readrawdata' s read size

[yagabey]

Hello friends,

I am trying to send serial data over com port. i am using qextserialport for serialport communication.

I thought that most suitable way for me is to use QDatastream . As below, when i clicked the send button, some part(1944 bytes ) of avatar.png is sent.

I say "some part of" ; because for the values more than 1944, i get an error about memcpy. I tried this on another machine; this time instead of 1944, at most 1976 bytes were being send without an error..

What is the problem with it?

void MainWindow::on_sendButton_clicked()
{
	QFile file("C:/avatar.png");
	QDataStream out(&file);
 
                if (!file.open(QIODevice::ReadOnly)) {
		cerr << "Cannot open file for writing: "
			<< qPrintable(file.errorString()) << endl;
		return;
	}
 
	char *datas = new char();
	out.readRawData(datas,1944);
	sp->send(1944,datas);	
}

void MainWindow::on_sendButton_clicked()
{
	QFile file("C:/avatar.png");
	QDataStream out(&file);
	
                if (!file.open(QIODevice::ReadOnly)) {
		cerr << "Cannot open file for writing: "
			<< qPrintable(file.errorString()) << endl;
		return;
	}

	char *datas = new char();
	out.readRawData(datas,1944);
	sp->send(1944,datas);	
}

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[marcel]

It's pretty obvious:

char* datas = new char();

char* datas = new char();

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allocates a SINGLE char. So it would crash no matter on what machine you run it. It depends on how much the heap is fragmented. In your particular case, there was an unallocated hole of 1944 bytes starting at the address pointed to by datas.

You should read the file in buffers.

[yagabey]

Thanks Marcel,

But valid arguements for readRawData are (char * s, int len) , how will read a buffer into QDatastream..
and also my send function (actually write(const char *data, qint64 len) ) has arguements : const char *data, qint64 len

How can i adopt all these?
I am a new programmer , so be tolerant toward me

[marcel]

Try this:

int maxSize = 1024;
char* datas = new char[maxSize];
while(!out->atEnd())
{
  int actuallyRead = out.readRawData(datas, maxSize);
  sp->send(actuallyRead, datas);
}
delete[] datas;

int maxSize = 1024;
char* datas = new char[maxSize];
while(!out->atEnd())
{
  int actuallyRead = out.readRawData(datas, maxSize);
  sp->send(actuallyRead, datas);
}
delete[] datas;

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[yagabey]

Ok thanks, it works correctly now!..

[yagabey]

A small qestion more:

How can this code progress to the next 1024 bytes, after reading first 1024. Does atEnd() do it for us?

int maxSize = 1024;
char* datas = new char[maxSize];
 
while(!out->atEnd()) //checks the end of file
{
  int actuallyRead = out.readRawData(datas, maxSize); //counts read bytes
  sp->send(actuallyRead, datas);  //send the data
}
delete[] datas;

int maxSize = 1024;
char* datas = new char[maxSize];

while(!out->atEnd()) //checks the end of file
{
  int actuallyRead = out.readRawData(datas, maxSize); //counts read bytes
  sp->send(actuallyRead, datas);  //send the data
}
delete[] datas;

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[marcel]

No, readRawData advances the iodevice marker.

[yagabey]

Hmmm ok I got it...

in the datalistener side, i used a similar code:

void MainWindow::dataListener()
{
	int maxSize = 1024;
	char *buffer = new char[maxSize];
	QFile file("C:/imgreceived.GIF");
	QDataStream in(&file);
 
	if (!file.open(QIODevice::WriteOnly)) {
		cerr << "Cannot open file for writing: "
			<< qPrintable(file.errorString()) << endl;
		return;
	}
 
	while(1){
		int rec=sp->receive(maxSize,buffer);
		in.writeRawData(buffer,rec);
		if(rec != maxSize)
			break;
	}
 
}

void MainWindow::dataListener()
{
	int maxSize = 1024;
	char *buffer = new char[maxSize];
	QFile file("C:/imgreceived.GIF");
	QDataStream in(&file);

	if (!file.open(QIODevice::WriteOnly)) {
		cerr << "Cannot open file for writing: "
			<< qPrintable(file.errorString()) << endl;
		return;
	}

	while(1){
		int rec=sp->receive(maxSize,buffer);
		in.writeRawData(buffer,rec);
		if(rec != maxSize)
			break;
	}
	
}

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This works correctly but i doubt using that break condition :if(rec != maxSize), because the last record may also be 1024 bytes(low possiblity;but perhaps )

How else i can end writing to the file ?

[marcel]

QExtSerialPort is a QIODevice so you can still use atEnd to test if there's something to read.

[yagabey]

do you mean something like:

while(!sp->atEnd()){
		int rec=sp->receive(maxSize,buffer);
		in.writeRawData(buffer,rec);
	}

while(!sp->atEnd()){
		int rec=sp->receive(maxSize,buffer);
		in.writeRawData(buffer,rec);
	}

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didn't work but i used bytesavailable()>0 it is ok now..

while(sp->bytesavailable()>0){
		int rec=sp->receive(maxSize,buffer);
		in.writeRawData(buffer,rec);
	}

while(sp->bytesavailable()>0){
		int rec=sp->receive(maxSize,buffer);
		in.writeRawData(buffer,rec);
	}

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[wysota]

QExtSerialPort is a QIODevice so you can still use atEnd to test if there's something to read.

QExtSerialPort is a sequencial device so atEnd() will probably not work properly.

[marcel]

QExtSerialPort is a sequencial device so atEnd() will probably not work properly.

Seems so...

[yagabey]

Any other way to decide end of reception?

[wysota]

The protocol you use should be able to send an "end of stream" marker or send the expected stream length at the beginning of the transmission. There are no ways of indicating the end of transmission on the transportation layer, because you might always write some more data into the stream.

[yagabey]

Is there anybody who can show me a sample code for datalistener side? It is working problemeatic ... First 4 kb of the data is being read; but the rest not !

The datalistener is connected to a signal which is emitted when bytesAvailable()>0.

On the serialport monitor, I observed that; first alll of the data is being sent and after that on the datalistener side, this mass data is being read. I mean; it is like a half duplex communiction; reading and writing are not simultaneous...May this be the cause for the problem?