libQuotient (former libQMatrixClient)

The Quotient project aims to produce a Qt5-based SDK to develop applications for Matrix. libQuotient is a library that enables client applications. It is the backbone of Quaternion, NeoChat and other projects. Before version 0.5.x it was called libQMatrixClient.

Contacts

You can find Quotient developers in the Matrix room: #quotient:matrix.org.

You can file issues at the project issue tracker. If you find what looks like a security issue, please use instructions in SECURITY.md.

Getting and using libQuotient

Depending on your platform, the library can be obtained from a package management system. Recent releases of Fedora, Debian and openSUSE already have it. Alternatively, you can build the library from the source and bundle it with your application, as described below.

Pre-requisites

To use libQuotient (i.e. build or run applications with it), you'll need:

A recent Linux, macOS or Windows system (desktop versions are known to work, and there's also limited positive experience with Android)
- Recent enough Linux examples: Debian Bullseye; Fedora 35; openSUSE Leap 15.4; Ubuntu 22.04 LTS
Qt 5.15 or 6 (experimental, as of libQuotient 0.7) - either Open Source or Commercial
QtKeychain (https://github.com/frankosterfeld/qtkeychain) - 0.12 or newer is recommended; the build configuration of both QtKeychain and libQuotient must use the same Qt major version

To build applications with libQuotient, you'll also need:

CMake 3.16 or newer
A C++ toolchain that supports at least some subset of C++20 (concepts, in particular):
- GCC 11 (Windows, Linux, macOS), Clang 11 (Linux), Apple Clang 12 (macOS) and Visual Studio 2019 (Windows) are the oldest officially supported
If using E2EE (beta, as of libQuotient 0.7):
- libolm 3.2.5 or newer (the latest 3.x strongly recommended)
- OpenSSL (both 1.1.x and 3.x are known to work)
Any build system that works with CMake should be fine; known to work are GNU Make and ninja (recommended) on any platform, NMake and jom on Windows

The requirements to build libQuotient itself are basically the same except that you should install development libraries for the dependencies listed above.

Linux

Just install the prerequisites using your preferred package manager. If your Qt package base is fine-grained you might want to run CMake and look at error messages. The library is entirely offscreen; however, aside from QtCore and QtNetwork it also depends on QtGui in order to handle avatar thumbnails, without any on-screen drawing.

macOS

brew install qt qtkeychain should get you the most recent versions of the runtime libraries. Note that by default it installs Qt 6 and that's the only available configuration for QtKeychain in Homebrew. If you need to go with Qt 5, use brew install qt@5 and build QtKeychain from the source.

You may need to add the output of brew --prefix qt and brew --prefix qtkeychain to CMAKE_PREFIX_PATH (see below) to make CMake aware of the library locations.

If using E2EE, you need to also brew install libolm [email protected] (it is strongly recommended to have OpenSSL of the version that was used to build Qt; as of this writing, it's 1.1.x but check https://formulae.brew.sh/formula/qt for the most current situation). If necessary, add path prefixes for these two to CMAKE_PREFIX_PATH as well.

Windows

Install Qt (and OpenSSL, if using E2EE) using The Qt Project official installer; make sure to also tick the CMake box in the list of components to install unless you already have. This will get you both the runtime libraries and the files necessary for building libQuotient or with libQuotient. If you go this way, you'll have to build QtKeychain from the source code.

Alternatively, you can use vcpkg to install Qt, OpenSSL 1.1, and QtKeychain. You're not getting Qt Creator, which is a very nice IDE to deal with Qt-based projects; but if you already use VSCode or CLion, you might prefer this route.

If you build from the command line: the commands in further sections imply that cmake is in your PATH, otherwise you have to prepend those commands with actual paths. It's a good idea to run the qtenv2.bat script that can be found in C:\Qt\<Qt version>\<toolchain>\bin (assuming you installed Qt to C:\Qt). This script adds necessary paths to PATH. You might not want to run that script on system startup but it's very handy to setup the environment before building.

If you use a C++ IDE: you should be able to configure CMake path and extra options (CMAKE_PREFIX_PATH, in particular) in its settings. It is recommended NOT to add the path for Qt (or any other library) to PATH explicitly; use CMAKE_PREFIX_PATH instead and leave PATH unchanged. If your IDE is Qt Creator, you shouldn't need to deal with Qt paths at all, just pick the right kit and go straight to building.

If you're trying out E2EE, you will also need libolm. You'll have to build it yourself - there's no official or semi-official binary library you can download. The source code is available at https://gitlab.matrix.org/matrix-org/olm; and you can use the same toolchain (CMake+MSVC, e.g.) you'll use for Quotient.

Using the library

If you're just starting a project using libQuotient from scratch, you can copy quotest/CMakeLists.txt to your project and change quotest to your project name. If you already have an existing CMakeLists.txt, you need to insert a find_package(Quotient REQUIRED) line to an appropriate place in it (use find_package(Quotient) if libQuotient is not a hard dependency for you) and then add Quotient to your target_link_libraries() line.

Building with dynamic linkage is only tested on Linux at the moment and is a recommended way of linking your application with libQuotient on this platform. Static linkage is the default on Windows/macOS; feel free to experiment with dynamic linking and submit PRs if you get reusable results.

As for the actual API usage, a (very basic) overview can be found at the respective wiki page. Beyond that, looking at Quotest - the test application that comes with libQuotient - may help you with most common use cases such as sending messages, uploading files, setting room state etc. For more extensive usage feel free to check out (and copy, with appropriate attribution) the source code of Quaternion (the reference client for libQuotient) or NeoChat.

As of Quotient 0.7.2, all header files of libQuotient except those ending with _p.h are considered public and covered by API/ABI stability guarantees as follows: the API and ABI are backwards compatible within every minor version (0.7.x releases) with every next minor version (0.8, e.g.) breaking the compatibility. Once we reach 1.0, this rule will apply to the major version, aligning with semantic versioning rules. _p.h files are not covered by these guarantees; definitions from those should not be directly used by clients.

Building the library

On platforms other than Linux you will have to build libQuotient yourself before usage - nobody packaged it so far (contributions welcome!). You may also want to build the library on Linux if you need an unreleased snapshot.

The source code is at GitHub. Checking out a certain commit or tag (rather than downloading the archive) along with submodules is strongly recommended. If you want to hack on the library as a part of another project (e.g. you are working on Quaternion but need to do some changes to the library code), it makes sense to make a recursive check out of that project (in this case, Quaternion) and update the library submodule (also recursively) within the appropriate branch. Be mindful of API compatibility restrictions: e.g., Quaternion 0.0.95 will not build with the dev branch of libQuotient, only with 0.6.x branch.

Tags consisting solely of digits and fullstops (e.g., 0.7.0) correspond to released versions; tags ending with -betaN or -rcN mark respective pre-releases. If/when packaging pre-releases, it is advised to replace the leading - with ~ (tilde).

libQuotient is a classic CMake-based project; assuming the dependencies are in place, the following commands issued in the root directory of the project sources:

cmake -B build -S . # [-D<cmake-variable>=<value>...], see below
cmake --build build --target all

will get you a compiled library in build (make sure it exists before running). Any C++ IDE that works with CMake should be able to do the same with minimal configuration effort.

Static builds are tested on all supported platforms, building the library as a shared object (aka dynamic library) is known to work on Linux and macOS, untested on Windows.

Before proceeding, double-check that you have installed development libraries for all prerequisites above. CMake will stop and tell you if something's missing.

The first CMake invocation above configures the build. You can pass CMake variables (such as -DCMAKE_PREFIX_PATH="path1;path2;..." and -DCMAKE_INSTALL_PREFIX=path) to that invocation if needed. CMake documentation (pick the CMake version at the top of the page that you use) describes the standard variables coming with CMake. On top of them, Quotient understands:

Quotient_INSTALL_TESTS=<ON/OFF>, ON by default - install quotest along with the library files when install target is invoked. quotest is a small command-line program that (assuming correct parameters, see quotest --help) that tries to connect to a given room as a given user and perform some basic Matrix operations, such as sending messages and small files, redaction, setting room tags etc. This is useful to check the sanity of your library installation.
Quotient_ENABLE_E2EE=<ON/OFF>, OFF by default - enable work-in-progress E2EE code in the library. As of 0.6, this code is very incomplete and buggy; you should NEVER use it. In 0.7, the enabled code is beta-quality and is generally good for trying the technology but really not for mission-critical applications.

Switching this on will define Quotient_E2EE_ENABLED macro (note the difference from the CMake switch) for compiler invocations on all Quotient and Quotient-dependent (if it uses find_package(Quotient)) code; so you can use #ifdef Quotient_E2EE_ENABLED to guard the code that depends on parts of Quotient that only get built for E2EE.
MATRIX_SPEC_PATH and GTAD_PATH - these two variables are used to point CMake to the directory with the matrix-doc repository containing API files and to a GTAD binary. These two are used to generate C++ files from Matrix Client-Server API description made in OpenAPI notation. This is not needed if you just need to build the library; if you're really into hacking on it, please read the respective section in CONTRIBUTING.md.
QUOTIENT_FORCE_NAMESPACED_INCLUDES=<ON/OFF>, since version 0.7.2, OFF by default (note that QUOTIENT is in caps here, unlike options above) - when this option is set to ON, CMake skips adding <top-level include prefix>/Quotient/ to include paths, thereby forcing the client code to use #include <Quotient/header.h> instead of historically accepted #include <header.h>. By default this is set to OFF for backwards compatibility; eventually this default may change so it is recommended to at least occasionally add -DQUOTIENT_FORCE_NAMESPACED_INCLUDES=1 to a CMake invocation (or set the variable in your IDE) and make sure your code uses prefixed #include paths.

You can install the library with CMake:

cmake --build . --target install

This will also install cmake package config files; once this is done, you should be able to use quotest/CMakeLists.txt to compile quotest with the installed library. As mentioned above, installation of the quotest binary along with the rest of the library can be skipped by setting Quotient_INSTALL_TESTS to OFF.

Troubleshooting

Building fails

If cmake fails with

CMake Warning at CMakeLists.txt:11 (find_package):
  By not providing "FindQt5Widgets.cmake" in CMAKE_MODULE_PATH this project
  has asked CMake to find a package configuration file provided by
  "Qt5Widgets", but CMake did not find one.

then you need to set the right -DCMAKE_PREFIX_PATH variable, see above.

If cmake fails with a message similar to:
```
CMake Error at /usr/lib64/cmake/Qt6Core/Qt6CoreVersionlessTargets.cmake:37 (message):
  Some (but not all) targets in this export set were already defined.

  Targets Defined: Qt::Core

  Targets not yet defined: Qt::CorePrivate
```
then you likely have both Qt 5 and Qt 6 on your system, and your code uses a different major version of Qt than Quotient. Make sure you configure the build so that the same major Qt version is used both by libQuotient and your code.
If you use GCC and get an "unknown declarator" compilation error in the file qtconcurrentthreadengine.h - unfortunately, it is an actual error in Qt 5.15 sources, see https://bugreports.qt.io/browse/QTBUG-90568 (or https://bugreports.qt.io/browse/QTBUG-91909). The Qt company did not make an open source release with the fix, therefore:
- if you're on Linux - try to use Qt from your package management system, as most likely this bug is already fixed in the packages
- if you're on Windows, or if you have to use Qt (5.15) from download.qt.io for any other reason, you should apply the fix to Qt sources: locate the file (the GCC error message tells exactly where it is), find the line with the ThreadEngineStarter constructor definition that looks like this:
```
ThreadEngineStarter<void>(ThreadEngine<void> *_threadEngine)
```
  and remove the template specialisation from the constructor name (but not from ThreadEngine). The result should look like this:
```
ThreadEngineStarter(ThreadEngine<void> *_threadEngine)
```
  This will fix your build (and any other build involving QtConcurrent from this installation of Qt - the fix is not specific to Quotient in any way).

Logging configuration

libQuotient uses Qt's logging categories to make switching certain types of logging easier. In case of troubles at runtime (bugs, crashes) you can increase logging if you add the following to the QT_LOGGING_RULES environment variable:

quotient.<category>.<level>=<flag>

where

<category> is one of: main, jobs, jobs.sync, jobs.thumbnail, events, events.state (covering both the "usual" room state and account data), events.members, events.messages, events.ephemeral, database, e2ee and profiler - you can always find the full list in Quotient/logging.cpp;
<level> is one of debug, info, and warning;
<flag> is either true or false.

You can use * (asterisk) as a wildcard for any part between two dots, and semicolon is used for a separator. Latter statements override former ones, so if you want to switch on all debug logs except jobs you can set

QT_LOGGING_RULES="quotient.*.debug=true;quotient.jobs.debug=false"

Cache format

In case of troubles with room state and caching it may be useful to switch cache format from binary CBOR to plaintext JSON. To do that, set the following value in your client's configuration file/registry key (you might need to create the libQuotient key for that): libQuotient/cache_type to json. This will make cache saving and loading work slightly slower but the cache will be in text JSON files (very long and with no indentation; prepare a good JSON viewer or text editor with JSON formatting capabilities).

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