Trivial Janet project with a bundled native C module

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Blog 2020/7/29

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Trivial Janet project with a bundled native C module

This is the simplest example of how to write a Janet project which is a mixture of C and Janet, which results in a single, statically-linked executable.

Run make to build and test the executable build/baz.

Details

foo.c is a Janet native C module which implements a namespace foo, which contains one function bar, which returns the number 42.

To build and test this module, run make test-foo. You can also start a interactive REPL with this module imported via make repl-foo (type (foo/bar) at the prompt).

baz.janet is a Janet program which imports the foo module, then prints foo/bar multiplied by 2.

To build and test the build/baz statically-linked executable, run make test-baz, or simply make.

baz.janet

(import build/foo :as foo)

(defn main [& args]
  (print (* 2 (foo/bar))))

foo.c

// Trivial Janet native module which contains one function which returns 42.

#include <janet.h>

static Janet cfun_foo_bar(int32_t argc, Janet *argv) {
    return janet_wrap_number(42);
}

static const JanetReg cfuns[] = {
    {
        "bar", cfun_foo_bar,
        "(foo/bar)\n\nReturns 42."
    },
    {NULL, NULL, NULL}
};

JANET_MODULE_ENTRY(JanetTable *env) {
    janet_cfuns(env, "foo", cfuns);
}

Makefile

default: test-foo test-baz

build/foo.a: foo.c
	jpm --verbose build

test-foo: build/foo.a
	test 42 = $(shell janet -e '(import build/foo :as foo) (print (foo/bar))')

repl-foo: build/foo.a
	janet -e '(import build/foo :as foo)' -r

build/baz: build/foo.a baz.janet
	jpm --verbose build

test-baz: build/baz
	test 84 = $(shell ./build/baz)

clean:
	rm -rf build

.PHONY: default test-foo repl-foo test-baz clean

project.janet

(declare-project
  :name "baz")

(declare-native 
  :name "foo"
  :source @["foo.c"]
  :cflags [;default-cflags "-Wno-unused-parameter"])

(declare-executable
  :name "baz"
  :entry "baz.janet")

This is really helpful. Thanks for sharing it!