Where is located the qDebug qWarning qCritical and qFatal log by default on Qt?

Question

When running my Qt5 application on linux, I don\'t see any output from qDebug, qWarning, qCritical or qFatal. I know that I can use qInstallMsgHandler to instal

Answer 1

They are still printed to standard error.

If you launch the application from the command line, it is usually printed there, or if using Qt Creator, it's displayed in the Application Output window.

Answer 2

Please do not make the mistake of assuming that qDebug, qWarning, qCritical and qFatal always log on standard error. That's absolutely not the case.

The actual destination varies depending on the Qt configuration and the targeting OS. Plus, 5.4 and then 5.11 introduced some behavioural changes. See here and here for discussions.

TL;DR:

On Qt >= 5.11

• If the process' stderr has a console attached, that's where the debug log will go.
• If you want to always log on stderr, set QT_FORCE_STDERR_LOGGING to 1.
• In alternative, set QT_ASSUME_STDERR_HAS_CONSOLE to 1. I suspect this one is meant to be used by a parent process that reads a child's stderr and shows it to the user somehow.
• QT_LOGGING_TO_CONSOLE to 1 still works, but Qt will complain.

On Qt >= 5.4 and < 5.11

• If you want to always log on stderr, set the QT_LOGGING_TO_CONSOLE environment variable to 1.
• If you do not want to log on stderr, the QT_LOGGING_TO_CONSOLE environment variable to 0 (this will force logging through the native system logger).
• If the QT_LOGGING_TO_CONSOLE environment variable is not set, then whether logging to the console or not depends on whether the application is running in a TTY (on UNIX) or whether there's a console window (on Windows).

On Qt < 5.4, the situation is more confusing.

• If Qt has been built with support for a specific logging framework (e.g. SLOG2, journald, Android log, etc.) then logging always goes to that framework
• Otherwise on UNIX it goes to stderr
• Otherwise on Windows OutputDebugString or stderr is used depending whether the app is a console app.

The problem with the pre-5.4 approach was that, f.i., under Unix IDEs would not capture an application's debug output if Qt had been built with journald support. That's because the output went to journald, not to the IDE. In 5.4 the approach has been made more flexible and uniform across OSes.

Answer 3

If you are using visual studio, qDebug etc are printed to the output window in the IDE.

Answer 4

If you happen to run Arch Linux, which compiles Qt with the -journald option, all debug output is per default directed to the systemd journal (display with journalctl).

You can override this behaviour by defining QT_LOGGING_TO_CONSOLE=1 as an environment variable.