Incorrect order of attributes in Qt XML

Question

I have the following code:

element.clear();
element.setTagName("accountpoint");
element.setAttribute("code", QString(ID_CONST)+serial);
element.setAttribute("name", QString());
element.setAttribute("serial", serial);

QFile file(filename);
file.open(QIODevice::ReadWrite);
QTextStream stream(&file);
doc.save(stream, 4);

I thought I will get an XML like:

<accountpoint code="4871583314750580" name="" serial="14750580">

But I have:

<accountpoint serial="14750580" code="4871583314750580" name="">

Why?

P.S. Yes, of course, in XML document it does not matter, but I want to get the right order of attributes.

Answer 1

I ran into this issue when trying to store xml setting data in git. In this case (in order to get a sane diff) it is important to store the xml using the same attribute ordering each time. The code base was several years old, using the deprecated Qt Xml instead of the newer QXmlStreamWriter.

The trick of setting the QT_HASH_SEED environment variable (from @MrEricSir's answer) works well in this case. However, it can also be done directly in code, like in this example:

qSetGlobalQHashSeed(0); // set a fixed hash value

QDomDocument doc = QDomDocument(); 
// add stuff to doc...
// ...

// save doc to file:
QFile file(filename);
QTextStream stream(&file);
stream << doc.toString(4);
file.close();

// reset hash seed with new random value.
qSetGlobalQHashSeed(-1);

This way, the rest of your application works as before, thus avoiding exposure to algorithmic complexity attacks.

Answer 2

I realize this is a partial answer to an old question, but if you just want the order of the attributes to be consistent every time there's a way to achieve this in Qt5.

The reason for the inconsistent ordering is that the attributes are stored in a hash map. By default QHash randomizes its seed every time your application starts to guard against certain types of DDOS attacks. But there is a workaround:

This randomization of QHash is enabled by default. Even though programs should never depend on a particular QHash ordering, there may be situations where you temporarily need deterministic behavior, e.g. for debugging or regression testing. To disable the randomization, define the environment variable QT_HASH_SEED. The contents of that variable, interpreted as a decimal value, will be used as the seed for qHash().

So all you need to do is set an environment variable named QT_HASH_SEED that has a consistent value. For example you can set this in Qt Creator in the Projects tab:

This may or may not solve your problem, but it's super handy for unit testing.