GetStringDescriptorASCII broken on Raspbian

[matthewrankin]

GetStringDescriptorASCII() on a Raspberry Pi running Raspbian Jessie Lite doesn't work.

[rojer]

getting the same on Ubuntu 16.04
the root cause seems to be handling of libusb_get_string_descriptor_ascii's return value (here).
looking at C code, it seems that return value in case of error will be negative, just like in the case of get_string_descriptor and should be treated in the same way.

[matthewrankin]

@rojer According to the cgo documentation:

Any C function (even void functions) may be called in a multiple assignment context to retrieve both the return value (if any) and the C errno variable as an error (use _ to skip the result value if the function returns void). For example:

n, err = C.sqrt(-1)
_, err := C.voidFunc()
var n, err = C.sqrt(1)

I need to do a little research and experimentation.

[matthewrankin]

Turns out the current code is reading bytes, but also returning a "resource temporarily unavailable" error. This test was performed on a Raspberry Pi 3 running Raspbian Jessie Lite with an MCC DAQ USB-1608FS-Plus attached.

When I perform the same test on my MacBook Pro with an MCC DAQ USB-1608FS-Plus, the error is nil instead of "resource temporarily unavailable". Not sure why the MBP is giving a different answer than the RPi3.

[matthewrankin]

When reading on the RPi3, when bytesRead = -2 the error is nil, so using two returns in the function ins't working as I had thought.