1. Introduction

Writing a HAL component can be a tedious process, most of it in setup calls to rtapi_ and hal_ functions and associated error checking. halcompile will write all this code for you, automatically.

Compiling a HAL component is also much easier when using halcompile, whether the component is part of the LinuxCNC source tree, or outside it.

For instance, when coded in C, a simple component such as "ddt" is around 80 lines of code. The equivalent component is very short when written using the halcompile preprocessor:

Simple Component Example
component ddt "Compute the derivative of the input function";
pin in float in;
pin out float out;
variable double old;
function _;
license "GPL"; // indicates GPL v2 or later
float tmp = in;
out = (tmp - old) / fperiod;
old = tmp;

2. Installing

To compile a component you need to install dev.

sudo apt install linuxcnc-dev
sudo apt install linuxcnc-uspace-dev

Another method is to use the Synaptic Package Manager to install linuxcnc-dev or linuxcnc-uspace-dev.

3. Using a Component

Components need to be loaded and added to a thread before it can be used.

loadrt threads name1=servo-thread period1=1000000
loadrt ddt
addf ddt.0 servo-thread

More information on loadrt and addf can be found in the Hal Basics.

To test your component you can follow the examples in the HAL Tutorial.

4. Definitions

  • component - A component is a single real-time module, which is loaded with halcmd loadrt. One .comp file specifies one component. The component name and file name must match.

  • instance - A component can have zero or more instances. Each instance of a component is created equal (they all have the same pins, parameters, functions, and data) but behave independently when their pins, parameters, and data have different values.

  • singleton - It is possible for a component to be a "singleton", in which case exactly one instance is created. It seldom makes sense to write a singleton component, unless there can literally only be a single object of that kind in the system (for instance, a component whose purpose is to provide a pin with the current UNIX time, or a hardware driver for the internal PC speaker)

5. Instance creation

For a singleton, the one instance is created when the component is loaded.

For a non-singleton, the count module parameter determines how many numbered instances are created. If count is not specified, the names module parameter determines how many named instances are created. If neither count nor names is specified, a single numbered instance is created.

6. Implicit Parameters

Functions are implicitly passed the period parameter which is the time in nanoseconds of the last period to execute the component. Functions which use floating-point can also refer to fperiod which is the floating-point time in seconds, or (period*1e-9). This can be useful in components that need the timing information.

7. Syntax

A .comp file consists of a number of declarations, followed by ;; on a line of its own, followed by C code implementing the module’s functions.

Declarations include:

  • component HALNAME (DOC);



  • function HALNAME (fp | nofp) (DOC);

  • option OPT (VALUE);

  • variable CTYPE STARREDNAME ([SIZE]);

  • description DOC;

  • notes DOC;

  • see_also DOC;

  • license LICENSE;

  • author AUTHOR;

  • include HEADERFILE;

Parentheses indicate optional items. A vertical bar indicates alternatives. Words in CAPITALS indicate variable text, as follows:

  • NAME - A standard C identifier

  • STARREDNAME - A C identifier with zero or more * before it. This syntax can be used to declare instance variables that are pointers. Note that because of the grammar, there may not be whitespace between the * and the variable name.

  • HALNAME - An extended identifier. When used to create a HAL identifier, any underscores are replaced with dashes, and any trailing dash or period is removed, so that "this_name_" will be turned into "this-name", and if the name is "_", then a trailing period is removed as well, so that "function _" gives a HAL function name like "component.<num>" instead of "component.<num>."

    If present, the prefix hal_ is removed from the beginning of the component name when creating pins, parameters and functions.

    In the HAL identifier for a pin or parameter, # denotes an array item, and must be used in conjunction with a [SIZE] declaration. The hash marks are replaced with a 0-padded number with the same length as the number of # characters.

    When used to create a C identifier, the following changes are applied to the HALNAME:

    1. Any "#" characters, and any ".", "_" or "-" characters immediately before them, are removed.

    2. Any remaining "." and "-" characters are replaced with "_".

    3. Repeated "_" characters are changed to a single "\_" character.

    A trailing "_" is retained, so that HAL identifiers which would otherwise collide with reserved names or keywords (e.g., min) can be used.

    HALNAME C Identifier HAL Identifier
















  • if CONDITION - An expression involving the variable personality which is nonzero when the pin or parameter should be created

  • SIZE - A number that gives the size of an array. The array items are numbered from 0 to SIZE-1.

  • MAXSIZE : CONDSIZE - A number that gives the maximum size of the array followed by an expression involving the variable personality and which always evaluates to less than MAXSIZE. When the array is created its size will be CONDSIZE.

  • DOC - A string that documents the item. String can be a C-style "double quoted" string, like:

    "Selects the desired edge: TRUE means falling, FALSE means rising"

    or a Python-style "triple quoted" string, which may include embedded newlines and quote characters, such as:

    """The effect of this parameter, also known as "the orb of zot",
    will require at least two paragraphs to explain.
    Hopefully these paragraphs have allowed you to understand "zot"

    Or a string may be preceded by the literal character r, in which case the string is interpreted like a Python raw-string.

    The documentation string is in "groff -man" format. For more information on this markup format, see groff_man(7). Remember that halcompile interprets backslash escapes in strings, so for instance to set the italic font for the word example, write:


    In this case, r-strings are particularly useful, because the backslashes in an r-string need not be doubled:

  • TYPE - One of the HAL types: bit, signed, unsigned, or float. The old names s32 and u32 may also be used, but signed and unsigned are preferred.

  • PINDIRECTION - One of the following: in, out, or io. A component sets a value for an out pin, it reads a value from an in pin, and it may read or set the value of an io pin.

  • PARAMDIRECTION - One of the following: r or rw. A component sets a value for a r parameter, and it may read or set the value of a rw parameter.

  • STARTVALUE - Specifies the initial value of a pin or parameter. If it is not specified, then the default is 0 or FALSE, depending on the type of the item.

  • HEADERFILE - The name of a header file, either in double-quotes (include "myfile.h";) or in angle brackets (include <systemfile.h>;). The header file will be included (using C’s #include) at the top of the file, before pin and parameter declarations.

7.1. HAL functions

  • fp - Indicates that the function performs floating-point calculations.

  • nofp - Indicates that it only performs integer calculations. If neither is specified, fp is assumed. Neither halcompile nor gcc can detect the use of floating-point calculations in functions that are tagged nofp, but use of such operations results in undefined behavior.

7.2. Options

The currently defined options are:

  • option singleton yes - (default: no) Do not create a count module parameter, and always create a single instance. With singleton, items are named component-name.item-name and without singleton, items for numbered instances are named component-name.<num>.item-name.

  • option default_count number - (default: 1) Normally, the module parameter count defaults to 1. If specified, the count will default to this value instead.

  • option count_function yes - (default: no) Normally, the number of instances to create is specified in the module parameter count; if count_function is specified, the value returned by the function int get_count(void) is used instead, and the count module parameter is not defined.

  • option rtapi_app no - (default: yes) Normally, the functions rtapi_app_main() and rtapi_app_exit() are automatically defined. With option rtapi_app no, they are not, and must be provided in the C code. Use the following prototypes:
    int rtapi_app_main(void);
    void rtapi_app_exit(void);
    When implementing your own rtapi_app_main(), call the function int export(char *prefix, long extra_arg) to register the pins, parameters, and functions for prefix.

  • option data TYPE - (default: none) deprecated If specified, each instance of the component will have an associated data block of type TYPE (which can be a simple type like float or the name of a type created with typedef). In new components, variable should be used instead.

  • option extra_setup yes - (default: no) If specified, call the function defined by EXTRA_SETUP for each instance. If using the automatically defined rtapi_app_main, extra_arg is the number of this instance.

  • option extra_cleanup yes - (default: no) If specified, call the function defined by EXTRA_CLEANUP from the automatically defined rtapi_app_exit, or if an error is detected in the automatically defined rtapi_app_main.

  • option userspace yes - (default: no) If specified, this file describes a userspace (ie, non-realtime) component, rather than a regular (ie, realtime) one. A userspace component may not have functions defined by the function directive. Instead, after all the instances are constructed, the C function void user_mainloop(void); is called. When this function returns, the component exits. Typically, user_mainloop() will use FOR_ALL_INSTS() to perform the update action for each instance, then sleep for a short time. Another common action in user_mainloop() may be to call the event handler loop of a GUI toolkit.

  • option userinit yes - (default: no) This option is ignored if the option userspace (see above) is set to no. If userinit is specified, the function userinit(argc,argv) is called before rtapi_app_main() (and thus before the call to hal_init() ). This function may process the commandline arguments or take other actions. Its return type is void; it may call exit() if it wishes to terminate rather than create a HAL component (for instance, because the commandline arguments were invalid).

  • option extra_link_args "…" - (default: "") This option is ignored if the option userspace (see above) is set to no. When linking a userspace component, the arguments given are inserted in the link line. Note that because compilation takes place in a temporary directory, "-L." refers to the temporary directory and not the directory where the .comp source file resides.

  • option extra_compile_args "…" - (default: "") This option is ignored if the option userspace (see above) is set to no. When compiling a userspace component, the arguments given are inserted in the compiler command line.

If an option’s VALUE is not specified, then it is equivalent to specifying option … yes. The result of assigning an inappropriate value to an option is undefined. The result of using any other option is undefined.

7.3. License and Authorship

  • LICENSE - Specify the license of the module for the documentation and for the MODULE_LICENSE() module declaration. For example, to specify that the module’s license is GPL v2 or later,

    license "GPL"; // indicates GPL v2 or later

    For additional information on the meaning of MODULE_LICENSE() and additional license identifiers, see <linux/module.h>. or the manual page rtapi_module_param(3)

    This declaration is required.

  • AUTHOR - Specify the author of the module for the documentation.

7.4. Per-instance data storage





    Declare a per-instance variable STARREDNAME of type CTYPE, optionally as an array of SIZE items, and optionally with a default value DEFAULT. Items with no DEFAULT are initialized to all-bits-zero. CTYPE is a simple one-word C type, such as float, u32, s32, int, etc. Access to array variables uses square brackets.

    If a variable is to be of a pointer type, there may not be any space between the "*" and the variable name. Therefore, the following is acceptable:

    variable int *example;

    but the following are not:

    variable int* badexample;
    variable int * badexample;


C++-style one-line comments (//… ) and

C-style multi-line comments (/* … */) are both supported in the declaration section.

8. Restrictions

Though HAL permits a pin, a parameter, and a function to have the same name, halcompile does not.

Variable and function names that can not be used or are likely to cause problems include:

  • Anything beginning with _comp.

  • comp_id

  • fperiod

  • rtapi_app_main

  • rtapi_app_exit

  • extra_setup

  • extra_cleanup

9. Convenience Macros

Based on the items in the declaration section, halcompile creates a C structure called struct __comp_state. However, instead of referring to the members of this structure (e.g., *(inst->name)), they will generally be referred to using the macros below. The details of struct __comp_state and these macros may change from one version of halcompile to the next.

  • FUNCTION(name) - Use this macro to begin the definition of a realtime function which was previously declared with function NAME. The function includes a parameter period which is the integer number of nanoseconds between calls to the function.

  • EXTRA_SETUP() - Use this macro to begin the definition of the function called to perform extra setup of this instance. Return a negative Unix errno value to indicate failure (e.g., return -EBUSY on failure to reserve an I/O port), or 0 to indicate success.

  • EXTRA_CLEANUP() - Use this macro to begin the definition of the function called to perform extra cleanup of the component. Note that this function must clean up all instances of the component, not just one. The "pin_name", "parameter_name", and "data" macros may not be used here.

  • pin_name or parameter_name - For each pin pin_name or param parameter_name there is a macro which allows the name to be used on its own to refer to the pin or parameter. When pin_name or parameter_name is an array, the macro is of the form pin_name(idx) or param_name(idx) where idx is the index into the pin array. When the array is a variable-sized array, it is only legal to refer to items up to its condsize.

    When the item is a conditional item, it is only legal to refer to it when its condition evaluated to a nonzero value.

  • variable_name - For each variable variable_name there is a macro which allows the name to be used on its own to refer to the variable. When variable_name is an array, the normal C-style subscript is used: variable_name[idx]

  • data - If "option data" is specified, this macro allows access to the instance data.

  • fperiod - The floating-point number of seconds between calls to this realtime function.

  • FOR_ALL_INSTS() {…} - For userspace components. This macro iterates over all the defined instances. Inside the body of the loop, the pin_name, parameter_name, and data macros work as they do in realtime functions.

10. Components with one function

If a component has only one function and the string "FUNCTION" does not appear anywhere after ;;, then the portion after ;; is all taken to be the body of the component’s single function. See the Simple Comp for and example of this.

11. Component Personality

If a component has any pins or parameters with an "if condition" or "[maxsize : condsize]", it is called a component with personality. The personality of each instance is specified when the module is loaded. Personality can be used to create pins only when needed. For instance, personality is used in the logic component, to allow for a variable number of input pins to each logic gate and to allow for a selection of any of the basic boolean logic functions and, or, and xor.

The default number of allowed personality items is a compile-time setting (64). The default applies to numerous components included in the distribution that are built using halcompile.

To alter the allowed number of personality items for user-built components, use the --personality option with halcompile. For example, to allow up to 128 personality times:

  [sudo] halcompile --personality=128 --install ...

When using components with personality, normal usage is to specify a personality item for each specified component instance. Example for 3 instances of the logic component:

loadrt logic names=and4,or3,nand5, personality=0x104,0x203,0x805
If a loadrt line specifies more instances than personalities, the instances with unspecified personalities are assigned a personality of 0. If the requested number of instances excedes the number of allowed personalities, personalities are assigned by indexing modulo the number of allowed personalities. A message is printed denoting such assignments.

12. Compiling

Place the .comp file in the source directory linuxcnc/src/hal/components and re-run make. Comp files are automatically detected by the build system.

If a .comp file is a driver for hardware, it may be placed in linuxcnc/src/hal/drivers and will be built unless LinuxCNC is configured as a userspace simulator.

13. Compiling realtime components outside the source tree

halcompile can process, compile, and install a realtime component in a single step, placing rtexample.ko in the LinuxCNC realtime module directory:

[sudo] halcompile --install rtexample.comp
sudo (for root permission) is needed when using LinuxCNC from a deb package install. When using a Run-In-Place (RIP) build, root privileges should not be needed.

Or, it can process and compile in one step, leaving example.ko (or example.so for the simulator) in the current directory:

halcompile --compile rtexample.comp

Or it can simply process, leaving example.c in the current directory:

halcompile rtexample.comp

halcompile can also compile and install a component written in C, using the --install and --compile options shown above:

[sudo] halcompile --install rtexample2.c

man-format documentation can also be created from the information in the declaration section:

halcompile --document rtexample.comp

The resulting manpage, example.9 can be viewed with

man ./example.9

or copied to a standard location for manual pages.

14. Compiling userspace components outside the source tree

halcompile can process, compile, install, and document userspace components:

halcompile usrexample.comp
halcompile --compile usrexample.comp
[sudo] halcompile --install usrexample.comp
halcompile --document usrexample.comp

This only works for .comp files, not for .c files.

15. Examples

15.1. constant

Note that the declaration "function _" creates functions named "constant.0" , etc. The file name must match the component name.

component constant;
pin out float out;
param r float value = 1.0;
function _;
license "GPL"; // indicates GPL v2 or later
FUNCTION(_) { out = value; }

15.2. sincos

This component computes the sine and cosine of an input angle in radians. It has different capabilities than the "sine" and "cosine" outputs of siggen, because the input is an angle, rather than running freely based on a "frequency" parameter.

The pins are declared with the names sin_ and cos_ in the source code so that they do not interfere with the functions sin() and cos(). The HAL pins are still called sincos.<num>.sin.

component sincos;
pin out float sin_;
pin out float cos_;
pin in float theta;
function _;
license "GPL"; // indicates GPL v2 or later
#include <rtapi_math.h>
FUNCTION(_) { sin_ = sin(theta); cos_ = cos(theta); }

15.3. out8

This component is a driver for a fictional card called "out8", which has 8 pins of digital output which are treated as a single 8-bit value. There can be a varying number of such cards in the system, and they can be at various addresses. The pin is called out_ because out is an identifier used in <asm/io.h>. It illustrates the use of EXTRA_SETUP and EXTRA_CLEANUP to request an I/O region and then free it in case of error or when the module is unloaded.

component out8;
pin out unsigned out_ "Output value; only low 8 bits are used";
param r unsigned ioaddr;

function _;

option count_function;
option extra_setup;
option extra_cleanup;
option constructable no;

license "GPL"; // indicates GPL v2 or later
#include <asm/io.h>

#define MAX 8
int io[MAX] = {0,};
RTAPI_MP_ARRAY_INT(io, MAX, "I/O addresses of out8 boards");

int get_count(void) {
    int i = 0;
    for(i=0; i<MAX && io[i]; i++) { /* Nothing */ }
    return i;

    if(!rtapi_request_region(io[extra_arg], 1, "out8")) {
        // set this I/O port to 0 so that EXTRA_CLEANUP does not release the IO
        // ports that were never requested.
        io[extra_arg] = 0;
        return -EBUSY;
    ioaddr = io[extra_arg];
    return 0; }

    int i;
    for(i=0; i < MAX && io[i]; i++) {
        rtapi_release_region(io[i], 1);

FUNCTION(_) { outb(out_, ioaddr); }

15.4. hal_loop

component hal_loop;
pin out float example;

This fragment of a component illustrates the use of the hal_ prefix in a component name. loop is the name of a standard Linux kernel module, so a loop component might not successfully load if the Linux loop module was also present on the system.

When loaded, halcmd show comp will show a component called hal_loop. However, the pin shown by halcmd show pin will be loop.0.example, not hal-loop.0.example.

15.5. arraydemo

This realtime component illustrates use of fixed-size arrays:

component arraydemo "4-bit Shift register";
pin in bit in;
pin out bit out-# [4];
function _ nofp;
license "GPL"; // indicates GPL v2 or later
int i;
for(i=3; i>0; i--) out(i) = out(i-1);
out(0) = in;

15.6. rand

This userspace component changes the value on its output pin to a new random value in the range (0,1) about once every 1ms.

component rand;
option userspace;

pin out float out;
license "GPL"; // indicates GPL v2 or later
#include <unistd.h>

void user_mainloop(void) {
    while(1) {
        FOR_ALL_INSTS() out = drand48();

15.7. logic

This realtime component shows how to use "personality" to create variable-size arrays and optional pins.

component logic "LinuxCNC HAL component providing experimental logic functions";
pin in bit in-##[16 : personality & 0xff];
pin out bit and if personality & 0x100;
pin out bit or if personality & 0x200;
pin out bit xor if personality & 0x400;
function _ nofp;
description """
Experimental general 'logic function' component.  Can perform 'and', 'or'
and 'xor' of up to 16 inputs.  Determine the proper value for 'personality'
by adding:
.IP \\(bu 4
The number of input pins, usually from 2 to 16
.IP \\(bu
256 (0x100)  if the 'and' output is desired
.IP \\(bu
512 (0x200)  if the 'or' output is desired
.IP \\(bu
1024 (0x400)  if the 'xor' (exclusive or) output is desired""";
license "GPL"; // indicates GPL v2 or later
    int i, a=1, o=0, x=0;
    for(i=0; i < (personality & 0xff); i++) {
        if(in(i)) { o = 1; x = !x; }
        else { a = 0; }
    if(personality & 0x100) and = a;
    if(personality & 0x200) or = o;
    if(personality & 0x400) xor = x;

A typical load line for this component might be

loadrt logic count=3 personality=0x102,0x305,0x503

which creates the following pins:

  • A 2-input AND gate: logic.0.and, logic.0.in-00, logic.0.in-01

  • 5-input AND and OR gates: logic.1.and, logic.1.or, logic.1.in-00, logic.1.in-01, logic.1.in-02, logic.1.in-03, logic.1.in-04,

  • 3-input AND and XOR gates: logic.2.and, logic.2.xor, logic.2.in-00, logic.2.in-01, logic.2.in-02

15.8. general functions

This example shows how to call functions from the main function.
it also shows how to pass reference of HAL pins to those functions.

component example;
pin in s32 in;
pin out bit out1;
pin out bit out2;

function _;
license "GPL";

// general pin set true function
void set(hal_bit_t *p){
    *p = 1;

// general pin set false function
void unset(hal_bit_t *p){
    *p = 0;

//main function
    if (in < 0){
    }else if (in >0){


This component uses two general function to manipulate a HAL bit pin referenced to it.

16. Command Line Usage

The halcompile man page gives details for invoking halcompile.

$ man halcompile

A brief summary of halcompile usage is given by:

$ halcompile --help