[libusb] libusb Asynchronous Bulk Transfer Timeout On Linux Ubuntu

[Ian M. <ian...@gm...>]

I am using libusb on Ubuntu and am attempting to send 4 bytes of data via
an asynchronous bulk transfer. I am able to successfully perform the
following steps:

Allocate the transfer via libusb_alloc_transfer.
Fill the bulk transfer via libusb_fill_bulk_transfer.
Detach the kernel driver via libusb_detach_kernel_driver.
Claim the interface via libusb_claim_interface.
Submit the transfer via libusb_submit_transfer.
Lastly, reattach the kernal driver via libusb_attach_kernel_driver.
The code will then run until the transfer times out and the callback
function is called. Enabling libusb debug provides the following messages:

[10.033374] [000026fe] libusb: debug [handle_events] timerfd triggered
[10.033375] [000026fe] libusb: debug [libusb_cancel_transfer] transfer
0x55555576de80
[10.036908] [000026fe] libusb: debug [disarm_timerfd]
[10.036919] [000026fe] libusb: debug [handle_events] poll() 3 fds with
timeout in 0ms
[10.036921] [000026fe] libusb: debug [handle_events] poll() returned 1
[10.036924] [000026fe] libusb: debug [reap_for_handle] urb type=3 status=-2
transferred=0
[10.036926] [000026fe] libusb: debug [handle_bulk_completion] handling
completion status -2 of bulk urb 1/1
[10.036927] [000026fe] libusb: debug [handle_bulk_completion] abnormal
reap: urb status -2
[10.036928] [000026fe] libusb: debug [handle_bulk_completion] abnormal
reap: last URB handled, reporting
[10.036930] [000026fe] libusb: debug [usbi_handle_transfer_cancellation]
detected timeout cancellation
[10.036931] [000026fe] libusb: debug [disarm_timerfd]
[10.036932] [000026fe] libusb: debug [usbi_handle_transfer_completion]
transfer 0x55555576de80 has callback 0x7ffff7bd4c78
Questions:

What does a URB status of -2 mean?
Based on your knowledge or experience, what could possibly prevent the
transfer from actually taking place?