I want to convert the libusb.h into PureBasic code and now I have this line:

```
typedef void (LIBUSB_CALL *libusb_transfer_cb_fn)(struct libusb_transfer *transfer);

```

What does this typedef do?

Later on there are lines like this:

```
void LIBUSB_CALL libusb_free_device_list(libusb_device **list, int unref_devices);

```

Thanks in advance!