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