clong

A wrapper for libclang and a generator that can turn c header files into clojure apis.

Currently, there is only a generator for jna, but support for other ffi libs is likely.

Rationale

Writing wrappers for c libraries is tedious and error prone. The goal of clong is to streamline the process of creating wrappers for c libraries. It is a non-goal to make the process 100% automatic. However, it should be possible to do 80-90% of the work and provide tools that are useful for building higher level APIs.

Usage

Leiningen dependency:

[com.phronemophobic/clong "1.2"]
;; only needed for parsing. not needed for generation
[org.bytedeco/llvm-platform "16.0.4-1.5.9"]

deps.edn dependency:

com.phronemophobic/clong {:mvn/version "1.1"}
;; only needed for parsing. not needed for generation
org.bytedeco/llvm-platform {:mvn/version "16.0.4-1.5.9"}

Note: If you receive an error like "Error building classpath. Could not acquire write lock for[...]" while downloading deps, try deleting the maven dep and retrying with -Sthreads 1 to workaround this issue.

rm -rf ~/.m2/repository/org/bytedeco/
clj -Sthreads 1 -P

Parsing

com.phronemophobic.clong.clang/parse takes a header filename and the command line args to pass to clang. The most common command line args are "-I/my/header/path". If you built clang locally, then you may also need to add system paths. You can see the system paths that you may by running clang -### empty-file.h from the command line.

parse returns a CXCursor. Further processing can be done via the raw api in com.phronemophobic.clong.clang.jna.raw. For basic usage, just use:

(require '[com.phronemophobic.clong.clang :as clang])

(def results (->> (clang/parse "my-header.h" clang/default-arguments)
                  clang/get-children
                  (map clang/cursor-info)))

To get a flavor of some of the data that can be extracted, check out clong's datafied version of the libclang API.

Generating APIs

Below is how clong can be used to generate a wrapper for libz (full example):

(def libz
  (com.sun.jna.NativeLibrary/getInstance "z"))

(def api (clong/easy-api "/opt/local/include/zlib.h"))

(gen/def-api libz api)

(zlibVersion) ;; "1.2.11"

(def source "clong!")

(def dest (byte-array 255))
(def dest-size* (doto (LongByReference.)
                  (.setValue (alength dest))))

(compress  dest dest-size* source (count source)) ;; 0

(.getValue dest-size*) ;; 14

(def dest2 (byte-array (count source)))
(def dest2-size* (doto (LongByReference.)
                   (.setValue (alength dest2))))
(uncompress dest2 dest2-size* dest (.getValue dest-size*)) ;; 0

(String. dest2) ;; "clong!"

The basic idea is to generate a description of the api with easy-api and generate the required structs, functions, and enums with def-api.

Examples

Examples can be found in the examples directory.

• libz
• freetype
• lmdb

Other projects using clong:

• clj-graphviz
• clj-libretro
• glfw
• llama.clj
• clj-media

For a more complicated example, clong's clang interface is generated by clong itself.

Additionally, clong was successfully able to generate a complete wrapper for the libav* libraries, (source).

Tips

• Structs will not have any fields reported if any of the struct's fields has an undetermined size. If fields for a struct are missing, you may need to include more headers or make sure the system's standard paths are included (try clang -### empty-file.h to see what system paths might be missing).
• Clong doesn't offer any special support for automatic memory management of ffi data.
• You can include additional headers using the -include argument when parsing
• Parsing headers can be slow and you probably don't want to ship headers and libclang with your wrapper library. Generate the wrapper ahead of time and save the result as a edn resource that can be loaded. In the future, support for AOT may be added to better support this.
• JNA has issues with writing to StructureByReference fields. In some cases, it's useful to override structure fields to be raw pointers.

Future Work

• Improve documentation.
• Add support for #define values.
• Add support for other ffi libraries besides jna.
• Improve support for references. Currently, there is support for structs by reference, but not for references to structs by reference.
• Implement clojure data interfaces over structs.
• Support AOT of wrappers.

License

Copyright © 2022 Adrian

Distributed under the Eclipse Public License version 1.0.