qInstallMessageHandler

QtMessageHandler qInstallMessageHandler(QtMessageHandler handler)

Installs a Qt message handler which has been defined previously. Returns a pointer to the previous message handler.

The message handler is a function that prints out debug messages, warnings, critical and fatal error messages. The Qt library (debug mode) contains hundreds of warning messages that are printed when internal errors (usually invalid function arguments) occur. Qt built in release mode also contains such warnings unless QT_NO_WARNING_OUTPUT and/or QT_NO_DEBUG_OUTPUT have been set during compilation. If you implement your own message handler, you get total control of these messages.

The default message handler prints the message to the standard output under X11 or to the debugger under Windows. If it is a fatal message, the application aborts immediately.

Only one message handler can be defined, since this is usually done on an application-wide basis to control debug output.

To restore the message handler, call qInstallMessageHandler(0).

Example:

  #include <qapplication.h>
  #include <stdio.h>
  #include <stdlib.h>
  void myMessageOutput(QtMsgType type, const QMessageLogContext &context, const QString &msg)
  {
      QByteArray localMsg = msg.toLocal8Bit();
      switch (type) {
      case QtDebugMsg:
          fprintf(stderr, "Debug: %s (%s:%u, %s)\n", localMsg.constData(), context.file, context.line, context.function);
          break;
      case QtInfoMsg:
          fprintf(stderr, "Info: %s (%s:%u, %s)\n", localMsg.constData(), context.file, context.line, context.function);
          break;
      case QtWarningMsg:
          fprintf(stderr, "Warning: %s (%s:%u, %s)\n", localMsg.constData(), context.file, context.line, context.function);
          break;
      case QtCriticalMsg:
          fprintf(stderr, "Critical: %s (%s:%u, %s)\n", localMsg.constData(), context.file, context.line, context.function);
          break;
      case QtFatalMsg:
          fprintf(stderr, "Fatal: %s (%s:%u, %s)\n", localMsg.constData(), context.file, context.line, context.function);
          abort();
      }
  }
  int main(int argc, char **argv)
  {
      qInstallMessageHandler(myMessageOutput);
      QApplication app(argc, argv);
      ...
      return app.exec();
  }

 

This function was introduced in Qt 5.0.