[SOLVED] Problem with QByteArray

willypuzzle

I have this code but it doesn't work properly because 'from' pointer content is set to "0\0".
'hold' variable is set properly.
What's wrong?

@
QSharedMemory sharedMemory;
QString uniqueID = "abcde";
sharedMemory.setKey(uniqueID);

    if(sharedMemory.attach()){

        QBuffer buffer;
        buffer.open( QBuffer::ReadWrite );
        QDataStream out( &buffer );
        out << "qadvice, raise window!";
        int size = buffer.size();

        if (sharedMemory.lock()){
          char *to = (char*)sharedMemory.data();
          QByteArray hold = buffer.data();
          char *from = new char[hold.size() + 1];
          strcpy(from, hold.data());
          memcpy( to, from, qMin( sharedMemory.size(), (size+1) ) );
          delete [] from;
          sharedMemory.unlock();
       }
  }

@

MuldeR

What exactly are you trying to do?

Why not simply:
@QSharedMemory sharedMemory;
QString uniqueID = "abcde";
sharedMemory.setKey(uniqueID);

if(sharedMemory.attach())
{
QBuffer buffer;
buffer.open( QBuffer::ReadWrite );

if (sharedMemory.lock())
{
    QByteArray &data = buffer.data();
    memcpy(
        sharedMemory.data(),
        data.constData(),
        qMin(data.size() + 1, sharedMemory.size())
    );
    sharedMemory.data()[sharedMemory.size() - 1] = '\0';
    sharedMemory.unlock();
}

}@

willypuzzle

I resolved the problem, I discovered that 'hold' variable holds 3 zeros at first three positions, and the number of elements in the fourth position.

So to recover the string I had to cut the first 4 chars of the variable 'hold', with this function:

@
const char* analizeStream(const QByteArray& originalArray, int* size){
size = originalArray.size() - 4;
char toReturn = new char[*size];
for (int i=0; i < (*size); i++){
char c = originalArray[i+4];
toReturn[i] = c;
}
return toReturn;
}
@

willypuzzle

MuldeR, I have to add these lines:

@
QDataStream out( &buffer );
out << "qadvice, raise window!";
@

To insert the text in the shared memory

MuldeR

If that's all, why not:

@static const QString text("qadvice, raise window!");

if(sharedMemory.attach())
{
//QBuffer buffer;
//buffer.open( QBuffer::ReadWrite );

if (sharedMemory.lock())
{
    QByteArray data = text.toUtf8();
    memcpy(
        sharedMemory.data(),
        data.constData(),
        qMin(data.size() + 1, sharedMemory.size())
    );
    sharedMemory.data()[sharedMemory.size() - 1] = '\0';
    sharedMemory.unlock();
}

}@

Why do you need the QBuffer, the QDataStream and the strcpy() ???

willypuzzle

So QBuffer buffer is useless ....
I read it on a tutorial http://www.developer.nokia.com/Community/Wiki/QSharedMemory_example

MuldeR

Wouldn't say it's "useless". Maybe just not needed/helpful in your specific case, as far as I can see.

The purpose of QBuffer is that you get a QIODevice backed by a QByteArray as its internal buffer. So if you need to access a QByteArray through a QIODevice-style interface, then QBuffer would be the way to go.

The same way, wrapping your QBuffer into a QDataStream is helpful if you need to serialize a sequence of elements of various types into a platform-independent byte stream. Just for pumping a simple string into a QByteArray, using a QBuffer+QDataStream is like taking a sledgehammer to crack a nut ;-)