the get_string_descriptor_ascii function documentation is unhelpful

[wchristian]

It indicates a descriptor index parameter and a length parameter, but entirely fails to describe how to choose which descriptor index when i want, for example, the vendor name, nor how to determine the correct length to pass.

Googling was singularly unhelpful as well.

[amba]

Hi wchristian,

USB::LibUSB is essentially a thin wrapper around the libusb C-library and it would not be feasible to duplicate
the documentation of libusb (which is quite extensive...).
The docs for all the USB descriptor functions are here: https://libusb.sourceforge.io/api-1.0/group__libusb__desc.html
Is this helpful for you?

Best,
Simon

[wchristian]

Not even slightly. Can you point out where that explains which index and length to choose to e.g. print out the name of the vendor of a device?

[amba]

I am not even sure that the name of the vendor can be obtained from the USB descriptors in every case.
In the lsusb program they do it with the udev hwdb. From the lsusb.c:

/*
 * Attempt to get friendly vendor and product names from the udev hwdb. If
 * either or both are not present, instead populate those from the device's
 * own string descriptors.
 */
static void get_vendor_product_with_fallback(char *vendor, int vendor_len,
					     char *product, int product_len,
					     libusb_device *dev)
{

[wchristian]

Honestly it seems to me that you're not talking to me under the assumption that i am trying to use bindings for a language mostly used by people operating at a high language level, but at a "knows all the bare metal stuff and every single little linux bit" level.

My /achieve here is: I have a CO2 sensor plugged in. I know it can be interfaced with USB. I don't have a manual. I want to write a script that identifies which plugged in device it is, and then explores its interface to get useful values out.

I'm hearing from you that USB::LibUSB, despite the name, cannot do that?

Edit: Oh yeah, and i'm doing this on Cygwin. I do not know how linux functions.

[amba]

Ok, in that case I would first use lsusb to find out more about the endpoints presented by your CO2 sensor.
It is available on Cygwin: https://cygwin.com/packages/summary/usbutils.html
What do you get from running 'lsusb -v' ?

[wchristian]

Appreciate your willingness to help with this. :)

Here's the outputs i'm getting. The current displayed values are roughly 23.9 °C and 880 ppm CO2.

Mithaldu@XENBOX /cygdrive/c/UDAG/vendorbox
$ lsusb
Bus 001 Device 007: ID 0b05:18f3 ASUSTek Computer, Inc.
Bus 001 Device 009: ID 0b05:1996 ASUSTek Computer, Inc.
Bus 001 Device 005: ID 08bb:2902 Texas Instruments PCM2902 Audio Codec
Bus 001 Device 001: ID 8086:43ed Intel Corp.
Bus 001 Device 008: ID 05e3:0610 Genesys Logic, Inc. 4-port hub
Bus 001 Device 003: ID 8087:0032 Intel Corp.
Bus 001 Device 009: ID 0b05:1996 ASUSTek Computer, Inc.
Bus 001 Device 008: ID 05e3:0610 Genesys Logic, Inc. 4-port hub
Bus 001 Device 020: ID 1532:00a8 Razer USA, Ltd
Bus 001 Device 011: ID 046d:c335 Logitech, Inc.
Bus 001 Device 006: ID 174c:2074 ASMedia Technology Inc.
Bus 001 Device 010: ID 046d:c539 Logitech, Inc.
Bus 001 Device 006: ID 174c:2074 ASMedia Technology Inc.
Bus 001 Device 004: ID 046d:082b Logitech, Inc.
Bus 001 Device 011: ID 046d:c335 Logitech, Inc.
Bus 001 Device 020: ID 1532:00a8 Razer USA, Ltd
Bus 001 Device 021: ID 04d9:a052 Holtek Semiconductor, Inc.
Bus 001 Device 002: ID 174c:3074 ASMedia Technology Inc.
Bus 001 Device 004: ID 046d:082b Logitech, Inc.
Bus 001 Device 010: ID 046d:c539 Logitech, Inc.

Mithaldu@XENBOX /cygdrive/c/UDAG/vendorbox
$ lsusb -s 021
Bus 001 Device 021: ID 04d9:a052 Holtek Semiconductor, Inc.

Mithaldu@XENBOX /cygdrive/c/UDAG/vendorbox
$ lsusb -d 04d9:a052
Bus 001 Device 021: ID 04d9:a052 Holtek Semiconductor, Inc.

Mithaldu@XENBOX /cygdrive/c/UDAG/vendorbox
$ lsusb -d 04d9:a052 -v

Bus 001 Device 021: ID 04d9:a052 Holtek Semiconductor, Inc.
Device Descriptor:
  bLength                18
  bDescriptorType         1
  bcdUSB               1.10
  bDeviceClass            0 (Defined at Interface level)
  bDeviceSubClass         0
  bDeviceProtocol         0
  bMaxPacketSize0         8
  idVendor           0x04d9 Holtek Semiconductor, Inc.
  idProduct          0xa052
  bcdDevice            1.00
  iManufacturer           1 Holtek
  iProduct                2 USB-zyTemp
  iSerial                 3 1.40
  bNumConfigurations      1
  Configuration Descriptor:
    bLength                 9
    bDescriptorType         2
    wTotalLength           34
    bNumInterfaces          1
    bConfigurationValue     1
    iConfiguration          0
    bmAttributes         0x80
      (Bus Powered)
    MaxPower              100mA
    Interface Descriptor:
      bLength                 9
      bDescriptorType         4
      bInterfaceNumber        0
      bAlternateSetting       0
      bNumEndpoints           1
      bInterfaceClass         3 Human Interface Device
      bInterfaceSubClass      0 No Subclass
      bInterfaceProtocol      0 None
      iInterface              0
        HID Device Descriptor:
          bLength                 9
          bDescriptorType        33
          bcdHID               1.10
          bCountryCode            0 Not supported
          bNumDescriptors         1
          bDescriptorType        34 Report
          wDescriptorLength      53
          Warning: incomplete report descriptor
          Report Descriptor: (length is 45)
            Item(Global): Usage Page, data= [ 0xa0 0xff ] 65440
                            (null)
            Item(Local ): Usage, data= [ 0x01 ] 1
                            (null)
            Item(Main  ): Collection, data= [ 0x01 ] 1
                            Application
            Item(Local ): Usage, data= [ 0x01 ] 1
                            (null)
            Item(Global): Logical Minimum, data= [ 0x00 ] 0
            Item(Global): Logical Maximum, data= [ 0xff ] 255
            Item(Global): Report Size, data= [ 0x08 ] 8
            Item(Global): Report Count, data= [ 0x08 ] 8
            Item(Main  ): Input, data= [ 0x00 ] 0
                            Data Array Absolute No_Wrap Linear
                            Preferred_State No_Null_Position Non_Volatile Bitfield
            Item(Local ): Usage, data= [ 0x02 ] 2
                            (null)
            Item(Global): Logical Minimum, data= [ 0x00 ] 0
            Item(Global): Logical Maximum, data= [ 0xff ] 255
            Item(Global): Report Size, data= [ 0x08 ] 8
            Item(Global): Report Count, data= [ 0x08 ] 8
            Item(Main  ): Output, data= [ 0x00 ] 0
                            Data Array Absolute No_Wrap Linear
                            Preferred_State No_Null_Position Non_Volatile Bitfield
            Item(Local ): Usage, data= [ 0x03 ] 3
                            (null)
            Item(Global): Logical Minimum, data= [ 0x00 ] 0
            Item(Global): Logical Maximum, data= [ 0xff ] 255
            Item(Global): Report Size, data= [ 0x08 ] 8
            Item(Global): Report Count, data= [ 0x08 ] 8
            Item(Main  ): Feature, data= [ 0x02 0x01 ] 258
                            Data Variable Absolute No_Wrap Linear
                            Preferred_State No_Null_Position Non_Volatile Buffered Bytes
            Item(Main  ): End Collection, data=none
      Endpoint Descriptor:
        bLength                 7
        bDescriptorType         5
        bEndpointAddress     0x81  EP 1 IN
        bmAttributes            3
          Transfer Type            Interrupt
          Synch Type               None
          Usage Type               Data
        wMaxPacketSize     0x0008  1x 8 bytes
        bInterval              10
Device Status:     0x4f70
  (Bus Powered)
  Debug Mode

[wchristian]

Addendum:

On earlier attempts i was unknowingly using a cable that did not allow data access, which lsusb fairly quickly pointed me at. Maybe it'd be a decent way to catch newbies like me to mention it early on in the docs? :)

Also, now i have the vendor name, i am finding quite a bit of prior art: https://www.google.com/search?q=Holtek+Semiconductor+co2

[amba]

Great, so this means that your sensor operates as a "Human Interface Device".
To get the sensor readout, you could try usbhid-dump. It is contained in the usbutils ( like lsusb), so you should already have it installed:
https://github.com/gregkh/usbutils/blob/master/usbhid-dump/README.md

For example this is what I need to get the x,y coordinates from my mouse:

usbhid-dump --model 045e:0083 --entity=all

Likely the most simple approach would be to capture and parse the output of this utility in your Perl program.

[wchristian]

Makes sense, sadly it doesn't seem to work on cygwin:

Mithaldu@XENBOX /cygdrive/c/Windows/system32
$ usbhid-dump --model 04d9:a052 --entity=all
001:006:000:Failed to detach from the kernel driver: Operation not supported or unimplemented on this platform
001:006:000:Failed to release: Entity not found