No longer unable to retrieve data from QIODevice after calling readAll(). Buffer flushed?

I've just noticed something when using QNetworkReply that I was unable to find the slightest hint in the Qt documentation for QIODevice::readAll() (which the QNetworkReply inherits this method from).

Here is what the documentation states:

Reads all remaining data from the device, and returns it as a byte array.

This function has no way of reporting errors; returning an empty QByteArray can mean either that no data was currently available for reading, or that an error occurred.

Let's say I have the following connection:

connect(this->reply, &QIODevice::readyRead, this, &MyApp::readyReadRequest);

Ths readyReadRequest() slot looks like this:

void MyApp::readyReadRequest()
{
    LOG(INFO) << "Received data from \"" << this->url.toString() << "\"";
    LOG(INFO) << "Data contents:\n" << QString(this->reply->readAll());
    this->bufferReply = this->reply->readAll();
}

The surprise came after I called this->bufferReply (which a QByteArray class member of MyApp). I passed it to a QXmlStreamReader and did:

while (!reader.atEnd())
{
    LOG(DEBUG) << "Reading next XML element";
    reader.readNext();
    LOG(DEBUG) << reader.tokenString();
}
if (reader.hasError())
{
    LOG(ERROR) << "Encountered error while parsing XML data:" << reader.errorString();
}

Imagine my surprise when I got the following output:

2017-10-17 16:12:18,591 DEBUG [default] [void MyApp::processReply()][...] Reading next XML element

2017-10-17 16:12:18,591 DEBUG [default] [void MyApp::processReply()] [...] Invalid

2017-10-17 16:12:18,591 ERROR [default] Encountered error while parsing XML data: Premature end of document

Through debugging I got that my bufferReply at this point is empty. I looked in the docs again but couldn't find anything that hints removing the data from the device (in my case the network reply) after reading it all.

Removing the line where I print the byte array or simply moving it after this->bufferReply = this->reply->readAll(); and then printing the contents of the class member fixed the issue:

void MyApp::readyReadRequest()
{
    LOG(INFO) << "Received data from \"" << this->url.toString() << "\"";
    this->bufferReply = this->reply->readAll();
    LOG(INFO) << "Data contents:\n" << QString(this->bufferReply);
}

However I would like to know if I'm doing something wrong or is the documentation indeed incomplete.

Since readAll() doesn't report errors or that data is not available at the given point in time returning an empty byte array is the only thing that hints towards the fact that something didn't work as intended.

Solution

Yes. When you call QIODevice::readAll() 2 times, it is normal that the 2nd time you get nothing. Everything has been read, there is nothing more to be read.

This behavior is standard in IO read functions: each call to a read() function returns the next piece of data. Since readAll() reads to the end, further calls return nothing.

However, this does not necessarily means that the data has been flushed. For instance when you read a file, it just moves a "cursor" around and you can go back to the start of the file with QIODevice::seek(0). For QNetworkReply, I'd guess that the data is just discarded.