QBuffer from void pointer and memory size

Question

I'm looking for the most simple and or elegant way to create a QBuffer in C++ Qt5.6 from void * data and long data_size.

I tried casting the void to a char pointer and using the QByteArray::fromRawData() as well as using QDataStream to fill a QByteArray. In both cases I didn't succeed.

What does "I didn't succeed" mean? Did you get an error or something? — dtech, Mar 29 '16 at 12:29
I'm using the QBuffer to play audio and in every case so far I get only noise or silence. When casting to a char pointer I get a buffer underrun somewhere in the process. — gro, Mar 29 '16 at 12:38
Do you know, what QByteArray::fromRawData did not copy buffer to QByteArray? With this method QByteArray uses your pointer. — Unick, Mar 29 '16 at 13:05
@gro - this question is about QBuffer from a void *. If you have a different question, you should post it as such. — dtech, Mar 29 '16 at 13:58

dtech · Answer 1 · 2016-03-29T13:00:55.653

QByteArray::QByteArray(const char *data, int size) will copy the data.

QByteArray::fromRawData(const char *data, int size) will use the already existing data.

Depending on your implementation, not copying the data might end up being problematic.

After you have the data in a byte array, there are several ways to go, you can directly construct a buffer:

QBuffer(QByteArray *byteArray, QObject *parent = Q_NULLPTR)

or more likely, since you are playing audio, you might want to reusing the same buffer and use one of the following to refill it:

setBuffer(QByteArray *byteArray)
setData(const QByteArray &data)

Lastly, there is also void QBuffer::setData(const char *data, int size) for which you don't even need the byte array step at all.

Lastly, remember that QBuffer is an IO device - it needs to be opened in order for it to work. A quick test shows that it works as expected:

char data[] = {1, 2, 3};
void * vdata = data;
QBuffer buffer;
buffer.setData((char *)vdata, sizeof(data));
buffer.open(QIODevice::ReadOnly);
qDebug() << buffer.readAll(); // outputs "\x01\x02\x03"

score 1 · Answer 2 · answered Mar 29 '16 at 13:19

Since you are asking for an elegant way to create a QBuffer from data and data_size, I would simply use this:

QBuffer buffer;
buffer.setData(static_cast<const char*>(data), data_size);

Note: this copies the memory pointed to by data.

I have no idea why you are unable to succeed, maybe you forgot to add the data_size as size argument for QByteArray::fromRawData? In that case strlen(data) would be used to calculate the size.

And if you are using something like this:

QByteArray byteArray = QByteArray::fromRawData(static_cast<const char*>(data), data_size);
QBuffer buffer(&byteArray);

Then the byteArray must remain valid until the QBuffer is destroyed. And since QByteArray::fromRawData(...) does not copy, the memory pointed to by data must remain valid as well. Failing to meet that requirement would also explain any failure.

QBuffer from void pointer and memory size

2 Answers2