QDataStream and readBytes

[pdoria]

Hi,

In the wake of this thread thread I'm trying to read binary data from a socket, hence the code:

unsigned int totalBytes = 0;
	totalBytes = tcpSocket.bytesAvailable();
	if (totalBytes < 4) return;		// bogus tx...
 
	uint headerSize=sizeof(uint);
	char * header = new char [headerSize];
 
	uint headerBytes=0;
	QDataStream inData(&tcpSocket);
 
	inData.readBytes(header, headerSize);
	headerBytes=atoi(header);

unsigned int totalBytes = 0;
	totalBytes = tcpSocket.bytesAvailable();
	if (totalBytes < 4) return;		// bogus tx...

	uint headerSize=sizeof(uint);
	char * header = new char [headerSize];

	uint headerBytes=0;
	QDataStream inData(&tcpSocket);

	inData.readBytes(header, headerSize);
	headerBytes=atoi(header);

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Unless I really don't know what I'm doing... (most probably) why does inData.readBytes(header, headerSize); always cause a segfault?

Any pointers welcome.
Thanks in advance,
Pedro Doria Meunier.

[jacek]

Use QDataStream::readRawData(). readBytes() is a complement of writeBytes() and it assumes that the data is in certain format (it also allocates the buffer itself).

Won't it be simplier to write:

uint headerBytes=0;
QDataStream inData(&tcpSocket);
inData >> headerBytes;

uint headerBytes=0;
QDataStream inData(&tcpSocket);
inData >> headerBytes;

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?

[pdoria]

Txs Jacek,

I already tried that only to discover that these devices are stupid enough to send the header and data all at once, so in >> is not an option. I have to read the 1st bytes of what comes to determine the length of the following data...

Kind regards,
Pedro Doria Meunier.

[jacek]

I already tried that only to discover that these devices are stupid enough to send the header and data all at once, so in >> is not an option.

Why?

quint16 header;
inData >> header;

quint16 header;
inData >> header;

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will read exactly two bytes from the stream into header, leaving the rest intact. Then you can check the header and read more data.

[pdoria]

Jacek,

You're absolutely right. This is working now. I don't remember what I did last time...
Man... what a ride!!!
I sincerely want to thank you for your valuable support... thank you!

Kind regards,
Pedro Doria Meunier.

[pdoria]

Ok. last step:

this doesn't work...

quint16 dataLength=0;
 
QDataStream inData(&tcpSocket);
 
while (zerocount < 4 && dataLength==0)
{
	inData >> dataLength;
	if (dataLength==0) zerocount++;
}
 
// exceed number of tries... bogus txs. return.
if (zerocount==3) return;
 
char * data = new char[dataLength];
inData >> data;

quint16 dataLength=0;

QDataStream inData(&tcpSocket);

while (zerocount < 4 && dataLength==0)
{
	inData >> dataLength;
	if (dataLength==0) zerocount++;
}

// exceed number of tries... bogus txs. return.
if (zerocount==3) return;

char * data = new char[dataLength];
inData >> data;

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the data is always 0x0 ...
So ... please tell me... after in >> dataLength the pointer in the stream is advanced or not?
the operator>> ( char *& s ) is able to convert the binary data into char?

Thanks in advance.

[pdoria]

bump.

Must one do this?

char * data = new char[dataLength];
	qint8 c;
	int n;
	for (n=0; n<dataLength; n++) {
		inData >> c;
		data[n]=c;
	}

char * data = new char[dataLength];
	qint8 c;
	int n;
	for (n=0; n<dataLength; n++) {
		inData >> c;
		data[n]=c;
	}

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It feels a bit archaic...

[jacek]

Must one do this?

No, you can use readRawData() instead.

[wysota]

Be aware that QDataStream is not a general purpose binary stream but a serialization method. Don't use it if you don't need it.