Writing HAL Components for Machinekit

Recommended pre-reading from the legacy docs.

Quite often there are existing components that can achieve your ends, without the need to write one yourself.

Certainly, maths and logic components are very well catered for.

This is a brief resume of the existing 'stock' components

This is an index of all component man pages, most of which are now instantiated.

C files

The most powerful and flexible method of writing components, is to write them in C.

If you are conversant with C, you can study other components to see the layout and function calls required and use one of them as a template to base your component upon.

The components written directly in C are to be found mainly in


There are device drivers in


Generated files

A much simpler method is to use the preprocessor / compiler tools provided with Machinekit.

With the minimum of knowledge of hal and rtapi internals, the user can create a working component.

The user does have to know how to write the main function code in C however, there are only a certain number of shortcuts to enlightenment.


comp is the legacy (linuxcnc) component generator.

comp will generate and compile components for rt or userspace.

It takes shorthand and macro options, plus the code to be used in the function proper of the component, and generates a C file from templates to contain the function code.

It also builds a full component.


instcomp is the new Machinekit instantiated component generator, developed and extended from comp.

The essential difference between normal realtime components and instantiable components, is that 'normal components' are loaded once only and have to create as many copies of the component as they need, all at once, there and then.

Instantiable components load the 'base component' and from that can create instances of the component whenever they are required. So instances can be created in separate hal files, on-the-fly or whatever.

Further info here

Amoungst many advantages, it opens up huge flexibility in configuration.

The options for instcomp differ from comp but are overall similar. The code generated differs hugely from that generated by comp.

There are over 80 components converted to instcomp from the legacy components, so there is a wealth of examples to follow and adapt

They can be found at src/hal/i_components

Python components

It is possible to write userspace components in python, using the python bindings for the C libraries.

It cannot be used for time sensitive or realtime components, because python is a scripted, interpreted language, but a lot of components have no need of milli-second poll timing and can quite happily be written as userspace using python

This link gives some basic information.

There are not that many examples, some in src/hal/user_comps

but the best well known one, is part of every basic machine config