# Module structure¶

This section explains the structure of an HPX module.

The tool create_library_skeleton.py can be used to generate a basic skeleton. To create a library skeleton, run the tool in the libs subdirectory with the module name as an argument:

./create_library_skeleton <lib_name>


This creates a skeleton with the necessary files for an HPX module. It will not create any actual source files. The structure of this skeleton is as follows:

• <lib_name>/

• README.rst

• CMakeLists.txt

• cmake

• docs/

• index.rst

• examples/

• CMakeLists.txt

• include/

• hpx/

• <lib_name>

• src/

• CMakeLists.txt

• tests/

• CMakeLists.txt

• unit/

• CMakeLists.txt

• regressions/

• CMakeLists.txt

• performance/

• CMakeLists.txt

A README.rst should be always included which explains the basic purpose of the library and a link to the generated documentation.

A main CMakeLists.txt is created in the root directory of the module. By default it contains a call to add_hpx_module which takes care of most of the boilerplate required for a module. You only need to fill in the source and header files in most cases.

add_hpx_module requires a module name. Optional flags are:

• DEPRECATION_WARNINGS: Enables deprecation warnings for the module.

Optional single-value arguments are:

• COMPATIBILITY_HEADERS: Can be ON, OFF, or left out. Enables compatibility headers. Creates a variable which can be turned on or off by the user when set to ON or OFF. If left out the option is completely disabled.

• INSTALL_BINARIES: Install the resulting library.

Optional multi-value arguments-are:

• SOURCES: List of source files.

• HEADERS: List of header files.

• COMPAT_HEADERS: List of compatibility header files.

• DEPENDENCIES: Libraries that this module depends on, such as other modules.

• CMAKE_SUBDIRS: List of subdirectories to add to the module.

The include directory should contain only headers that other libraries need. For each of those headers, an automatic header test to check for self containment will be generated. Private headers should be placed under the src directory. This allows for clear separation. The cmake subdirectory may include additional CMake scripts needed to generate the respective build configurations.

Compatibility headers (forwarding headers for headers whose location is changed when creating a module, if moving them from the main library) should be placed in an include_compatibility directory. This directory is not created by default.

Documentation is placed in the docs folder. A empty skeleton for the index is created, which is picked up by the main build system and will be part of the generated documentation. Each header inside the include directory will automatically be processed by Doxygen and included into the documentation. If a header should be excluded from the API reference, a comment // sphinx:undocumented needs to be added.

Tests are placed in suitable subdirectories of tests.

When in doubt, consult existing modules for examples on how to structure the module.

## Finding circular dependencies¶

Our CI will perform a check to see if there are circular dependencies between modules. In cases where it’s not clear what is causing the circular dependency, running the cpp-dependencies tool manually can be helpful. It can give you detailed information on exactly which files are causing the circular dependency. If you do not have the cpp-dependencies tool already installed, one way of obtaining it is by using our docker image. This way you will have exactly the same environment as on the CI. See Using docker for development for details on how to use the docker image.

To produce the graph produced by CI run the following command (HPX_SOURCE is assumed to hold the path to the HPX source directory):

cpp-dependencies --dir $HPX_SOURCE/libs --graph-cycles circular_dependencies.dot  This will produce a dot file in the current directory. You can inspect this manually with a text editor. You can also convert this to an image if you have graphviz installed: dot circular_dependencies.dot -Tsvg -o circular_dependencies.svg  This produces an svg file in the current directory which shows the circular dependencies. Note that if there are no cycles the image will be empty. You can use cpp-dependencies to print the include paths between two modules. cpp-dependencies --dir$HPX_SOURCE/libs --shortest <from> <to>


prints all possible paths from the module <from> to the module <to>. For example, as most modules depend on config, the following should give you a long list of paths from algorithms to config:

cpp-dependencies --dir $HPX_SOURCE/libs --shortest algorithms config  The following should report that it can’t find a path between the two modules: cpp-dependencies --dir$HPX_SOURCE/libs --shortest config algorithms