QtMessageHandler context contains no information

tmag

We have a QtMessageHandler registered with qInstallMessageHandler and when messages are logged the context.file and context.function do not seem to be defined.

void handle_message(QtMsgType type, const QMessageLogContext &context, const QString &msg) {
  QByteArray localMsg = msg.toLocal8Bit();
  const char *file = context.file ? context.file : "???";
  const char *func = context.function ? context.function : "???";

  switch (type) {
    case QtDebugMsg:
      fprintf(stderr, "[DEBUG %s:%u#%s] %s\n", file, context.line, func, localMsg.constData());
      break;
  }
  // ...
}

This always produces ??? in the place of the function and file name when called with qDebug() << "Message";

Would anyone know why this might be the case? Thanks in advance!

JonB

@tmag
I would guess qDebug() does not say anywhere it will actually pass a file/function name in QMessageLogContext from wherever you call it, does it? (Unlike, say, Q_ASSERT() which I know prints them, whether it goes via qInstallMessageHandler.) You would have to pass in from caller __FILE__, __LINE__, __WHATEVER_FOR_FILENAME somehow to achieve this, probably not via qDebug()?

This is only a guess. But put a breakpoint on handle_message(), look back up the stack view to where qDebug() is calling it, and see what parameters are being passed/how the QMessageLogContext context is set up.

mpergand

@JonB

void DocCenterApp::initialize()
{
    qDebug()<<"Sorry @jonB you're wrong";
....
}

11-06-2022 15:51:50.236 [eDocCenter2 from initialize() in DocCenterApp.cpp line 59] Sorry @jonB you're wrong

JonB

@mpergand
Err, OK, thanks!
The OP should still put a breakpoint on his handler and have a look up the stack/look in the context variable.

kkoehne

@tmag said in QtMessageHandler context contains no information:

Would anyone know why this might be the case? Thanks in advance!

Your project most likely was compiled with either QT_NO_MESSAGELOGCONTEXT or QT_NO_DEBUG defines. If the latter, you can force message log contexts by defining QT_MESSAGELOGCONTEXT.

hskoglund

Also, to add to what @kkoehne says, perhaos you compiled in Release mode? Usually the QMessageLogContext stuff is available only in Debug builds.

tmag

@JonB
Thanks for the response. Whenever the breakpoint was hit the context struct just had null values for the file and line number.

tmag

@kkoehne
Brilliant thanks, this worked :)