This is a text-only version of the following page on https://raymii.org:

This is a text-only version of the following page on https://raymii.org:
---
Title : Disable (debug) logging in QT and QML
Author : Remy van Elst
Date : 27-02-2021
URL : https://raymii.org/s/articles/Disable_logging_in_QT_and_QML.html
Format : Markdown/HTML
---

In QT you can use a few functions from the `qDebug.h` header like `qDebug()` and `qWarning()` to log information to the console. In QML you can use the likes of `console.log()`, `console.error()`. It's also very easy to implement your own logger (eg. `SyslogMessageHandler`) if you want something different, like logging to syslog and the console.
In this post I'll show you how to disable both forms of logging in a release build, qml and qt have different ways to manage their output.

 Recently I removed all Google Ads from this site due to their invasive tracking, as well as Google Analytics. Please, if you found this content useful, consider a small donation using any of the options below: <a href="https://leafnode.nl">I'm developing an open source monitoring app called Leaf Node Monitoring, for windows, linux & android. Go check it out!</a> <a href="https://github.com/sponsors/RaymiiOrg/">Consider sponsoring me on Github. It means the world to me if you show your appreciation and you'll help pay the server costs.</a> <a href="https://www.digitalocean.com/?refcode=7435ae6b8212">You can also sponsor me by getting a Digital Ocean VPS. With this referral link you'll get $100 credit for 60 days. </a> 

Information online mostly suggested to define `QT_NO_DEBUG_OUTPUT`, but that failed to disable
QML logging. Not sure if that's because my sample project had it's own `MessageHandler`, but in
the end I figured out how to disable everything.

The QT documentation pages on [debugging][1] and [qml logging][3] are helpful, but as most often
with their documentation, it's so comprehensive that it is not easy to find how to do just one
specific thing, in this case, just disabling logging in a release build.

### Disable QT logging

In your `.pro` file add the following lines:

# Suppress logging output for release build.
CONFIG(release, debug|release): DEFINES += QT_NO_DEBUG_OUTPUT
CONFIG(release, debug|release): DEFINES += QT_NO_INFO_OUTPUT
CONFIG(release, debug|release): DEFINES += QT_NO_WARNING_OUTPUT

Each flag disables the appropriate `qLog()` method, for example, `QT_NO_DEBUG_OUTPUT` disables `qDebug()`.

After editing your `.pro` file, remember to clean and rebuild your project.

Why not just `CONFIG(release):`? If you have multiple options, this only triggers if the last option is `release`.

`CONFIG(release, debug|release)` evaluates to `true` if `CONFIG` contains `release` but not `debug`, or if it contains both `debug` and `release` but `debug` doesn't appear after the last occurrence of `release`. For example:

CONFIG += release debug release

Because the last `debug` comes after the last `debug`, `CONFIG(release, debug|release)` is `true`.

### Disable QML logging

In `main.cpp` include the `QLoggingCategory` header:

#include <QLoggingCategory>

Before your other logging handlers (like `qInstallMessageHandler()`), add this piece of code:

#ifdef QT_NO_DEBUG
QLoggingCategory::setFilterRules("*.debug=false\n"
"*.info=false\n"
"*.warning=false\n"
"*.critical=true");
fprintf(stderr, "Disabling QML logging in release build.\n");
#else
fprintf(stderr, "QML logging enabled.\n");
#endif

`QT_NO_DEBUG` is exported automatically by QT when compiling a release build. Filter rules
allow for more control over what is logged when and where, but this code just disables
everything except critical. It must be newline separated (`\n` in the method parameters),
comma's do not work.

The `fprintf` line is optional but does help to know what's going on and when there
is logging and when logging is disabled.

I tried using an `#if defined(QT_NO_DEBUG_OUTPUT)` check for more granular control,
that failed to work however. Every compile (debug or release) disabled the logging, not
sure why and didn't dig in too much either.

The other way is with an environment variable. [Here][2] they suggest the following flag:

QT_LOGGING_RULES=qml=false

This failed to work for me, both exported on the command line or in `main.cpp` as below:

#ifdef QT_NO_DEBUG
qputenv("QT_LOGGING_RULES", "qml=false");
#endif

Your mileage may vary. If it does work for you, please let me know.

[1]: https://web.archive.org/web/20210226104508/https://doc.qt.io/qt-5/debug.html
[2]: https://web.archive.org/web/20210226103620if_/https://forum.qt.io/topic/100098/how-i-can-disable-qdebug-and-console-log-logs-in-final-release/5
[3]: https://web.archive.org/web/20210226103259/https://doc.qt.io/qt-5.12/qloggingcategory.html

---

License:
All the text on this website is free as in freedom unless stated otherwise.
This means you can use it in any way you want, you can copy it, change it
the way you like and republish it, as long as you release the (modified)
content under the same license to give others the same freedoms you've got
and place my name and a link to this site with the article as source.

This site uses Google Analytics for statistics and Google Adwords for
advertisements. You are tracked and Google knows everything about you.
Use an adblocker like ublock-origin if you don't want it.

All the code on this website is licensed under the GNU GPL v3 license
unless already licensed under a license which does not allows this form
of licensing or if another license is stated on that page / in that software:

This program is free software: you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation, either version 3 of the License, or
(at your option) any later version.

This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.

You should have received a copy of the GNU General Public License
along with this program. If not, see <http://www.gnu.org/licenses/>.

Just to be clear, the information on this website is for meant for educational
purposes and you use it at your own risk. I do not take responsibility if you
screw something up. Use common sense, do not 'rm -rf /' as root for example.
If you have any questions then do not hesitate to contact me.

See https://raymii.org/s/static/About.html for details.