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comp: a tool for creating HAL modules


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

Compiling a realtime HAL component is also much easier when using comp, whether the component is part of the emc2 source tree, or outside it.

For instance, the ``ddt'' portion of blocks is around 80 lines of code. The equivalent component is very short when written using the comp preprocessor:

component ddt ``Compute the derivative of the input function''; 
pin in float in; 
pin out float out; 
option data float; 
function _; 
    float tmp = in; 
    out = (tmp - data) / fperiod; 
    data = tmp; 
and it can be compiled and installed very easily: by simply placing ddt.comp in src/hal/components and running 'make', or by placing it anywhere on the system and running comp -install ddt.comp


A component is a single real-time module, which is loaded with halcmd loadrt. One .comp file specifies one component.
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.
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)

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.


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 realtime functions.

Declarations include:

component NAME [DOC]; 
function NAME [fp | nofp] [DOC]; 
option NAME [VALUE];
Brackets indicate optional items. A vertical bar indicates alternatives. Words in CAPITALS indicate variable text, as follows:

A C identifier.
When used to create a HAL identifier, any underscores are replaced with dashes, and any trailing dash is removed, so that ``this_name_'' will be turned into ``this-name''. 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.
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:

param rw bit zot=TRUE 
"""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"  
One of the HAL types: bit, s32, u32, or float.
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.
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.
Specifies the initial value of a parameter. If it is not specified, then the default is 0 or FALSE, depending on the type of the parameter.
Indicates that the function performs floating-point calculations.
Indicates that it only performs integer calculations. If neither is specified, fp is assumed. Neither comp nor gcc can detect the use of floating-point calculations in functions that are tagged nofp.
Depending on the option name, the valid VALUEs vary. 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
(default: 1)
Normally, the module parameter count defaults to 0. 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.
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)
If specified, each instance of the component will have an associated data block of type (which can be a simple type like float or the name of a type created with typedef).
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.
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.

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

Other restrictions on comp files

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

Convenience Macros

Based on the items in the declaration section, comp creates a C structure called struct 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 state and these macros may change from one version of comp to the next.

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.
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.
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.
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.
If 'option data' is specified, this macro allows access to the instance data.
The floating-point number of seconds between calls to this function.

Compiling .comp files in the source tree

Place the .comp file in the source directory emc2/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 emc2/src/hal/components and will be built except if emc2 is configured as a userspace simulator.

Compiling components outside the source tree

comp can process, compile, and install a component in a single step, placing example.ko in the emc2 realtime module directory:

comp -install example.comp
Or, it can process and compile in one step, leaving example.ko in the current directory:

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

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

comp -install example2.c
man-format documentation can also be created from the information in the declaration section:

comp -document example.comp
The resulting manpage, example.9 can be viewed with

man ./example.9
or copied to a standard location for manual pages.



This component functions like the one in 'blocks', including the default value of 1.0. The declaration ``function _'' creates functions named 'constant.0', etc.

component constant; 
pin out float out; 
param r float value = 1.0; 
function _; 
option extra_setup yes; 
FUNCTION(_) { out = value; }


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_ out; 
pin out float cos_ out; 
pin in float theta in; 
function _; 
#include <rtapi_math.h> 
FUNCTION(_) { sin_ = sin(theta); cos_ = cos(theta); }


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 u32 out_ "Output value; only low 8 bits are used";
param r u32 ioaddr;

function _;

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

#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); }


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.