qDebug() doesn't show messages in Qt Creator/Application Output

[aha_1980]

@Axel-Spoerl In principle you are right, but I don't think thats the problem here. int main() is even the default in the C++ template in Creator and that works.

Interesting though, what happens under the hood so the linker does not complain.

Regards

[JonB]

It does not matter whether you declare main() with parameters or not. It's a C function. It gets linked/called (and caller passes parameters even if not declared as received) by its name being main(), regardless.

[Christian Ehrlicher]

I would guess there is an external logging config somewhere. Please read https://doc.qt.io/qt-6/qloggingcategory.html#configuring-categories

[JonB]

@Christian-Ehrlicher
To be fair the OP reports

Running in debug mode, qDebug() doesn't show messages in Qt Creator/Application Output, but when running a terminal manually, it shows.

Assuming the executable is compiled the same (e.g. either debug or release) whether it is run from Creator or from a terminal how would some "external logging config" allow for output to terminal in the latter case but not the former? Unless you mean the environment differs such that this logging file is picked up when running from Creator but not outside?

@jronald
You might verify that if compiled for release instead the behaviour is the same? Also if executed from Creator but as Run rather than Debug does that make any difference?

[jronald]

@Axel-Spoerl said in qDebug() doesn't show messages in Qt Creator/Application Output:

Hi and welcome @jronald !
please add qInfo() << QT_VERSION_STR << QT_DEBUG and post the output here.
Cheers
Axel

#include <QDebug>
#include <QtVersion>

int main()
{
#ifdef QT_DEBUG
    qDebug() << "Test123";
    qInfo() << QT_VERSION_STR;
#else
    qDebug() << "Test456";
    qInfo() << QT_VERSION_STR;
#endif
    return 0;
}

can't output QT_DEBUG as QT_VERSION_STR does, so do it as above.
Still no output in "Qt Creator/Application Output", but when run it in a terminal manually, it outputs as below:

Test123
6.10.1

[jronald]

@JonB said in qDebug() doesn't show messages in Qt Creator/Application Output:

You might verify that if compiled for release instead the behaviour is the same? Also if executed from Creator but as Run rather than Debug does that make any difference?

I've tried debug only.
I tried release just now, same as debug, no output in "Qt Creator/Application Output", output in a terminal.

[jronald]

If check "Projects/Run Settings/Run in terminal", it will show the messages by qDebug() and qInfo().

[jronald]

It works after adding QT_ASSUME_STDERR_HAS_CONSOLE=1 in "Projects/Run Settings/Environment".
But I don't know why.

[jronald]

https://bbs.archlinux.org/viewtopic.php?id=259035

Log from Qt Creator is redirected to systemd journal.
Maybe Arch Linux maintainers should change some build flag(s) for Qt Creator.

I've checked journalctl, the outputs are there.
It should be the cause of archlinux, not intuitive, and leaves unnecessary footprints.

[JonB]

@jronald
That looks like it to me for Arch, well found! I can only say I always have Qt from Ubuntu repo and it works, you don't need to set an environment variable.

[jronald]

@JonB said in qDebug() doesn't show messages in Qt Creator/Application Output:

I can only say I always have Qt from Ubuntu repo and it works, you don't need to set an environment variable.

It's the default behaivor of Qt to output to journelctl on linux, https://doc.qt.io/qt-6/qtlogging.html#qDebug

qDebug(const char *format, ...)
Logs debug message format to the central message handler.

I think ubuntu has changed this in a good way.

[JonB]

@jronald said in qDebug() doesn't show messages in Qt Creator/Application Output:

It's the default behaivor of Qt to output to journelctl on linux, https://doc.qt.io/qt-6/qtlogging.html#qDebug

I do not see anything at that link or on that page which mentions journalctl or the systemd journal. I think the "central message handler" in the docs refers to Qt's own QtMessageHandler qInstallMessageHandler(QtMessageHandler handler), which is how qDebug() etc. output their messages. When I Googled for your issue and QT_ASSUME_STDERR_HAS_CONSOLE it came across that this behaviour is specifically in the Arch pre-built. We would come across a lot more questions about this if it applied widely across Linuxes.

[jronald]

@JonB said in qDebug() doesn't show messages in Qt Creator/Application Output:

this behaviour is specifically in the Arch pre-built

But the build script is clean: https://gitlab.archlinux.org/archlinux/packaging/packages/qtcreator/-/blob/main/PKGBUILD?ref_type=heads

[JonB]

Then for all I know other non-Arch distros compile with QT_ASSUME_STDERR_HAS_CONSOLE=1. Whatever, if you Google like I did it seems to me this is Arch but not typical Linux distro, that's all I know.

FWIW, if you don't wish to recompile Qt itself and you don't wish to have to set environment variable QT_ASSUME_STDERR_HAS_CONSOLE before running to get desired behaviour, be ware you might either:

• qputenv("QT_ASSUME_STDERR_HAS_CONSOLE", "1") as the very first statement in main(), before Q[Core]Application creation; or
• Use qInstallMessageHandler() to install your own message handler which does write to stderr or similar.

[jronald]

reported: https://qt-project.atlassian.net/browse/QTCREATORBUG-33823

[aha_1980]

@jronald And to add to @JonB: You can set the environment variables for a Qt Creator Kit in Edit > Preferences > Kits > Kits. It will then be used for all new projects automatically.

Regards