How to invoke native C-functions in Clojure via JNR-FFI?

Fri, 04 Dec 2020 13:57:56 +0000

Hi Everyone,

I want to call native functions in a C-Library from Clojure via JNR-FFI without having to write Java code.

(defproject mypkg
	:dependencies [[org.clojure/clojure "1.10.1"]
				   [com.github.jnr/jnr-ffi "2.2.0"]])

Based on the jnr-ffi examples here and here a Java wrapper might look like

package blahblah;

public class BlahNative {
	private static int[] intDummy;
	private static double[] doubleDummy;

	public BlahNative() {
	}

	public static native void native_a(char var0, char var1, int var2, int var3, int var4, double[] var7, int var8, int var9);

	public static native void native_b(char var0, char var1, int var2, int var3, int var4, double[] var7, int var8, int var9);

	static {
		lib = LibraryLoader
            .create(LibSodium.class)
            .search("/usr/local/lib")
            .search("/opt/local/lib")
            .search("/usr/lib")
            .search("/lib")
            .load(LIBRARY_NAME);
        initializeLibrary(lib);
    }
}

I presume my Clojure namespace should use the gen-class feature for implementing Java classes.

(ns mypkg.native
	(:import [jnr.ffi LibraryLoader])
	(:import [jnr.ffi.annotations IgnoreError])
	(:import [jnr.ffi.provider FFIProvider])
    (:gen-class))

Then,

How do I write a native method in Clojure? In other words, do I even need to specify the method type (as native) when using Clojure's Java interop features?
After loading the native library I do not know how to access the C-functions in the library. How do I do it?
To avoid reloading the library for every Clojure wrapper function of each C-functions, can I write a method like

(defn lib (. LibraryLoader load LIBRARY_NAME))

and then access the native functions in it?

I am neither a C nor a Java expert so any guidance will be much appreciated.

Thanks.

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How to invoke native C-functions in Clojure via JNR-FFI?