2.1 Invoking Gforth

Gforth is made up of two parts; an executable “engine” (named gforth or gforth-fast) and an image file. To start it, you will usually just say gforth – this automatically loads the default image file gforth.fi. In many other cases the default Gforth image will be invoked like this:

gforth [file | -e forth-code] ...

This interprets the contents of the files and the Forth code in the order they are given.

In addition to the gforth engine, there is also an engine called gforth-fast, which is faster, but gives less informative error messages (see Error messages) and may catch some errors (in particular, stack underflows and integer division errors) later or not at all. You should use it for debugged, performance-critical programs.

Moreover, there is an engine called gforth-itc, which is useful in some backwards-compatibility situations (see Direct or Indirect Threaded?).

In general, the command line looks like this:

gforth[-fast] [engine options] [image options]

The engine options must come before the rest of the command line. They are:

--image-file file
-i file

Loads the Forth image file instead of the default gforth.fi (see Image Files).

--appl-image file

Loads the image file and leaves all further command-line arguments to the image (instead of processing them as engine options). This is useful for building executable application images on Unix, built with gforthmi --application ....

--path path
-p path

Uses path for searching the image file and Forth source code files instead of the default in the environment variable GFORTHPATH or the path specified at installation time and the working directory . (e.g., /usr/local/share/gforth/0.2.0:.). A path is given as a list of directories, separated by ‘:’ (previous versions had ‘;’ for other OSes, but since Cygwin now only accepts /cygdrive/<letter>, and we dropped support for OS/2 and MS-DOS, it is ‘:’ everywhere).

--dictionary-size size
-m size

Allocate size space for the Forth dictionary space instead of using the default specified in the image (typically 256K). The size specification for this and subsequent options consists of an integer and a unit (e.g., 4M). The unit can be one of b (bytes), e (element size, in this case Cells), k (kilobytes), M (Megabytes), G (Gigabytes), and T (Terabytes). If no unit is specified, e is used.

--data-stack-size size
-d size

Allocate size space for the data stack instead of using the default specified in the image (typically 16K).

--return-stack-size size
-r size

Allocate size space for the return stack instead of using the default specified in the image (typically 15K).

--fp-stack-size size
-f size

Allocate size space for the floating point stack instead of using the default specified in the image (typically 15.5K). In this case the unit specifier e refers to floating point numbers.

--locals-stack-size size
-l size

Allocate size space for the locals stack instead of using the default specified in the image (typically 14.5K).


Normally, Gforth tries to start up even if there is not enough virtual memory for the dictionary and the stacks (using MAP_NORESERVE on OSs that support it); so you can ask for a really big dictionary and/or stacks, and as long as you don’t use more virtual memory than is available, everything will be fine (but if you use more, processes get killed). With this option you just use the default allocation policy of the OS; for OSs that don’t overcommit (e.g., Solaris), this means that you cannot and should not ask for as big dictionary and stacks, but once Gforth successfully starts up, out-of-memory won’t kill it.


Print a message about the command-line options


Print version and exit


Print some information useful for debugging on startup.


Start the dictionary at a slightly different position than would be used otherwise (useful for creating data-relocatable images, see Data-Relocatable Image Files).


Start the dictionary at the normal position.


Initialize all bytes in the dictionary to 0 before loading the image (see Data-Relocatable Image Files).


Normally Gforth handles most signals (e.g., the user interrupt SIGINT, or the segmentation violation SIGSEGV) by translating it into a Forth THROW. With this option, Gforth exits if it receives such a signal. This option is useful when the engine and/or the image might be severely broken (such that it causes another signal before recovering from the first); this option avoids endless loops in such cases.


Disable or enable dynamic superinstructions with replication (see Dynamic Superinstructions).


Disable dynamic superinstructions, use just dynamic replication; this is useful if you want to patch threaded code (see Dynamic Superinstructions).


Use only the first N static superinstructions compiled into the engine (default: use them all; note that only gforth-fast has any). This option is useful for measuring the performance impact of static superinstructions.


Use specified metric for determining the cost of a primitive or static superinstruction for static superinstruction selection. Codesize is the native code size of the primive or static superinstruction, ls is the number of loads and stores, lsu is the number of loads, stores, and updates, and nexts is the number of dispatches (not taking dynamic superinstructions into account), i.e. every primitive or static superinstruction has cost 1. Default: codesize if you use dynamic code generation, otherwise nexts.


This option is useful for measuring the performance impact of static superinstructions. By default, an optimal shortest-path algorithm is used for selecting static superinstructions. With --ss-greedy this algorithm is modified to assume that anything after the static superinstruction currently under consideration is not combined into static superinstructions. With --ss-min-nexts this produces the same result as a greedy algorithm that always selects the longest superinstruction available at the moment. E.g., if there are superinstructions AB and BCD, then for the sequence A B C D the optimal algorithm will select A BCD and the greedy algorithm will select AB C D.


Prints some metrics used during static superinstruction selection: code size is the actual size of the dynamically generated code. Metric codesize is the sum of the codesize metrics as seen by static superinstruction selection; there is a difference from code size, because not all primitives and static superinstructions are compiled into dynamically generated code, and because of markers. The other metrics correspond to the ss-min-... options. This option is useful for evaluating the effects of the --ss-... options.

As explained above, the image-specific command-line arguments for the default image gforth.fi consist of a sequence of filenames and -e forth-code options that are interpreted in the sequence in which they are given. The -e forth-code or --evaluate forth-code option evaluates the Forth code. This option takes only one argument; if you want to evaluate more Forth words, you have to quote them or use -e several times. To exit after processing the command line (instead of entering interactive mode) append -e bye to the command line. You can also process the command-line arguments with a Forth program (see OS command line arguments).

If you have several versions of Gforth installed, gforth will invoke the version that was installed last. gforth-<version> invokes a specific version. If your environment contains the variable GFORTHPATH, you may want to override it by using the --path option.

On startup, before processing any of the image option, the user initialization file either specified in the environment variable GFORTH_ENV or, if not set, ~/.config/gforthrc0 is included, if it exists. If GFORTH_ENV is “off,” nothing is included. After processing all the image options and just before printing the boot message, the user initialization file ~/.config/gforthrc from your home directory is included, unless the option --no-rc is given.

Warning levels can be set with


Turn off warnings


Turn on warnings (level 1)


Turn on beginner warnings (level 2)


Turn on pedantic warnings (level 3)


Turn warnings into errors (level 4)