libusb_init() fails with LIBUSB_ERROR_IO on Android if there are no devices attached

[matswebjorn]

I'm using libusb 1.0.21 with a Samsung SM-T580 (Android 6.0.1 API23) rooted tablet. If I have at least one external device attached to the bus then libusb_init()returns ok, but if there aren't any external devices attached, then libusb_init() fails with LIBUSB_ERROR_IO.

The call chain is

libusb_init: usbi_backend->init()
    op_init:linux_scan_devices()
        linux_scan_devices:linux_default_scan_devices()
            linux_default_scan_devices:sysfs_get_device_list()
                sysfs_get_device_list()

The reason is that when there aren't any devices attached, then readdir(devices) in sysfs_get_device_list() just returns two entries "." and ".." and this is considered an IO-error.

But if I attach a single device then readdir(devices) returns ".", "..", "1-1", "usb1", "usb2", "1-0:1.0", "1-1:1.0" and "2-0:1.0". This is an OTG device so it might explain why no devices (even no root hub) are reported when no external devices are attached.

But shouldn't it be allowed to initialize libusb even if there are no devices on the bus, or if this is an OTG device?