How to perform file IO using QDataStream in Qt

Comments

Dor666 - Memory leak

Hi,

I've tried this code like this in boost::test:

BOOST_AUTO_TEST_CASE( QFileAndQDataStreamBasicUsage)
{
	QFile file("c://test.txt");
	file.open(QIODevice::WriteOnly);
	QDataStream out(&file);   // we will serialize the data into the file
	out << "welcome to the Qt World";   // serialize a string
	file.close();
 
 
	QFile read("c://test.txt");
	read.open(QIODevice::ReadOnly);
	QDataStream in(&read);    // read the data serialized from the file
	QString str;
	in >> str;
}

And it gives me memory leaks:

Detected memory leaks!

Dumping objects ->
{9762} normal block at 0x009A4108, 40 bytes long.
 Data: <        @B      > 00 00 00 00 CC CD CD CD 40 42 0F 00 00 00 00 00 
{9761} normal block at 0x009BC1B8, 120 bytes long.
 Data: <|cO\        L d\> 7C 63 4F 5C B0 06 9A 00 00 00 00 00 4C B9 64 5C 
{9760} normal block at 0x009A06B0, 8 bytes long.
 Data: <<cO\    > 3C 63 4F 5C B8 C1 9B 00 
{9759} normal block at 0x009A40B0, 40 bytes long.
 Data: <        @B      > 00 00 00 00 CC CD CD CD 40 42 0F 00 00 00 00 00 
{9758} normal block at 0x009AFE78, 60 bytes long.
 Data: <                > 01 00 00 00 B0 06 9A 00 C4 1E 00 00 00 CD CD CD 
Object dump complete.

dor666 16:39, 20 November 2012 (EET)

Hamishwillee - Fixed up text

Hi Dor666

Firstly, thanks for posting this feedback. I've tidied your layout - you can use Help:Code Syntax Highlighting for marking up code -see Help:Formatting for other wiki markup. To edit the text look for "Admin" link next to the "Comment" title.

I suspect the problem is that QFile read is opened but never closed. Can you try replacing

in >> str;

with

in >> str;
read.close()

Regards

Hamish

hamishwillee 05:45, 21 November 2012 (EET)

Dor666 -

Hi Hammish. Thank you for quick reply and layout fix.

I think, that file is closed automatically in QFile destructor.

I've tried also this:

	for (int i = 0; i < 100; ++i)
	{
		QString fileName = QString("test%1.txt").arg(i);
		QFile file(fileName);
		file.open(QIODevice::WriteOnly);
		QDataStream out(&file);   // we will serialize the data into the file
		out << "welcome to the Qt World";   // serialize a string
		file.close();
 
 
		QFile read(fileName);
		read.open(QIODevice::ReadOnly);
		QDataStream in(&read);    // read the data serialized from the file
		QString str;
		in >> str;
		read.close();
	}

It gives same amount of leak (40-+120+8+40+60) even repeated 1000 times on different files. So it can be just some global qt data, I guess it's not bug, but it triggers alarm in my test :)

dor666 09:12, 21 November 2012 (EET)

Hamishwillee - I think you are probably right!

I have heard similar things before, but I can't find a reference sorry.

hamishwillee 07:40, 22 November 2012 (EET)

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This code snippet demonstrates how to use QDataStream for performing file read and write in Qt.

Article Metadata

---

Tested with

Devices(s): Emulator

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Compatibility

Platform(s): Qt

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Platform Security

Signing Required: Self-Signed

Capabilities: None

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Article

Keywords: QDataStream

Created: james1980 (27 Jan 2009)

Last edited: hamishwillee (11 Oct 2012)

Various Functions

• Sets the serialization byte order to bo.

in.setByteOrder(QDataStream::LittleEndian);

• Sets the version number of the data serialization format to v

QDataStream out(file);
out.setVersion(QDataStream::Qt_4_0);

Source File

More About QDataStream: http://pepper.troll.no/s60prereleases/doc/qdatastream.html

#include "fileIO.h"
#include <QFile>
#include <QLabel>
#include <QDataStream>
fileIO::fileIO(QWidget *parent)
    : QWidget(parent)
{
	QFile file("c://test.txt");
	file.open(QIODevice::WriteOnly);
	QDataStream out(&file);   // we will serialize the data into the file
	out << "welcome to the Qt World";   // serialize a string
	file.close();
 
 
	QFile read("c://test.txt");
	read.open(QIODevice::ReadOnly);
	QDataStream in(&read);    // read the data serialized from the file
	QString str;
	in >> str;
}
 
fileIO::~fileIO()
{
 
}