uLisp Builder


The uLisp Builder is a set of programs written in Common Lisp to allow you to build a version of uLisp for a particular platform from a common repository of source files.

I currently run it on LispWorks on a MacBook Pro, but it should run on any version of Common Lisp, such as SBCL.

Note that you don't need to run the uLisp Builder to use uLisp. I wrote it for my own use in maintaining uLisp, and I'm making it available now in case it's of interest to anyone else.

Get it from GitHub here: https://github.com/technoblogy/ulisp-builder


4th April 2023 - The uLisp Builder has been updated to uLisp Version 4.4b, and I've updated the description accordingly.

13th June 2024 - The uLisp Builder has been updated to uLisp Version 4.6.


The aim of the Builder was to make it easier to maintain uLisp across multiple platforms. Where the C function for a particular uLisp feature is identical on all platforms there is just a single occurrence of that source in the Builder repository. The Builder also takes care of constructing the lookup table of function names and entry points used by uLisp for the built-in functions.

Why is the uLisp source for each platform a single file?

C and C++ projects are usually divided into a multiple file structure, with each logical set of functions in a separate .ino or .c file, and a separate .h header file for each.

I can see the benefits of this where a project has multiple people maintaining it, but I tried working with this sort of structure and found it much less efficient to work with. It’s also easier for users to install uLisp from a single file; there’s no setup involved, aside from double-clicking the .ino file.

Why not use the C preprocessor?

It's been suggested that uLisp could be provided as a single set of multi-platform source files, with C preprocessor statements to select the sections of code specific to each platform.

However, this would involve a much larger source file, or set of source files, and the presence of all the preprocessor statements would make it very hard to follow.

Also, I believe that some functions performed by the Builder, such as constructing the table of function names and entry points, would be beyond the capabilities of the C preprocessor.

Current state of the Builder

The version of the Builder on GitHub currently builds the latest version of uLisp for the AVR, ARM, ESP8266/32, and RISC-V platforms.


The Builder was written for my own use, and has grown as I needed to add new features to uLisp. It isn't designed to be easy to use or extend, and isn't written in elegant Lisp. Feel free to ask me questions about it on the uLisp Forum.

Using the Builder

Here's a step by step set of instructions for using the Builder:

  • Open the file Load Builder.lisp.
  • Change the first line in the file to the name of the platform you what to build for. For example:
(push :arm *features*)

The supported options are: :avr-nano, :avr, :arm, :esp, or :riscv

  • Update the lines:
(defparameter *release* "4.4b")
(defparameter *date* "3rd April 2023")
  • Compile the file. This will compile the files specified by the system builder.
  • In the Listener give the command:
(build :arm)

where the parameter should specify the same platform you specified in the first step.

If you want to build a source file containing comments describing each of the C functions, use:

(build :arm t)

You will be prompted to enter a name for the file:


  • Enter a name and click Save.

The source file for the specified platform will be saved.

What the Builder does

The Builder's main function is (build) which performs the following actions to build the uLisp source for a particular platform:


The Builder merges the sections of source for the chosen platform and outputs them to the source file.

Sources are represented in the Builder source files as strings, using the #" ... "# sharp-double-quote reader macro syntax to avoid the need to escape double-quote marks within each string. I am grateful to Doug Hoyte's excellent book Let Over Lambda for providing this reader macro.

Table of functions

The Builder processes the uLisp function definitions in the file functions.lisp, and uses these to generate the following entries in the uLisp source for each function:

For any keywords in the *enums* list, a keyword in the enum function { }; list. For example:


The source of the function. For example:

object *fn_list (object *args, object *env) {
  (void) env;
  return args;

The name of the function as a string in program memory; for example:

const char string87[] PROGMEM = "list";

An entry in lookup_table[] giving pointers to the name of the function and the function's entry point, an uint8_t giving the minimum and maximum number of parameters, and a pointer to the documentation string for the function; for example:

  { string87, fn_list, 0207, doc87 },

Documentation strings

The documentation string for each Lisp function is extracted from the comment preceding the C function that implements the Lisp function in the file functions.lisp. For example:

  (list item*)
  Returns a list of the values of its arguments.
object *fn_list (object *args, object *env) {
  (void) env;
  return args;

This is inserted into the source file as a string in program memory; for example:

const char doc87[] PROGMEM = "(list item*)\n"
"Returns a list of the values of its arguments.";

which, in turn, is referenced in the entry for the function in lookup_table[].

For more information about the 


The Builder incorporates definitions for keywords from the list:


or equivalent on other platforms in the platform-specific source file.

This gives keywords that apply to all variants, followed by variant-specific keywords; for example:

(defparameter *keywords-avr*


The following files contain utilities used by the Builder:

  • extras.lisp - general Lisp utilities used by the Builder
  • build.lisp - the main build function and routines it uses

Common sources

The sources common to all platforms are given in the files:

  • preface.lisp - the C macros and constant definitions
  • utilities.lisp - the C typedefs, global variables, and utility functions
  • saveload.lisp - the definitions for save-image, load-image, and autorun
  • assembler.lisp - the assembler for the ARM and RISC-V platforms
  • prettyprint.lisp - the prettyprinter
  • postscript.lisp - the definitions for the function table lookup, eval, read, print, and the REPL

Platform-specific features

Sources specific to a particular platform are identified using two alternative mechanisms:

Platform-specific sources

The files avr.lisp, arm.lisp, esp.lisp, and riscv.lisp contain definitions for the following uLisp platform-specific features:

  • The source file preamble
  • The platform-specific settings: WORKSPACESIZE, etc.
  • The stream definitions: I2C, SPI, serial, etc.
  • The analogread and analogwrite I/O pin definitions
  • Definitions for note, sleep, and keywords

Platform-specific includes

The main source files also include read-time conditional statements to select platform-specific versions of code, as a more economical way of incorporating platform-specific differences. For example:

void pfstring (PGM_P s, pfun_t pfun) {
  char str[strlen_P(s)+1];
  strcpy_P(str, s);
  pstring(str, pfun);

incorporates a version of pfstring on the esp platform.