Question

Im using this code to get data from QTcpSocket. I read data in buffer and then if I free it immediately its fine. But if I free it later the application crashes.
QtDocs says that I have to clean up memory with delete[] after using method readBytes

void Widget::slotReadClient()
{
    QTcpSocket* pClientSocket = (QTcpSocket*)sender();
    QDataStream in(pClientSocket);
    in.setVersion(QDataStream::Qt_4_7);
    for (;;)
    {
        if (!m_nNextBlockSize)
        {
            if (pClientSocket->bytesAvailable() < sizeof(quint16))
            {
                break;
            }
            in >> m_nNextBlockSize;
        }
        if (pClientSocket->bytesAvailable() < m_nNextBlockSize)
        {
            break;
        }
      QString attribute;
     quint16 count=0;
      in>>count;//count of massives
     quint16 els=count;
     uint len=0;
     char** wkbs;
     if(count)
          wkbs=new char*[els];
     int j=0;
     char* buf=0;

      while(count)
      {
          in>>attribute;//description its ok
          buf=wkbs[j];
          buf=0;
          in.readBytes(buf,len);
          ui->plainTextEdit->appendPlainText(QString::number(len));//length of buffer ok
          j++;
          count--;
          processGeom(buf);//data is OK
          //delete [] buf; if I use this it works
      }
      ui->plainTextEdit->appendPlainText(QString::number(els));
      j=0;
      while(els)
      {
          buf=wkbs[j];
          delete[] buf;//here I get crash
          els--;
          j++;
      }
      delete[] wkbs;
      m_nNextBlockSize = 0;
    }
}

Answer 1

You are never assigning anything to wkbs, so you are trying to delete[] an uninitialized pointer!

buf=wkbs[j];
buf=0;
in.readBytes(buf,len);

Here, you are assigning to buf three times! The first lines are absolutely pointless, since readBytes will overwrite the value in buf. If you want to keep the allocated buffer, store it in wkbs after readBytes:

in.readBytes(buf, len);
wkbs[j] = buf;

Next you are attempting:

buf=wkbs[j];
delete[] buf;//here I get crash

With your original code, wkbs[j] is uninitialized, its value could be anything. Deleting this pointer is unsafe.