Read/write to memory that was previously allocated via JNI - java-native-interface

Question

I use macros to allocate memory via JNI and then write to that memory.
#MacroAllocator
case class SomeCaseClass(a: Int, b: Int, c: Int)
3 * INT.BYTES = 12, so it means I need to allocate 12 bytes of memory.
I wrote a buffer that stores offheap objects. and if i already get address of buffer i can reuse that address to write/read.
typedef struct {
uint8_t *buf;
size_t size;
} buf_t;
via jni i'm allocating memory (assume it's at compile time)
JNIEXPORT jlong JNICALL Java_some_package_com_1alloc(JNIEnv *env, jobject self, jint size) {
globalenv = env;
buf_t buf;
buf_alloc(&buf, (size_t) size);
jlong buf_addr = (jlong)buf.buf;
return buf_addr;
}
and then i'm trying to write something to that buf, it's okay too (compile time too)
JNIEXPORT void JNICALL Java_some_package_com_1write_1int(JNIEnv *env, jobject self, jint value, jint idx, jlong buf_addr) {
globalenv = env;
uint8_t *buf = (uint8_t*) buf_addr;
write_int_to_buf_at(buf, idx, value);
return;
}
and when everything is done, i'm trying to read (runtime):
JNIEXPORT jint JNICALL Java_some_package_com_read_1int(JNIEnv *env, jobject self, jint idx, jlong buf_addr) {
globalenv = env;
uint8_t *buf = (uint8_t*)buf_addr;
int32_t value = read_int_in_buf_at(buf, (size_t) idx);
return value;
}
UPD: read_int_in_buf_at func impl
int32_t read_int_in_buf_at(uint8_t *buf, size_t idx) {
uint8_t b3 = buf[idx + 3];
uint8_t b2 = buf[idx + 2];
uint8_t b1 = buf[idx + 1];
uint8_t b0 = buf[idx];
return (b3 << 24) | ((b2 & 255) << 16) |
((b1 & 255) << 8) | (b0 & 255);
}
i get some weird results when trying to read: 0, 1234125360, -1342346458, etc.

Answer 1

Continuing from the comments, you have to keep in mind that Java_some_package_com_1alloc returns the address of allocated memory (buf.buf) NOT the buf_t struct itself.
This means your functions have to change accordingly:
JNIEXPORT void JNICALL Java_some_package_com_1write_1int(JNIEnv *env, jobject self, jint value, jint idx, jlong buf_addr) {
globalenv = env;
// The `jlong` value is the address of allocated memory
uint8_t *buf = (uint8_t*) buf_addr;
write_int_to_buf_at(buf, idx, value);
}
and the reader:
JNIEXPORT jint JNICALL Java_some_package_com_read_1int(JNIEnv *env, jobject self, jint idx, jlong buf_addr) {
globalenv = env;
uint8_t *buf = (uint8_t*) buf_addr;
int32_t value = read_int_in_buf_at(buf, (size_t) idx);
return value;
}