QStream data serialization problem

GregWilsonLindberg

I'm trying to write a data serializer for some presistent data. I'm using an example from "mimec.org":http://http://www.mimec.org/node/348

Here is the write startup code:
@
bool openForWriting()
{
if (false == m_file.open( QIODevice::WriteOnly | QIODevice::Truncate ))
{
LOG_ERROR("openForWriting(): '%s'' not opened!!!", QByteArray(QString(m_file.fileName()).toUtf8()).data());

        return false;
    }

    m_stream.setDevice( &m_file );
    m_stream.setVersion( QDataStream::Qt_4_8 );

    m_stream << MagicHeader;
    m_stream << CurrentVersion;

    m_dataVersion = CurrentVersion;

    return true;
}

@

For now I'm calling this and then closing the file. If the file already exists, then the end result is a zero (0) length file. If I erase the file first, then it works correctly. In my file open I'm asking that an existing file be truncated to zero (0) length, that should have it restart as a zero length file. Is there a problem with truncating a file? Do I need to delete the file explicitly before I start writing, or maybe I need to seek to zero first?

Also, I'm printing our the file size @m_file.size()@ and the values that I get are basically garbage.

Any insights will be greatly appreciated.

SGaist

Hi,

Are you sure the file is properly closed ? What version of Qt on what OS are you using ?

GregWilsonLindberg

First, I'm running on Qt4.8 Embedded on a BeagleBone Black Angstrom Linux.

Second, I thought that I was properly closed, I open the file for reading on startup and I print out some messages in the routine that calls close() on the file and I'm seeing the messages so I assumed that the close() call was working.

I commented out the file read on startup and the write works with an existing zero length file.

Here is the code that opens for reading:

@
bool openForReading()
{
if (false == m_file.open( QIODevice::ReadOnly ))
{
LOG_ERROR("openForReading(): '%s'' not opened!!!", QByteArray(QString(m_file.fileName()).toUtf8()).data());

        return false;
    }

    m_stream.setDevice( &m_file );
    m_stream.setVersion( QDataStream::Qt_4_8 );

    qint32 header;
    m_stream >> header;

    if ( header != MagicHeader )
    {
        LOG_ERROR("openForReading(): '%s'' bad MagicHeader!!!", QByteArray(QString(m_file.fileName()).toUtf8()).data());

        return false;
    }

    qint32 version;
    m_stream >> version;

    if ( version < MinimumVersion || version > CurrentVersion )
    {
        LOG_ERROR("openForReading(): '%s'' Bad Version:%d!!!", QByteArray(QString(m_file.fileName()).toUtf8()).data(), version);

        return false;
    }

    m_dataVersion = version;

    return true;
}

@

For a zero length file the test on the MagicHeader fails and then exits.

Here is the close() code:

@
void close()
{
LOG_INFO("WorkingData bytes to write %d", m_file.bytesToWrite());
LOG_INFO("WorkingData save file size %d", m_file.size());

    m_file.close();

    m_dataVersion = -1;
}

@

And here is the code that calls everything on startup:

@
void loadWorkingData(void)
{
if (false == workingDataSave->openForReading())
{
LOG_WARNING("WorkingData save file not found!!!");

        workingDataSave->close();
    }
    else
    {
        workingDataSave->stream() >> workingData;

        workingDataSave->close();
    }
}

@

So if the openForReading() call fails, I call the close() routine that then prints out the messages and calls close() on the file.

I'll try rearranging things to see if I can get it to work.

GregWilsonLindberg

I added the file close() call into the openForReading() routine where it returns false with no change in behavior, the write still fails for an existing zero length file.

SGaist

One thing that might also come in play is that you set the same device again and again on m_stream which is never reseted so there might be an initialization problem there

GregWilsonLindberg

Very good point, I'll look into it.

Thanks,

Greg

GregWilsonLindberg

I tried setting the stream device to 0 after closing the device but that didn't have any effect.

I then changed the code to create a new dataserializer and therefore a new stream every time I load or save data and that seems to have done the trick, thanks for the tip.

Greg