heap corrupted

I have timer to emit signal to trigger sendMessage every 30 millisecond. after hours , i got heap corruptions

it said

Quote:

Windows has triggered a breakpoint in matx_migrate.exe.

This may be due to a corruption of the heap, and indicates a bug in
matx_migrate.exe or any of the DLLs it has loaded.

The output window may have more diagnostic information

Code:

void ClientThread::sendMessage(){
  #ifdef _DEBUG
  qWarning("sendMessage %d\n", QThread::currentThreadId()); 
#endif
  numOfMsg++;
  if(numOfMsg>CHECK_PERIOD){
    sendCheckMessage();
    numOfMsg=0;
  }
  QList<QByteArray> dataToSend= cannonPtr->getDataToSend();
  if(!dataToSend.isEmpty()){
    QByteArray data;
    QDataStream out( &data, QIODevice::ReadWrite );
    out.setVersion(QDataStream::Qt_4_4);
    out << (quint16)0;//reserve for size
    out << (quint16)dataToSend.count();
    for(int i=0; i<dataToSend.count(); i++){
 
      data.append(dataToSend[i]);
      //out<<dataToSend[i];
 
    }
    out.device()->seek(0);
    out<< (quint16)(data.size() - sizeof(quint16));
    #ifdef _DEBUG
    quint16 size;
    quint16 num;
    quint16 kind;
    qint32 frame;
    qint32 id;
    qint32 x;
    qint32 y;
    bool dead;
    QTime time;
    out.device()->seek(0);
    out>>size>>num>>kind>>frame>>id>>x>>y>>dead>>time;
    qWarning()<<"size: "<<size<<"num: "<<num<<"frame: "<<frame; 
#endif
    socketPrt->write(data);
    socketPrt->waitForBytesWritten();
 
    dataToSend.clear();
  }
}

any comments thanks very much