CMake support for Xenomai

This project aims to make the Xenomai libraries easily accessible with CMake, and hopefully in the longterm be added to upstream CMake.

Concepts

the use of the Module is aimed to be largely optional, means projects should be able to build-able without Xenomai with minimal (or no) changes. This has the effect that potentially required flags or libraries for Xenomai aren't provided automatically, to leave the full flexibility to the user. For example the Thread support has to be explicitly stated and linking to -librt is a good idea, but both are not absolutely necessary for every project so they are left out.

The integration aims to be "modern CMake", means you call find_package(Xenomai), which defines import libraries you can use to describe dependencies. Use of variables is not recommended, and existing variables might be removed at any time.

Support is provided in 2 variants, one as a set of config-files that can be installed with an Xenomai installation. The other is a FindXenomai script, which can be included in projects (or upstreamed to CMake).

The config-files are highly preferable, ideally this will end up upstream in Xenomai.

Minimal required CMake version is 3.0 (because of INTERFACE libraries), but full functionality required version 3.1.

Installation

Patching Xenomai

the patchset folder contains the patches needed to add the bootstrap header and install the cmake config files to Xenomai. Easiest solution if you don't mind re-compiling/installing

Config-files

Those come with placeholders, which get replaced through the included script. You would need to install the config files into <libdir>/cmake/xenomai of your (potentially staged) Xenomai installation.

Example for a custom buildroot installation:

TARGETDIR=$HOME/buildroot/staging/usr/xenomai/lib/cmake/xenomai
mkdir -p $TARGETDIR
config/install_cmakeconfig.sh --prefix=/usr/xenomai --includedir=/usr/include/xenomai --version 3.0.4 --bitness 8 -- $TARGETDIR

FindXenomai script (Currently OUTDATED!)

This needs to be found by CMake, typically you copy the directory cmake to your project, and then adjust the path CMake uses to search for Modules.

cp -r cmake <your_project_dir>

In your CMakeList.txt file, you add this early:

cmake_minimum_required(VERSION 3.0 FATAL_ERROR)
set(CMAKE_MODULE_PATH ${CMAKE_MODULE_PATH} "${CMAKE_CURRENT_SOURCE_DIR}/cmake/modules")

Bootstrap header

To allow multiple variants of creating bootstrapping code, a header is included, and needs to be found with

#include <xenomai/bootstrap-template.h>

that means you either have to copy it into the correctly directory of your xenomai installation <includedir>/xenomai or you have to make it available in some other directory and add this to the include path.

Usage

Basic recommended usage

This is the authors recommended usage of the bootstrapping code. Further variants will be described.

To allow Xenomai to consume commandline arguments (remove them so the application does not see them), you need to retrieve the modified argv.

This can be done by calling a function defined by the bootstrap code:

#if !defined(__COBALT__) && !defined(__MERCURY__)
static int xenomai_bootstrap_getargv(int *argc, char *const** argv)
{
    (void)argc; (void)argv;
    return 0;
}
#else
int xenomai_bootstrap_getargv(int *argc, char *const** argv);
#endif

int main(int argc, char *const argv[], char * const envp[])
{

    xenomai_bootstrap_getargv(&argc, &argv);
...

# this bootstrapping variant requires CMake 3.1
cmake_minimum_required(VERSION 3.1 FATAL_ERROR)

# Set the path so FindXenomai.cmake can be resolved (only if not using config files)
# set(CMAKE_MODULE_PATH ${CMAKE_MODULE_PATH} "${CMAKE_CURRENT_SOURCE_DIR}/cmake/modules")

# potential define an option to compile with/out xenomai
set(CMAKE_USE_XENOMAI true)

project(myproject VERSION 0.1)

# most applications will need threads anyway
set(THREADS_PREFER_PTHREAD_FLAG true)
find_package(Threads REQUIRED)

unset(USE_COBALT)
if(CMAKE_USE_XENOMAI)
    # we require Xenomai 3 or higher
    find_package(Xenomai 3.0 REQUIRED)
    # set a helper variable whether we can use the realtime API
    if(TARGET Xenomai::cobalt)
        set(USE_COBALT True)
    endif()
endif()

add_executable(myexec
    src/myexec.c
)

if(CMAKE_USE_XENOMAI)
    # use posix skin
    target_link_libraries(myexec
        PRIVATE
        Xenomai::posix
    )

    # add bootstrap code
    xenomai_target_bootstrap(myexec)
endif()

# always link all libraries that provide used functions explicitly
# do not depend on other modules pulling them in automatically!
target_link_libraries(myexec
    PRIVATE
    Threads::Threads rt
)

CMake Configuration allows specifying a toolchain/build environment. This is an real example using a Buildroot installation:

mkdir /tmp/build; cd /tmp/build
cmake -DCMAKE_TOOLCHAIN_FILE=$HOME/buildroot/host/share/buildroot/toolchainfile.cmake  $HOME/code/xeno_cmake/examples/alchemy
make -j4 VERBOSE=1

In case xenomai is installed in a location CMake does not automatically search, you need to add the path to the <prefix>, like -DXenomai_DIR=$HOME/buildroot/staging/usr/xenomai. Under some circumstances its necessary to specify the full path to config-files: -DXenomai_DIR=$HOME/buildroot/staging/usr/xenomai/lib/cmake/xenomai.

bootstrapping

"bootstrapping" is the method to automatically initialise an Xenomai Application, by interposing an xenomai_init and passing it the plain commandline argument. the original main is replaced with linker flags and then called with preprocessed commandline arguments.

some concepts map rather badly to CMake (for example there is no way to cleanly introduce compiled object files), and is lacking some flexibility. The obvious upside is that POSIX Applications can be compiled without touching the code.

Currently there is no equivalent in this project.

The aim is to require just a few modifications around the main routine, this explicit modification would allow dropping of some intransparent linker-remapping and some customization/variants for the boostrap code

• distribute (install) the boostrap as source. this is more flexible, especially in regards to different compilers + settings
• either require the application to rename the main routine to something else (predefined name).
• or do so in a hacky way by defining main as macro.

More detailed description

The boostrapping code does multiple tasks

• parse parameters from the commandline, create a new reduced argv with those options removed
• setup Xenomai subsystems (potentially depending on the parameters)
• promote the main thread to realtime (otherwise most cobalt calls will fail)
• a wrapper that interposes on the ragular main function and call it with the reduced argv

The first three points can be called the early initialisation happen relatively early with gcc's attribute constructor(220), see the file include/boilerplate/setup.h. Specifically it runs before normal (non-priority) constructor functions and C++ global constructors, which thus could depend on Xenomai already been initialised. A fully explicit call to xenomai_init from the main function would have the downside of not supporting these constructs.

The wrapping of the main function is (perhaps just subjectively) pretty complicated. My opionion (nolange) would be to easily be able to add the early initialisation to a project, while not providing an intransparent option for wrapping the main function.

some possible alternatives for the main wrap follow.

Variant A "Linkersymbol"

Has the advantage, that no macros are needed, the linker will hook up the correct function if available. Failure to link the correct function (like using a static library) will go unnoticed however.

int xenomai_init_getargv(int *argc, char *const** argv);

 __attribute__((weak)) int xenomai_init_getargv(int *argc, char *const** argv)
{
    return 0;

}
int main(int argc, char *const argv[])
{
    xenomai_init_getargv(&argc, &argv);
}

Variant B "Macro Guard"

Depends on Macros Xenomai will have to define anyway for the Posix wrappers to work. Failure to link bootstrapping code will be visible during linking.

int xenomai_init_getargv(int *argc, char *const** argv);

int main(int argc, char *const argv[])
{
#if defined(__COBALT__) || defined(__MERCURY__)
    xenomai_init_getargv(&argc, &argv);
#endif
}

Variant C "Dont Care"

This variant just links the bootstrap code and main will be called with the unaltered argv verctor.

int main(int argc, char *const argv[])
{
    /* argv might have arguments that were already consumed in early initialisation */
}

Variant D "Macro Wrap"

the application main function will be "renamed" with a macro definition. the bootstrap-code defines the real main function.

Variant E "Linker wrap Wrap"

the application main function will be "renamed" with a linker declaration, the bootstrap-code defines the real main function.

State

Posix over Cobalt with the preferred bootstrap variant is tested, the Config-files Method should stay stable.

Multiple other bootstrap variants are supported, including the original one with linker wrapping.

Documentation is lacking, FindXenomai script out-dated

Licenses

Sources genuine to this project are covered by the 3-Clause-BSD (identical to CMake), while the examples are lifted from Xenomai itself and are under GPL 2 unless noted otherwise.