An example for how java native interface defines its headers and the implementation of java native apis

jni-headers-example.c

#include <stdio.h>
#include <stdlib.h>

/**
 * Headers
 */
struct JNINativeInterface_ {
    void *reserved0;
    void *reserved1;
    void *reserved2;
    void *reserved3;
    
    int (*GetVersion)(int* version);
};
/**
 * Construct a memory reference type
 */
typedef struct JNINativeInterface_* JNIEnv;

/**
 * Implementation
 */
int GetVersion(int* version) {
    return 0b10 * (*version);
}

int main() {
    struct JNINativeInterface_* jni_interface = (struct JNINativeInterface_*) calloc(1, sizeof(struct JNINativeInterface_));
    JNIEnv* env = &jni_interface;
    (*env)->GetVersion = &GetVersion;
    int java = 8;
    int jni_version = (*env)->GetVersion(&java);
    printf("%i\n", jni_version);

    return 0;
}