Question on @Direct struct parameters

[SebRevol]

Hi I'm new using JNR and I have a question regarding the "Direct" annotation on struct parameters.
When I read the comment in the code, I understood that adding this annotation would allow to store the struct in direct memory before passing it to native code, to make it persistent for later access.

But I have made the following experiment :

My testlib.c:

#include <stdint.h>
#include <stdio.h>
#include <stdbool.h>

struct SimpleStruct {
	double doubleMember;
};

//local copy of doubleMember value
double  localDoubleMember;

//local pointer to the struct
struct SimpleStruct * localStruct;

bool compareDoubleMemberAndLocalCopy(){
	return localStruct->doubleMember == localDoubleMember;
}
bool setStruct(struct SimpleStruct *simpleStruct) {
	localStruct = simpleStruct;
	localDoubleMember = simpleStruct->doubleMember;
	return compareDoubleMemberAndLocalCopy();
}
bool setDirectStruct(struct SimpleStruct *simpleStruct){
	 return setStruct(simpleStruct);
}
bool setStructByPointer( struct SimpleStruct *simpleStruct){
	 return setStruct(simpleStruct);
}

Corresponding java lib :

public interface TestLib {	
	
	public class SimpleStruct extends Struct{
		protected SimpleStruct(Runtime runtime) {
			super(runtime);
		}
		public Double doubleMember = new Double();	
	}
	public boolean setStructByPointer(Pointer structPointer);
	public boolean setDirectStruct(@Direct SimpleStruct struct);
	public boolean setStruct(SimpleStruct struct);
	public boolean compareDoubleMemberAndLocalCopy();
	
}

And finally my test case:

public class TestCase {

	public static void main(String[] args) {
		TestLib testLib = LibraryLoader.create(TestLib.class).load(args[0]);
		
		SimpleStruct simpleStruct = new SimpleStruct(Runtime.getSystemRuntime());
		simpleStruct.doubleMember.set(42.0);
		
		boolean areEqual = false;
		
		areEqual = testLib.setStruct(simpleStruct);
		System.out.println("Simple set struct : " + areEqual);
		
		areEqual= testLib.compareDoubleMemberAndLocalCopy();
		System.out.println("After simple set struct : " + areEqual);
		
		areEqual=testLib.setDirectStruct(simpleStruct);
		System.out.println("Direct set direct struct : " + areEqual);
		
		areEqual=testLib.compareDoubleMemberAndLocalCopy();
		System.out.println("After set direct struct : " + areEqual);
		
		//here we explicitly allocate direct memory for the struct
		Pointer structPointer = Memory.allocateDirect(Runtime.getRuntime(testLib), Struct.size(simpleStruct));
		structPointer.putDouble(simpleStruct.doubleMember.offset(),42.0);
		
		areEqual=testLib.setStructByPointer(structPointer);
		System.out.println("Set Pointer: " + areEqual);
		
		areEqual=testLib.compareDoubleMemberAndLocalCopy();
		System.out.println("After Set Pointer : " + areEqual);
	}
}

I have the following output :

Simple set struct : true
After simple set struct : false
Direct set direct struct : true
After set direct struct : false
Set Pointer: true
After Set Pointer : true

We see that simple setStruct and setDirectStruct seem to behave the same, in both case the struct content is lost in native code. It is only preserved if I explicitly allocate Direct memory for the struct (setStructByPointer).

Is it normal? If yes what is finally the effect of the "Direct" annotation?

Thanks in advance,

Best regards,

Sebastien