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JNI - return struct from C

Ben Snyder

Joined: Jul 02, 2002
Posts: 6
Hi folks

I'm having some trouble returning a C struct to Java using JNI. Currently I'm trying to use the javolution Struct library, though the documentation seems to be lacking. I have a small struct now, but will eventually have a
struct which contains pointers to other structs, char arrays, etc.

Here's what I have:

C Code:
typedef struct {
long count;
} Results;

JNIEXPORT jobject JNICALL Java_JTest_CTest (JNIEnv *env, jobject job) {
Results results;
results.count = 42;
printf("%li\n", results.count);
return (*env)->NewDirectByteBuffer(env, &results, sizeof(Results));

Java code:
class TestStruct extends Struct {
Signed32 count = new Signed32();

class JTest {
private native TestStruct CTest();
public static void main (String args[]) {

JTest jt = new JTest();

TestStruct ts = jt.CTest();

static {

Can someone either point me in the right direction, tell me what I'm doing wrong, or maybe there is something completely different I should be doing?

Ernest Friedman-Hill
author and iconoclast

Joined: Jul 08, 2003
Posts: 24199

I can't critique the whole program, as I've never used this "javalution struct library" of which you speak, but I can tell you one bad, bad thing about your native code: the argument to NewDirectByteBuffer is supposed to be a pointer to a block of memory that will be staying put; but you're passing the address of a struct on the stack, which will turn into garbage as soon as the native function returns. You need to dynamically allocate (malloc, or new) your struct, or allocate it from a static array of structs, or something -- but you can't return stack-based objects from your JNI functions any more than you can return them within normal C(++) code.

[Jess in Action][AskingGoodQuestions]
I agree. Here's the link:
subject: JNI - return struct from C
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