort);

but the first argument is a char.

qint64 readDatagram(char *data, qint64 maxlen, QHostAddress *host = nullptr, quint16 *port = nullptr);

and I get bytes (uint8_t) - so my data will be trancated?

sierdzio

No, it won't. The first argument is a pointer which may point to a string of arbitrary length.

But better just use receiveDatagram() https://doc.qt.io/qt-5/qudpsocket.html#receiveDatagram, with that function you can easily read the result as a QByteArray.

JonB

@jenya7 said in Problem with a data from UDP:

but the first argument is a char.

It isn't. It's a char *.

and I get bytes (uint8_t)

That will be: typedef unsigned char uint8_t;.

Size is OK. There will be no "truncation". What type does your udp_buffer.data() return anyway?

jenya7

It's

QByteArray udp_buffer;

Now I pass it to a parse function

uint32_t MSGPARSER::ParseMessage(QByteArray data, MESSAGE * sens_msg)  
{
    if (data[0] == INTERFACE_ID_NONE || data[1] > INTERFACE_ID_RS485) // error: use of erloaded operator '==' is ambiguous (with operand types 'QByteRef' and 'int')
        return MSG_PARSE_ERROR_WRONG_INTFACE_ID;
    else
       sens_msg->interface_id = data[0];  // warning: implicit conversion changes signedness: 'char' to 'uint8_t' (aka 'unsigned char')
          
    return 0;
}

So data[0] is a char.

sierdzio

@jenya7 said in Problem with a data from UDP:

So data[0] is a char.

It's just a collection of 8 bits. UDP makes no assumptions about the data it is transporting. You can cast it to proper type if you know what it is.

jenya7

@sierdzio
So every time I have to cast?

if ( static_cast<uint8_t>(data[0]) == INTERFACE_ID_NONE || static_cast<uint8_t>(data[0]) > INTERFACE_ID_RS485)
        return MSG_PARSE_ERROR_WRONG_INTFACE_ID;
    else
        sens_msg->interface_id = static_cast<uint8_t>(data[0]);

It's not convenient at all. I have hundred bytes to parse. The first development environment I see that points char* to a network buffer.

socket->readDatagram(  (uint8_t *) udp_buffer.data(),  udp_buffer.size(),  &sender,  &senderPort);

error: cannot initialize a parameter of type 'char *' with an rvalue of type 'uint8_t *' (aka 'unsigned char *')

It should be (void *) in the function prototype.

JonB

@jenya7
QByteArray holds chars. uint8_t is unsigned char. If you compare them directly you are liable to get "unsigned/signed comparison" warnings. As you have seen.

If you don't like having to explicitly do casting each time, you could, say, write your own utility function for "the i'th element of a QByteArray as unsigned char/uint_t", or the whole data as uint_t *.

Separately, your parameter in ParseMessage(QByteArray data would be better declared as const QByteArray &data. But that won't change the casting issue.

jenya7

@JonB
socket->readDatagram( (uint8_t * ) udp_buffer.data(), udp_buffer.size(), &sender, &senderPort);

error: cannot initialize a parameter of type 'char *' with an rvalue of type 'uint8_t *' (aka 'unsigned char *')

KroMignon

@jenya7 said in Problem with a data from UDP:

socket->readDatagram( *(uint8_t ) udp_buffer.data(), udp_buffer.size(), &sender, &senderPort);

This should be: ~~socket->readDatagram( (uint8_t *) udp_buffer.data(), udp_buffer.size(), &sender, &senderPort);~~

EDIT:
socket->readDatagram( udp_buffer.data(), udp_buffer.size(), &sender, &senderPort);

jenya7

@KroMignon said in Problem with a data from UDP:

@jenya7 said in Problem with a data from UDP:

socket->readDatagram( *(uint8_t ) udp_buffer.data(), udp_buffer.size(), &sender, &senderPort);

This should be: socket->readDatagram( (uint8_t *) udp_buffer.data(), udp_buffer.size(), &sender, &senderPort);

It is. The editor's problem

JonB

@jenya7 said in Problem with a data from UDP:

The editor's problem

What does this mean?! :)

KroMignon

@jenya7 said in Problem with a data from UDP:

It is. The editor's problem

Sorry, posted to quickly: remove the cast which is false / not required

jenya7

@KroMignon said in Problem with a data from UDP:

socket->readDatagram( udp_buffer.data(), udp_buffer.size(), &sender, &senderPort);

This way I get array of chars I have to cast every byte.

socket->readDatagram((uint8_t *)udp_buffer.data(), udp_buffer.size(),  &sender, &senderPort);

This way - error: cannot initialize a parameter of type 'char *' with an rvalue of type 'uint8_t *' (aka 'unsigned char *')

sierdzio

You can use QDataStream to automatically convert all data to whatever format you require. To get a suitable device for it, use QBuffer.

JonB

@jenya7
You are misunderstanding C++. Going readDatagram((uint8_t *)udp_buffer.data() does not "make" the data actually be unsigned chars, it has no effect on "This way I get array of chars I have to cast every byte".

KroMignon

@jenya7 said in Problem with a data from UDP:

his way - error: cannot initialize a parameter of type 'char *' with an rvalue of type 'uint8_t *' (aka 'unsigned char *')

What are you doing?
socket is as instance of QUdpSocket and udp_buffer an instance of QByteArray or not?

This must work, as I always do it!

    QByteArray datagram(socket->pendingDatagramSize(), 0);
    QHostAddress sender;
    quint16 senderPort;

    socket->readDatagram(datagram.data(), datagram.size(), &sender, &senderPort);

jenya7

What are you doing?
socket is as instance of QUdpSocket and udp_buffer an instance of QByteArray or not?

Yes it is.
But taking char by char form datagram.data()
I have to cast - uint8_t b0 = static_cast<uint8_t>(datagram[0]);

KroMignon

@jenya7 said in Problem with a data from UDP:

Yes it is.
But taking char by char form datagram.data()
I have to cast - byte b0 = static_cast<uint8_t>(datagram[0]);

No:

for(const auto b : datagram)
{
   qDebug() << "Byte value:" << quint8(b);
}

EDIT
or

uint8_t* myPoint = static_cast<uint8_t*>(datagram.data());

JonB

@jenya7
I will contribute one more time. I already told you what to do if you want to reduce repeated casting:

If you don't like having to explicitly do casting each time, you could, say, write your own utility function for "the i'th element of a QByteArray as unsigned char/uint_t", or the whole data as uint_t *.

Same applies anywhere else.

For the record: I believe there have been discussions over the years about how some people would have preferred QByteArray to hold unsigned chars instead of chars. It stays with chars due (at least partly) to it's (slightly weird) determination to end the data with \0 and allow it to interchange fairly free with QString. This is not convenient for your case, but it is what it is, so you're going to have to work with it.

jenya7

@JonB said in Problem with a data from UDP:

@jenya7
I will contribute one more time. I already told you what to do if you want to reduce repeated casting:

If you don't like having to explicitly do casting each time, you could, say, write your own utility function for "the i'th element of a QByteArray as unsigned char/uint_t", or the whole data as uint_t *.

Same applies anywhere else.

To cast each element in a loop? It makes even worse, waste of run time.