gimp/devel-docs/interpreters.txt

# Interpreters for GIMP plugins



## About this document

This describes how GIMP invokes interpreters for GIMP plugin files.

This doesn't discuss the architecture of GIMP's interpreters,
or how to write an interpreted plugin.

The audience is mainly GIMP developers.
This may also interest users who want to use different interpreters.


## Brief summary

On Linux (except for Lua) and MacOS, a shebang in a GIMP plugin
text file is enough to indicate what interpreter to start.
On Windows (and Linux for Lua), you also need an .interp file installed with GIMP.

It can get complicated;
there are many combinations of envirnoment variables, shebangs, file suffixes, and .interp files that can work.

*To insure a GIMP interpreted plugin works across platforms,
it should have a shebang.*

*Except that ScriptFu plugin files installed to /scripts do not need a shebang
since the ScriptFu extension reads them.*

## Partial history of interpreters in GIMP

Rarely are interpreters addded to GIMP.
GIMP 2 offers Perl, Scheme, and Python2 interpreters.
GIMP 3 offers Python3, lua, javascript, and the gimp-script-fu-interpreter interpreters.


## Background

An interpreter usually reads a text file.
A user often launches an interpreter and passes a text file.
But users can also double-click on a text file to launch the corresponding interpreter.

Similarly, GIMP launches an interpreter on GIMP plugin text files.
GIMP must figure out the "corresponding" interpreter.

The general mechanism for launching interpreters from their text files is built into the operating system.
On Linux and MacOS, the mechanism is called a shebang or sh-bang.
On Windows, the mechanism "associates" file extensions with programs.

GIMP uses similar mechanisms to launch interpreters.
See the code in /app/plug-ins/gimpinterpreterdb.c .
*The exception is the ScriptFu extension.
GIMP starts it when GIMP starts and it reads its ".scm" plugin files from the /scripts directory without benefit
of the shebang mechanism.*

GIMP uses the mechanism when it queries plugin files at startup.
Subsequently, GIMP knows the interpreter to launch,
for example when a user clicks on a menu item implemented by an interpreter.
A user should not click on a GIMP plugin file in a file browser;
only one of the GIMP apps should launch interpreted GIMP plugin files.


## Platform differences

On Linux (except for Lua) and MacOS, you simply need a shebang in a plugin text file.

On Windows (and Linux, at least for Lua), you must also define an .interp file.
The .interp files are part of GIMP's installation on Windows (and Linux for Lua)
(in both installer and Microsoft Store versions).
The .interp files are built when the Windows installer is built.
See the source file: /build/windows/installer/base_gimp3264.iss .

A user can optionally create .interp files on Linux and MacOS.
But they are not usually part of a Linux installation.
Sophisticated users can edit .interp files to change which interpreters GIMP launches.


## shebangs

A shebang is text in the first line of a text file to be interpreted.
A shebang starts with "#!",
followed by the name or path of an interpreter,
or followed by "/usr/bin/env", a space, and the name or path of an interpreter.

!!! Shebangs for GIMP plugins always use UNIX notation, i.e. forward slashes in path strings.
Even on Windows, the shebangs are in UNIX notation.

Recommended examples for GIMP 3 (see repo directory /extensions/goat-exercises):

    #!/usr/bin/env   python3
    #!/usr/bin/env   lua
    #!/usr/bin/env   gjs
    #!/usr/bin/env   gimp-script-fu-interpreter-3.0

Other examples:

    #!python
    #!/usr/bin/python
    #!/usr/bin/env   python

Whether the other examples actually work depends on:

- the platform
- the user's environment, namely search PATH's
- any .interp files


## .interp files

Again, .interp files are necessary on Windows (and Linux, at least for Lua).
They tell GIMP which executable interpreter to launch for a GIMP plugin text file.

You usually have one .interp file for each interpreter.
For example:
- python.interp
- lua.interp
- gimp-script-fu-interpreter.interp

The repo file /data/interpreters/default.interp is a non-functioning template
for a <foo>.interp file.

.interp files are installed on Windows to, for example:

    C:\Users\foo\AppData\Programs\GIMP 3.0\lib\gimp\3.0\interpreters


interp files have three kinds of lines:
- "program" in the form lhs=rhs
- "extension" in the "binfmt" format
- "magic" in the "binfmt" format

### "program" lines in an .interp file

These lines associate a shebang with a path to an executable.

These are in the form: "lhs=rhs"
where lhs/rhs denotes "left hand side" and "right hand side."

The lhs matches the full text of a shebang after the "#!"
For example, the lhs can be "/usr/bin/env python", having a space.
Since a shebang is always in UNIX notation, any slashes are forward.

The rhs specifies a path to an interpreter.
The rhs on the Windows platform is in Windows notation, using back slashes.
For example, the rhs can be "C:\Users\foo\AppData\Programs\GIMP 3.0\bin\python"


### "extension" lines in an .interp file

These lines associate a three-letter (sic) file extension (suffix) with a path to an executable.

These lines are in binfmt format.
See https://en.wikipedia.org/wiki/Binfmt_misc.

Informally the format is:  ":name:type:offset:magic: mask:interpreter:flags"
!!! Note the field delimiter is usually ":" but can be another character.
GIMP parses the binfmt using the first character as the delimiter.
 The first field is a name or identifier and has little significance.
The second field is an "E".
The third, fifth, and seventh fields are usually empty.
The fourth field is an up-to-three letter suffix.
The sixth field "interpreter" is a name or path to an executable interpreter.

If the sixth field is a Windows path that has a ":"
then the fields must be delimited with another character, say a ",".

Examples:

    :python:E::py::python3:
    :luajit:E::lua::luajit:
    ,python,E,,py,,C:\Users\foo\AppData\GIMP 3.0\bin\python3,

Note the examples are not necessarily working examples.
They might not work if the name or path is not found,
for example if luajit was not installed to the Windows system directory of executables.

Note one example shows a path in Windows notation,
having a ":", back slashes, and a space in the path.


### "magic" lines in an .interp file

These lines associate "magic" bytes (inside a binary file) with a path to an executable.

These lines are in binfmt format.
The second field is an "M".

We won't discuss these further, since they are little used.
Binary files on Windows might not have "magic" bytes.
Usually interpreters read text files, and rarely binary files.


## Building .interp files for windows

If a GIMP developer adds an interpreter to the GIMP package,
they must modify GIMP's build for Windows
to ensure proper .interp files are installed.

See the repo file: /build/windows/installer/base_gimp3264.iss .

For the convenience of users, we usually install an .interp file having many lines.
Only one "program" line is needed if users only install canonical plugin text files
having a recommended shebang
using the actual filename of the target interpreter.
But since users may install non-canonical plugin text files by copying files,
for convenience we have more lines in the .interp file.
An extra "extension" line allows plugin text files without any shebang but a proper extension.
An extra "program" line allows plugin text files
having shebangs with alternate names for an interpreter.