libforth: A small Forth interpreter that can be used as a library written in c99
C Forth Groff Makefile Shell
Latest commit db2605f Jul 25, 2016 @howerj Minor improvements, removal of core.c
EVALUATE will now return if the Forth core has been rendered invalid.

The start time is no longer put into a register (which never really made any
sense to do that).

"core.c" has been removed and the "core" function within "forth.fth" continues
to be improved. {#mainpage}

This small Forth interpreter is based on a de-obfuscated entrant into the IOCCC by buzzard. The entry described a Forth like language which this derives from. You can use this library to evaluate Forth strings or as an embeddable interpreter. Work would need to be done to get useful information after doing those evaluations, but the library works quite well.

main.c is simply a wrapper around one the functions that implements a simple REPL.

This project implements a Forth interpreter library which can be embedded in other projects, it is incredibly minimalistic, but usable. To build the project a C compiler is needed, and a copy of Make, type:

make help

For a list of build options.

This implementation is released under the MIT license.


Forth is an odd language that has a loyal following groups, but it is admittedly not the most practical of language as it lacks nearly everything the modern programmer wants in a language; safety, garbage collection, modularity and clarity. It is however possible to implement a fully working interpreter in a one to two kilobytes of assembly, those kilobytes can make a functional and interactive programming environment, giving a high ratio of utility memory used.

From the Wikipedia article we can neatly summarize the language:

    "Forth is an imperative stack-based computer programming language 
    and programming environment. 

    Language features include structured programming, reflection (the
    ability to modify the program structure during program execution), 
    concatenative programming (functions are composed with juxtaposition) 
    and extensibility (the programmer can create new commands).


    A procedural programming language without type checking, Forth features
    both interactive execution of commands (making it suitable as a shell
    for systems that lack a more formal operating system) and the ability 
    to compile sequences of commands for later execution." 

Given the nature of the Forth language it does not make for a terribly good embeddable scripting language, but it is simple to implement and can be fun to use. This interpreter is based off a previous IOCCC in a file called buzzard.2.c, it is a descendant of that file.

Before using and understanding this library/interpreter it is useful to checkout more literature on Forth such as Thinking Forth by Leo Brodie for a philosophy of the language, Starting Forth (same Author), Jonesforth which is a specific implementation of the language in x86 assembly and Gforth, a more modern and portable implementation of the language.

It is important to realize that Forth is really more a philosophy and collection of ideas than a specific reference implementation or standard. It has been said that an intermediate Forth user is one who has implemented a Forth interpreter, something which cannot be said about other languages nor is possible given their complexity.

The saying "if you have seen one Forth implementation, you have seen one Forth implementation" applies, nearly every single Forth implementation has its own idea of how to go about things despite standardization efforts - in keeping with this, this library has its own idiosyncrasies.

This implementation, written in C, can be thought of as a hybrid between a fairly dumb stack based virtual machine with instructions such as "pop two values off the stack, add them, and push the result" and a small interpreter/compiler for the virtual machine. This simple kernel is then used to build a more compliant and usable Forth implementation by defining words that build upon those provided by the base system.

Other documentation

Apart from this file there are other sources of information about the project:

The manual pages can be consulted:

As can the code, which is small enough to be comprehensible:

And the forth startup code:

The startup code is well commented and shows how the core interpreter is extended to a more function Forth environment.

Using the interpreter

main.c simple calls the function main_forth() in libforth.c, this function initializes a Forth environment and puts the user in a REPL where you can issue commands and define words. See the manual pages for list of command line options and library calls. All commands are given using Reverse Polish Notation (or RPN),




    4 2 * 2 +

And brackets are no longer needed. Numbers of pushed on to the variable stack automatically and commands (such as '*' and '+') take their operands off the stack and push the result. Juggling variables on the stack becomes easier over time. To pop a value from the stack and print it there is the '.' word.


    2 2 + . 



The simplicity of the language allows for a small interpreter, the loop looks something like this:

    1) Read in a space delimited Forth WORD.
    2) Is this WORD in the dictionary?
       FOUND)      Are we in IMMEDIATE mode?
                   IMMEDIATE-MODE) Execute WORD.
                                   goto 1;
                   COMPILE-MODE)   Compile WORD into the dictionary.
                                   goto 1;
       NOT-FOUND)  Is this actually a number?
                   YES) Are we in IMMEDIATE mode?
                        IMMEDIATE-MODE) Push Number onto the stack.
                                        goto 1;
                        COMPILE-MODE)   Compile a literal number.
                                        goto 1;
                   NO)  Error! Handle error
                        goto 1;

Given that we are reading in space delimited words if follows that the above expression:

    2 2 + .

Would not work if we did:

    2 2+ .


    2 2 +.

As "2+" and "+." would be parsed as words, which may or may not be defined and if they are do not have the behavior that we want. This is more apparent when we do any kind of string handling.

A Forth Word

The Forth execution model uses Threaded Code, the layout of a word header follows from this.

A Forth word is defined in the dictionary and has a particular format that varies between implementations. A dictionary is simply a linked list of Forth words, the dictionary is usually contiguous and can only grow. The format for our Forth words is as follows:


  • Word Header:
  • field <0 = Word Name (the name is stored before the main header)
  • field 0 = Previous Word
  • field 1 = Code Word (bits 0 - 7) | Hidden Flag (bit 8) | Word Name Offset (bit 9 - 15)
  • field 3 = Code Word or First Data field Entry
  • field 4+ = Data Field

And in more detail:

    |                   Word Header                  |   Word Body    |
    | NAME ...      | PWD | MISC | CODE WORD or DATA | DATA ...       |

    NAME        = The name, or the textual representation, of a Forth
                  word, it is a variable length field that is ASCII NUL
                  terminated, the MISC field has an offset that points
                  to the begining of this field if taken off the PWD
                  position (not value). The offset is in machine words,
                  not characters.
    PWD         = A pointer to the previously declared word.
    MISC        = A complex field that can contains a CODE WORD, a
                  "hide" bit and the offset from the PWD field to the
                  beginning of NAME
    _________    ____
    CODE WORD or DATA = This will be RUN if the following DATA is a pointer
                  to the CODE WORDs of previously defined words. But it
                  could be any CODE WORD.
    DATA        = This could be anything, but it is most likely to be
                  a list of pointers to CODE WORDs of previously defined
                  words if this optional DATA field is present.

All fields are aligned on the Forth virtual machines word boundaries.

The MISC field is laid out as so:

    | <- Least Significant Bit                              Most Significant Bit -> |
    |  0 |  1 |  2 |  3 |  4 |  5 |  6 |  7 |  8 |  9 | 10 | 11 | 12 | 13 | 14 | 15 |
    |            CODE WORD             | HD |          NAME OFFSET                  |
    CODE WORD    = Bits 0-6 are a code word, this code word is always run
                   reguardless of whether we are in compiling or command
    HD           = Bit 7 is the Hide Bit, if this is true then when
                   compiling or executing words the word will be hidden from the 
    NAME OFFSET  = Bits 8 to 15 are the offset to the words name. To find the 
                   beginning of the words name we take this value away from
                   position of this words PWD header. This value is in
                   machine words, and so the beginning of the NAME must be aligned 
                   to the virtual machine words boundaries and not character, or byte, 
                   aligned. The length of this field, and the size of the input buffer, 
                   limit the maximum size of a word.

Depending on the virtual machine word size, or cell size, there may be more bits above bit '15', the most significant bit, in the MISC field. These bits are not used and should be set to zero.

And the dictionary looks like this:

   [ Special 'fake' word ]
   | NAME  | PWD | Rest of the word ... |
     The rest of the dictionary
   | NAME  | PWD | Rest of the word ... |
   | NAME  | PWD | Rest of the word ... |
   [ Previous Word Register ]

Searching of the dictionary starts from the Previous Word Register and ends at a special 'fake' word.

Defining words adds them to the dictionary, we can defined words with the ':' words like this:

    : two-times 2 * ;

Which defined the word "two-times", a word that takes a value from the stack, multiplies it by two and pushes the results back onto the stack.

The word ':' performs multiple actions; it is an immediate word that reads in the next space delimited word from the input stream and creates a header for that word. It also switches the interpreter into compile mode, compiling words will be compiled into that word definition instead of being executed, immediate words are executed as normal. ';' is also an immediate word, it compiles a special word exit into the dictionary which returns from a word call and switches the interpreter back into command mode. This type of behavior is typical of Forth implementations.

Memory Map and Special Registers

The way this interpreter works is that is emulates an idealized machine, one built for executing Forth directly. As such it has to make compromises and treats certain sectors of memory as being special, as shown below (numbers are given in hexadecimal and are multiples of the virtual machines word-size which is either 16, 32 or 64 bit depending on compile time options.

Where the dictionary ends and the variable and return stacks begin depends on how much memory was allocated to the interpreter (with a minimum of 2048 words), the default is 32768 words, and the following diagram assumes this:

    | 0-3F      | 40-7BFF       |7C00-7DFF|7E00-7FFF|
    | Registers | Dictionary... | V stack | R stack |

    V stack = The Variable Stack
    R stack = The Return Stack

Each may be further divided into special sections:


At the beginning of the Forth virtual machine there is a section used for registers, modifying them arbitrary can cause undefined behavior to occur which will most likely cause the virtual machine to be terminated.

                  DECIMAL  HEX
                  0-1      0-1    Unused
                  2-5      2-5    Push integer word
    DIC           6        6      Dictionary pointer
    RSTK          7        7      Return stack pointer
    STATE         8        8      Interpreter state; compile or command mode
    BASE          9        9      Base conversion variable
    PWD           10       A      Pointer to last defined word 
    SOURCE_ID     11       B      Input source selector (-1 = string input, 
                                  0 = file input)
    SIN           12       C      String input pointer
    SIDX          13       D      String input index  (index into SIN)
    SLEN          14       E      String input length (length of SIN)
    START_ADDR    15       F      Pointer to start of VM
    FIN           16       10     File input pointer
    FOUT          17       11     File output pointer 
    STDIN         18       12     File pointer to stdin, if available
    STDOUT        19       13     File pointer to stdout, if available
    STDERR        20       14     File pointer to stderr, if available
    ARGC          21       15     Count of arguments passed to program,
                                  if available
    ARGV          22       16     An array of pointers to NUL terminated
                                  ASCII strings, if available, of ARGC
DEBUG         23       17     Turn debugging on/off if enabled
INVALID       24       18     If non zero, this interpreter is invalid 
TOP           25       19     Stored version of top of stack
INSTRUCTION   26       1A     Stored version of instruction pointer
STACK_SIZE    27       1B     Size of the variable stack
ERROR_HANDLER 28     1C     Action to take on error
                  29-31    1D-3F  Reserved / used for other purposes


Apart from the constraints that the dictionary begins after where the registers are and before where V stack is there are no set demarcations for each region, although currently the defined word region ends before 0x200 leaving room between that and 0x7BFF for user defined words.

    | 40-???            | ???-???          | ???-7BFF                |
    | Special read word | Interpreter word | Defined word ...        |

    Special read word = A word called on entrance to the interpreter, 
                        it calls itself recursively (as a tail call). This
                        word cannot be 'found', it does not have a name.
    Interpreter word  = Any named (not 'invisible' ones) interpreter word 
                        gets put here.
    Defined word      = A list of words that have been defined with ':'

Glossary of Forth words

Each word is also given with its effect on the variable stack, any other effects are documented (including the effects on other stacks). Each entry looks like this:

  • word ( y -- z )

Where 'word' is the word being described, the contents between the parenthesis describe the stack effects, this word expects one number to be one the stack, 'y', and returns a number to the stack 'z'.

Internal words

There are three types of words.

'Invisible' words

These invisible words have no name but are used to implement the Forth. They are all immediate words.

  • push ( -- x)

Push the next value in the instruction stream onto the variable stack, advancing the instruction stream.

  • compile ( -- )

Compile a pointer to the next instruction stream value into the dictionary.

  • run ( -- )

Save the current instruction stream pointer onto the return stack and set the pointer instruction stream pointer to point to value after run.

Immediate words

These words are named and are immediate words.

  • ':' ( -- )

Read in a new word from the input stream and compile it into the dictionary.

  • 'immediate' ( -- )

Make the previously declared word immediate. Unlike in most Forth implementations this is used after the words name is given not after the final ';' has been reached.


    : word immediate ... ;

Instead of:

    : word ... ; immediate
  • '\' ( -- )

A comment, ignore everything until the end of the line.

Compiling words

  • 'read' ( -- )

read is a complex word that implements most of the user facing interpreter, it reads in a Forth word (up to 31 characters), if this word is in the dictionary it will either execute the word if we are in command mode or compile a pointer to the executable section of the word if in compile mode. If this word is not in the dictionary it is checked if it is a number, if it is then in command mode we push this value onto the variable stack, if in compile mode then we compile a literal into the dictionary. If it is none of these we print an error message and attempt to read in a new word.

  • '@' ( address -- x )

Pop an address and push the value at that address onto the stack.

  • '!' ( x address -- )

Given an address and a value, store that value at that address.

  • 'c@' ( char-address -- char )

Pop a character address and push the character value at that address onto the stack. Note that this access is not checked for being within range of the virtual machines memory, but it is still relative to the start address of virtual machine memory. This can be used to access memory outside of the interpreters memory range, although this access is unsafe.

  • 'c!' ( char char-address -- )

Given a character address, store a character value at that address, like 'c@' the address is relative to the virtual machines starting address and it is unchecked - so it is also an unsafe operation.

  • '-' ( x y -- z )

Pop two values, subtract 'y' from 'x' and push the result onto the stack.

  • '+' ( x y -- z )

Pop two values, add 'y' to 'x' and push the result onto the stack.

  • 'and' ( x y -- z )

Pop two values, compute the bitwise 'AND' of them and push the result on to the stack.

  • 'or' ( x y -- z )

Pop two values, compute the bitwise 'OR' of them and push the result on to the stack.

  • 'xor' ( x y -- z )

Pop two values, compute the bitwise 'XOR' of them and push the result on to the stack.

  • 'invert' ( x y -- z )

Perform a bitwise negation on the top of the stack.

  • 'lshift' ( x y -- z )

Pop two values, compute 'y' shifted by 'x' places to the left and push the result on to the stack.

  • 'rshift' ( x y -- z )

Pop two values, compute 'y' shifted by 'x' places to the right and push the result on to the stack.

  • '*' ( x y -- z )

Pop two values, multiply them and push the result onto the stack.

  • '/' ( x y -- z )

Pop two values, divide 'x' by 'y' and push the result onto the stack. If 'y' is zero and error message is printed and 'x' and 'y' will remain on the stack, but execution will continue on as normal.

  • 'u<' ( x y -- z )

Pop two unsigned values, compare them (y < x) and push the result onto the stack, the comparison will be unsigned.

  • 'u>' ( x y -- z )

Pop two values, compare them (y > x) and push the result onto the stack. The comparison will be unsigned.

  • '<' ( x y -- z )

Pop two unsigned values, compare them (y < x) and push the result onto the stack, the comparison will be signed.

  • '>' ( x y -- z )

Pop two values, compare them (y > x) and push the result onto the stack. The comparison will be signed.

  • 'exit' ( -- )

Pop the return stack and set the instruction stream pointer to that value.

  • '_emit' ( char -- status )

Pop a value and emit the character to the output. This pushes return status, negative means failure of some sort.

  • 'key' ( -- char )

Get a value from the input and put it onto the stack.

  • 'r>' ( -- x )

Pop a value from the return stack and push it to the variable stack.

  • '>r' ( x -- )

Pop a value from the variable stack and push it to the return stack.

  • 'branch' ( -- )

Jump unconditionally to the destination next in the instruction stream.

  • '?branch' ( bool -- )

Pop a value from the variable stack, if it is zero the jump to the destination next in the instruction stream, otherwise skip over it.

  • 'pnum' ( x -- status )

Pop a value from the variable stack and print it to the output either as a ASCII decimal or hexadecimal value depending on the BASE register. A return status is pushed onto the stack, greater or equal to zero is a success, negative is a failure. Failure can occur because of an invalid base in the BASE register, or because the output could not be written to.

  • ''' ( -- )

Push the next value in the instruction stream onto the variable stack and advance the instruction stream pointer over it.

  • ',' ( x -- )

Write a value into the dictionary, advancing the dictionary pointer.

  • '=' ( x y -- z )

Pop two values, perform a test for equality and push the result.

  • 'swap' ( x y -- y z )

Swap two values on the stack.

  • 'dup' ( x -- x x )

Duplicate a value on the stack.

  • 'drop' ( x -- )

Drop a value.

  • 'over' ( x y -- x y x )

Duplicate the value that is next on the stack.

  • 'bsave' ( char-address block-number -- )

Given an address, attempt to write out the values addr to addr+1023 values out to disk, the name of the block will be 'XXXX.blk' where the 'XXXX' is replaced by the hexadecimal representation of blocknum.

  • 'bload' ( char-address block-number -- )

Like bsave, but attempts to load a block of 1024 words into an address in memory of a likewise blocknum derived name as in bsave.

  • 'find' ( -- execution-token )

Find a word in the dictionary pushing a pointer to that word onto the variable stack.

  • 'print' ( char-address -- )

This prints a NUL terminate string at charptr. charptr is a character aligned pointer not a machine-word aligned pointer.

  • 'depth' ( -- depth )

Push the current stack depth onto the stack, the value is the depth of the stack before the depth value was pushed onto the variable stack.

  • 'clock' ( -- x )

Push the difference between the startup time and now, in milliseconds. This can be used for timing and implementing sleep functionality, the counter will not increase the interpreter is blocking and waiting for input, although this is implementation dependent.

  • 'evaluator' ( char-address -- x )

This word is a primitive used to implement 'evaluate'. It takes a pointer to a string to be evaluated as if it had been typed in. It pushes a status code, zero on success, anything else on failure of some sort.

  • 'system' ( c-addr u -- status )

Execute a command with the systems command interpreter.

File Access Words

The following compiling words are part of the File Access Word set, a few of the fields need explaining in the stack comments. "fileid" refers to a previously opened file as returned by "open-file", "ior" refers to a return status provided by the file operations. "fam" is a file access method,

  • 'close-file' ( fileid -- ior )

Close an already opened file.

  • 'open-file' ( c-addr u fam -- fileid ior )

Open a file, given a Forth string (the 'c-addr' and the 'u' arguments), and a file access method, which is defined within "forth.fth". Possible file access methods are "w/o", "r/w" and "r/o" for read only, read-write and write only respectively.

  • 'delete-file' ( c-addr u -- ior )

Delete a file on the file system given a Forth string.

  • 'read-file' ( c-addr u fileid -- ior )

Read in 'u' characters into 'c-addr' given a file identifier.

  • 'write-file' ( c-addr u fileid -- ior )

Write 'u' characters from 'c-addr' to a given file identifier.

  • 'file-position' ( fileid -- ud ior )

Get the file position offset from the beginning of the file given a file identifier.

  • 'reposition-file' ( ud fileid -- ior )

Reposition a files offset relative to the beginning of the file given a file identifier.

  • flush-file ( fileid -- ior )

Attempt to flush any buffered information written to a file.

  • rename-file ( c-addr1 u1 c-addr2 u2 -- ior )

Rename a file on the file system named by the first string ('c-addr1' and 'u1') to the second string ('c-addr2' and 'u2').

Defined words

Defined words are ones which have been created with the ':' word, some words get defined before the user has a chance to define their own to make their life easier.

  • 'state' ( -- addr )

Push the address of the register that controls the interpreter state onto the stack, this value can be written to put the interpreter into compile or command modes.

  • ';' ( -- )

Write 'exit' into the dictionary and switch back into command mode.

  • 'base' ( -- addr )

This pushes the address of a variable used for input and output conversion of numbers, this address can be written to and read, valid numbers to write are 0 and 2 to 36 (not 1).

  • 'pwd' ( -- pointer )

Pushes a pointer to the previously define word onto the stack.

  • 'h' ( -- pointer )

Push a pointer to the dictionary pointer register.

  • 'r' ( -- pointer )

Push a pointer to the register pointer register.

  • 'here' ( -- dictionary-pointer )

Push the current dictionary pointer (equivalent to "h @").

  • '[' ( -- )

Immediately switch into command mode.

  • ']' ( -- )

Switch into compile mode

  • '>mark' ( -- location )

Write zero into the head of the dictionary and advance the dictionary pointer, push a address to the zero written into the dictionary. This is usually used after in a word definition that changes the control flow, after a branch for example.

  • ':noname' ( -- execution-token )

This creates a word header for a word without a name and switches to compile mode, the usual ';' finishes the definition. It pushes a execution token onto the stack that can be written into the dictionary and run, or executed directly.

  • 'if' ( bool -- )

Begin an if-else-then statement. If the top of stack is true then we execute all between the if and a corresponding 'else' or 'then', otherwise we skip over it.

Abstract Examples:

    : word ... bool if do-stuff ... else do-other-stuff ... then ... ;

    : word ... bool if do-stuff ... then ... ;

and a concrete examples:

    : test-word if 2 2 + . cr else 3 3 * . cr ;
    0 test-word
    4             # prints 4
    1 test-word
    9             # prints 9

Is a simple and contrived example.

  • 'else' ( -- )

See 'if'.

  • 'then' ( -- )

See 'if'.

  • 'begin' ( -- )

This marks the beginning of a loop.

  • 'until' ( bool -- )

Loop back to the corresponding 'begin' if the top of the stack is zero, continue on otherwise.

  • "')'" ( -- char )

Push the number representing the ')' character onto the stack.

  • 'tab' ( -- )

Print a tab.

  • 'cr' ( -- )

Prints a newline.

  • '(' ( -- )

This will read the input stream until encountering a ')' character, it is used for comments.

  • 'allot' ( amount -- )

Allocate a number of cells in the dictionary.

  • 'tuck' ( x y -- y x y )

The stack comment documents this word entirely.

  • 'nip' ( x y -- y )

The stack comment documents this word entirely.

  • 'rot' ( x y z -- z x y )

The stack comment documents this word entirely. This word rotates three items on the variable stack.

  • '-rot' ( x y z -- y z x )

The stack comment documents this word entirely. This word rotates three items on the variable stack, in the opposite direction of "rot".

  • 'emit' ( x -- )

The is like "_emit", it writes a single character out to the output stream, however it drops the resulting status.


The file forth.fth contains many defined words, however those words are documented within that file and so as to avoid duplication will not be mentioned here. This file is not loaded automatically, and so should be run like this:


    ./forth -t forth.fth 


forth.exe -t forth.fth

Glossary of Forth terminology

  • Word vs Machine-Word

Usually in computing a 'word' refers to the natural length of integer in a machine, the term 'machine word' is used to invoke this specific meaning, a word in Forth is more analogous to a function, but there are different types of Forth words; immediate and compiling words, internal and defined words and finally visible and invisible words.

The distinction between a machine word and a Forth word can lead to some confusion.

  • The dictionary

There is only one dictionary in a normal Forth implementation, it is a data structure that can only grow in size (or at least it can in this implementation) and holds all of the defined words.

  • The stack

When we referring to a stack, or the stack, we refer to the variable stack unless otherwise stated (such as the return stack). The variable, or the stack, holds the result of recent operations such as addition or subtraction.

  • The return stack

Forth implementations are two (or more) stack machines. The second stack is the return stack which holds the usual function call return values as well as temporary variables.

  • Defined Words

A defined word is one that is not implement directly by the interpreter but has been create with the ':' word. It can be an immediate word, but does not have to be.

  • Compile mode

In this mode we compile words unless those words are immediate words, if the are then we immediately execute them.

  • Command mode

In this mode, regardless of whether we are in command or compile mode we execute words or push them on to the stack.

  • A block.

A Forth block is primitive way of managing persistent storage and this version of block interface is more primitive than most. A block is a contiguous range of bytes, usually 1024 of them as in this instance, and they can be written or read from disk. Flushing of dirty blocks is not performed in this implementation and must be done 'manually'.

Porting this interpreter

The interpreter code is written in C99, and is written to be portable, however porting it to embedded platforms that lack a C standard library (which is most of them) would mean replacing the most of the C standard library functions used, and implementing a new I/O mechanism for reading, printing and block storage.

The interpreter has been tested on the following platforms:

  • Linux ARM 32-bit Little Endian
  • Linux x86-64
  • Windows 7 x86-64

And the different virtual machine word size options (16, 32 and 64 bit machine words) have been tested.

Standards compliance

This Forth interpreter is in no way compliant with any of the standards relating to Forth, such as ANS Forth, previous Forth standardization efforts. However attempts to support words and behavior typical of these standards are made.

Some important deviations are:

  • immediate

In most Forths the "immediate" word goes after a words definition instead of inside it like this:

    : word ... ; immediate

Instead of how this interpreter does it:

    : word immediate ... ;

This behavior will not be changed for the foreseeable future, although it is the biggest difference.

  • recursion and definition hiding

A word can be called immediately before the terminating semi-colon has been reached, in the middle of a word definition. This makes the recurse keyword redundant but means using a previous definition of a word with the same name more difficult (but can be done). This might be a candidate for behavior that should be made more compliant.

  • ok

'ok' is not printed after a successful command execution , this is for two reasons, firstly because of limitations in the implementation, and secondly there is no reason for cluttering up the output window with this. The implementation should be silent by default.


Bugs and features

If you find a bug, or would like to request a new feature, please Email me at:

    howe.r.j.89 [ at ] gmail . com

The interpreter has not been battle hardened yet so there is likely behavior that is non-standard (for no reason) or just outright incorrect.


  • Port this to a micro controller, and a Linux kernel module device
  • The "check_bounds" function, whilst useful for debugging, limits the memory addressable by the virtual machine to only within its memory limits
  • A way to integrate calls to arbitrary functions that can be loaded at run time could be added, this would be technically non portable as uintptr_t is not guaranteed to be able to hold a function pointer (although it could index into a table of function pointers. The interface would take a pointer to the forth object, a pointer to the stack and the stack depth.
  • A few environment variables could be used to specify start up files for the interpreter and user specific startup files.
  • For a more complete "To-Do" list see the end of the file forth.fth.


  • The compilation should result in a small executable, and when statically linked against musl under Linux (x86-84), the stripped executable is around 50kb in size.
  • It is quite possible to make Forth programs that corrupt memory that they should, this is not a design flaw in this interpreter but more part of the Forth philosophy. If you want memory safety (and most of the time you should) you should use a different language, or implementation.
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