How to pass a member function pointer to libusb? - c++

Question

I'm writing a simple library that uses libusb to connect to my custom hardware that sends signals to a host device every 50ms. It's designed to provide a simple abstraction layer so that users are not bothered with libusb at all. I need to pass a pointer to a non-static class member function to libusb_fill_bulk_transfer. I.e. I need to have a separate callback for every instance of MyDevice without exposing libusb logic to my users.
The basic design is:
init the lib
int mylib_init(){
libusb_init(&context);
}
find all compatible devices
int mylib_get_valid_devices(vector devs){
//...
libusb_device **devs1;
int countAll = libusb_get_device_list(context, &devs1);
//... fill devs
}
int mylib_print_device_info(MyDevice* dev);
connect to as many devices as the user likes
int mylib_init_device(MyDevice* dev){
libusb_open(dev->device, &dev->dev_handle);
// check for kernel driver & remove if needed
libusb_claim_interface(dev->dev_handle, 0);
//...
}
set up callbacks for incoming data to all instances of connected devices
int mylib_start_transmission_async(MyDevice* dev, MyLibCallbackIn user_cb, unsigned char* buffer_in, int bufferSize){
libusb_control_transfer(dev->dev_handle, p1, p2, p3, p4, p5, p6, p7);
// .. rest of libusb_control_transfer -s
//start device:
int actual;
unsigned char startSig = 0b00110111;
libusb_bulk_transfer(dev->dev_handle, (1 | LIBUSB_ENDPOINT_OUT), &tosend, 1, &actual, 100);
dev->user_callback = user_cb;
dev->buffer_in = buffer_in;
dev->bufferSize = bufferSize;
//setup transfer
dev->transfer_in = libusb_alloc_transfer(0);
libusb_fill_bulk_transfer(dev->transfer_in, dev->dev_handle, LIBUSB_ENDPOINT_IN, dev->buffer_in,dev->bufferSize, cb_in, NULL, 0);
libusb_submit_transfer(dev->transfer_in);
}
disconnect when finished
int mylib_disconnect_device(MyDevice* device);
exit the lib
int mylib_exit();
MyDevice is defined as:
typedef int (*MyLibdCallbackIn)(unsigned char* buffer, int length);
class MyDevice{
unsigned char* buffer_in;
unsigned char* buffer_out;
libusb_device* device = nullptr;
libusb_device_handle* dev_handle = nullptr;
struct libusb_transfer* transfer_in = nullptr;
struct libusb_transfer* transfer_out = nullptr;
//this obviously wouldn't work because libusb doesn't accept it as a param:
void LIBUSB_CALL cb_in(struct libusb_transfer* transfer);
MyLibCallbackIn user_callback;
void run();
//...
}
and cb_in is defined as:
void LIBUSB_CALL MyDevice::cb_in(libusb_transfer* transfer){
int r = libusb_submit_transfer(this->transfer_in);
callback_in(transfer->buffer, transfered);
}
I can't pass MyDevice::cb_in to libusb_fill_bulk_transfer because the function pointer types are incompatible. In the same time I don't want my users to have to write a callback function with libusb_transfer* as a parameter (exposing to libusb) to pass directly to libusb_fill_bulk_transfer.
EDIT:
I tried with
void LIBUSB_CALL callback_wrapper(struct libusb_transfer* transfer){
MyDevice* dev = reinterpret_cast<MyDevice*>(transfer->user_data);
dev->cb_in(transfer);
}
but get Sementation fault (Core dumped) error

Answer 1

As per Max Langhof and Sam Varshavchik's comments the solution is to pass the instance of MyDevice to libusb_fill_bulk_transfer . So:
int mylib_start_transmission_async(MyDevice* dev, MyLibCallbackIn user_cb, unsigned char* buffer_in, int bufferSize){
//...
// setup transfer:
dev->transfer_in = libusb_alloc_transfer(0);
libusb_fill_bulk_transfer(dev->transfer_in, dev->dev_handle, LIBUSB_ENDPOINT_IN, dev->buffer_in,dev->bufferSize, cb_in, dev, 0);
libusb_submit_transfer(dev->transfer_in); ^^^
}
and then use the callback_wrapper as in the Edit. Thanks!