Passing Pointers as parameters to Native C Functions correctly

[L4m4L]

Hi all,

Firstly, thank-you for this library - it is very helpful and much easier than using Java Native Interface!

I am attempting to write a Java BLE library which calls out to the native C functions in https://github.com/OpenBluetoothToolbox/SimpleBLE. The specific function I am having trouble with is "simpleble_peripheral_write_command" which takes in a simpleble_peripheral_t handle, two simpleble_uuid_t pointers to the service and characteristic uuids, const uint8_t pointer to the data to write, and a size_t data_length.

SIMPLEBLE_EXPORT simpleble_err_t simpleble_peripheral_write_command(simpleble_peripheral_t handle, simpleble_uuid_t service, simpleble_uuid_t characteristic, const uint8_t* data, size_t data_length);

I have implemented this function in my "libsimpleble" public interface:

public class simpleBLE {
	
	// Interface to define what functions we would like to use
	public interface libSimpleBLE {
		// from adapter.h
		// https://github.com/OpenBluetoothToolbox/SimpleBLE/blob/v0.7.1/simpleble/include/simpleble_c/adapter.h
		boolean simpleble_adapter_is_bluetooth_enabled();
		...
		// from peripheral.h
		// https://github.com/OpenBluetoothToolbox/SimpleBLE/blob/v0.7.1/simpleble/include/simpleble_c/peripheral.h
		boolean simpleble_peripheral_write_command(Pointer handle, Pointer service, Pointer characteristic, Pointer data, int length);
	}

and have defined the function further in the file:

public boolean writeCommand(Pointer peripheralHandle, Pointer serviceUuid, Pointer characteristicUuid, Pointer data, int length) 
{
return libsimpleble.simpleble_peripheral_write_command(peripheralHandle, serviceUuid, characteristicUuid, data, length);
}

But the C program is crashing (on a memcpy call) whenever I try to call this function. It is currently working for all functions where I do not need to pass a user-defined pointer, which leads me to believe that there is something wrong with how I am passing my pointer parameters.

// Adding null bytes as Java strings are not null terminated
char[] service = addNullByte("65333333-a115-11e2-9e9a-0800200ca100");
char[] characteristic = addNullByte("65333333-a115-11e2-9e9a-0800200ca102");
		
// Data to send
ByteBuffer buffer = ByteBuffer.allocateDirect(3);
buffer.put(0x8a);
buffer.put(0x03);
buffer.put(0xff);
		
// Get the runtime in order to generate the custom structs
Runtime runtime = simpleBLE.libsimplebleRuntime();
		
simpleble_uuid_t service_uuid = new simpleble_uuid_t(runtime);
service_uuid.set(service);
Pointer service_p = Struct.getMemory(service_uuid);
		
simpleble_uuid_t characteristic_uuid = new simpleble_uuid_t(runtime);
characteristic_uuid.set(characteristic);
Pointer characteristic_p = Struct.getMemory(characteristic_uuid);
		
Pointer data_p = Pointer.wrap(runtime, buffer);
System.out.println("Data Pointer addr: " + data_p.address());
		
// Crashes on this call
simpleBLEwriteCommand(peripheral, service_p, characteristic_p, data_p, 3);
System.out.println("Connected!");

When I print out both the service_p.address() and characteristic_p.address these give me "0" which I think is my problem, however I have no idea how to allocate these structs in a memory address which the native library has access to? I have set the data_p using ByteBuffer.allocateDirect so I am hoping that this is okay.

Wondering if anyone has experience in passing user defined JNI-FFI Structs and pointers to data to native functions with success and could let me know where I am going wrong?

Thanks