QTextStream\QFile and zero bytes in file

QFile file;
QTextStream str;
qulonglong pos;
QString line;
...
file.close();
file.setFileName(path);
file.open(QFile::ReadOnly|QIODevice::Text);
str.setDevice(&file);
str.read(5); // to recognize file encoding (bug too?) 
str.seek(pos);
...
line = str.readLine(); \\ok while no zero chars in file
...
// if zero char meets in this line
line = str.readLine(); \\ bug: skips one char just after zero. But zero and others reads fine
...
// if zero char meets in some previous line
line = str.readLine(); \\ bug: may skip some first simbols if line starts with 'D'
...

forever
{
     QChar c;
     str >> c;
     if(c.isNull()) {
          str.seek(str.pos()-1); continue;// comment it out to get same as readLine result
     }
     else if(c == QChar('\n'))
          break;
     
     ret+=c;
}