Backing up a file.

[munna]

Hi,

I have a application that uses Sqlite to store data and also the option to backup this data.

I wrote the following code but is not working.

QString filter = tr("Database Backup (*.dbb)");
	QString fileName = QFileDialog::getSaveFileName(this,tr("Backup Database"),QDir::homePath(),filter);
 
if(!fileName.isEmpty()){
	QString filePath = QDir::homePath();
	filePath.append("/database");
	QFile sourceFile(filePath);
	if(!sourceFile.open(QIODevice::ReadOnly)){
               //Error Message
		return;
	}
	QFile destFile(fileName);
	if(!destFile.open(QIODevice::WriteOnly)){
		//Error Message
		return;
	}
	QTextStream sourceStream(&sourceFile);
	QTextStream destStream(&destFile);
        QByteArray data;
	sourceStream>>data;
	data = qCompress(data);
	destStream<<data;
	sourceFile.close();
	destFile.close();
}

QString filter = tr("Database Backup (*.dbb)");
	QString fileName = QFileDialog::getSaveFileName(this,tr("Backup Database"),QDir::homePath(),filter);

if(!fileName.isEmpty()){
	QString filePath = QDir::homePath();
	filePath.append("/database");
	QFile sourceFile(filePath);
	if(!sourceFile.open(QIODevice::ReadOnly)){
               //Error Message
		return;
	}
	QFile destFile(fileName);
	if(!destFile.open(QIODevice::WriteOnly)){
		//Error Message
		return;
	}
	QTextStream sourceStream(&sourceFile);
	QTextStream destStream(&destFile);
        QByteArray data;
	sourceStream>>data;
	data = qCompress(data);
	destStream<<data;
	sourceFile.close();
	destFile.close();
}

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The destination file is getting created but it's size is only 10 Bytes.

Can someone please tell me what is wrong with the code ?

Thanks a lot.

[wysota]

Is this a text file? Because if not, you shouldn't use QTextStream. And even if it is a text file, you shouldn't use QTextStream with the stream operator (you'll lose whitespaces, etc.).

I guess this would be a bit better:

QByteArray data;
while(!sourceFile.atEnd()){
  data = sourceFile.read(64*1024); // read 64kB
  data = qCompress(data);
  destFile.write(data);
}

QByteArray data;
while(!sourceFile.atEnd()){
  data = sourceFile.read(64*1024); // read 64kB
  data = qCompress(data);
  destFile.write(data);
}

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[munna]

With your code, how will the reverting back of the data work ?

Can I use the following code to get back the original file

QByteArray umcompressedData;
while(!compressedFile.atEnd()){
   umcompressedData = compressedFile.read(64*1024); // read 64kB. Can I do this ??
   umcompressedData = qUncompress(umcompressedData);
   destFile.write(umcompressedData);
}

QByteArray umcompressedData;
while(!compressedFile.atEnd()){
   umcompressedData = compressedFile.read(64*1024); // read 64kB. Can I do this ??
   umcompressedData = qUncompress(umcompressedData);
   destFile.write(umcompressedData);
}

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I think reverting back the data will be a problem.

What can I do ?

Thanks.

[wysota]

Try to uncompress with 64k blocks. zlib works on streams, afaik, so there is a chance this will work. But if it doesn't just use sourceFile.readAll() instead of reading 64k at a time. It'll use more memory but will certainly work correctly when uncompressing.