I have a driver project where I'm opening and closing a connection to a custom driver. If I do what I think I need to be doing to unmap the memory, when I try to open the service again, it fails. If I skip the step where I do that unmapping, the service opens successfully. If I call unmap() before trying to call openConnection() again, it will fail with a -308 error return code. If I skip that call to unmap(), it works and I'm able to communicate with my device. Here's the code where I open the service: public func openConnection() throws { guard !isOpen else { return } // Open device var connection: io_connect_t = IO_OBJECT_NULL var result = IOServiceOpen(device, mach_task_self_, 0, &connection) if result != kIOReturnSuccess { NSLog("Failed opening device with error: 0x%08x.\n", result); throw NSError.cdc_kernelReturnErrorWithError(result) } defer { IOConnectRelease(connection) } if device == IO_OBJECT_NULL || connection == IO_OBJECT_NULL { throw NSError.cdc_kernelReturnErrorWithError(result) } let receiveDataMappedMemory = ClientDriverMappedMemory(connection: connection, memoryType: MappedMemoryType_ReceiveDataBuffer) try receiveDataMappedMemory.map() let transmitDataMappedMemory = ClientDriverMappedMemory(connection: connection, memoryType: MappedMemoryType_TransmitDataBuffer) try transmitDataMappedMemory.map() // Setup async notification IONotificationPortSetDispatchQueue(dataReceivedPort, dataReceivedQueue) let callbackPort = IONotificationPortGetMachPort(dataReceivedPort) let input = DataStruct(foo: 0, bar: 0) var output = DataStruct(foo: 0, bar: 0) var outputSize = MemoryLayout<DataStruct>.size // Trampoline to C function because I don't quite know how to make this work in Swift result = setupCallback(self, connection, callbackPort, input, &output, &outputSize) if result != kIOReturnSuccess { NSLog("Error registering async callback with driver: \(result)"); throw NSError.cdc_kernelReturnErrorWithError(result) } self.connection = connection self.receivedDataMappedMemory = receiveDataMappedMemory self.transmitDataMappedMemory = transmitDataMappedMemory } map() and unmap() functions: - (BOOL)mapWithError:(NSError **)error { error = error ?: &(NSError * __autoreleasing){ nil }; kern_return_t result = IOConnectMapMemory64(self.connection, self.memoryType, mach_task_self(), &_address, &_size, kIOMapAnywhere); if (result != kIOReturnSuccess) { *error = [NSError cdc_kernelReturnErrorWithError:result]; return NO; } self.mapped = YES; return YES; } - (BOOL)unmapWithError:(NSError **)error { error = error ?: &(NSError * __autoreleasing){ nil }; kern_return_t result = IOConnectUnmapMemory64(self.connection, self.memoryType, mach_task_self(), _address); if (result != kIOReturnSuccess) { *error = [NSError cdc_kernelReturnErrorWithError:result]; return NO; } self.mapped = NO; return YES; } Any insights? What all should I be doing to close the service? Why would the unmapping create this issue or what else could the -308 error be indicated has gone wrong?