Re: [Libusb-devel] libusb_get_device_list Question

[<Phi...@mi...>]

Thanks.

________________________________

From: Nathan Hjelm [mailto:hj...@ma...] 
Sent: Friday, April 30, 2010 1:51 PM
To: Philip Joslin - C10554
Cc: lib...@li...
Subject: Re: [Libusb-devel] libusb_get_device_list Question


This is a known bug in all releases. The bug is fixed in the current git
head. 

-Nathan

On Apr 30, 2010, at 2:42 PM, Phi...@mi... wrote:


	I am running into a problem with libusb_get_device_list on the
Mac (OSX version 10.6 and using libusb-1.0.3).
	 
	Let me give an example so that my steps are clear. Let me start
with Linux, since my process works just fine in Linux.
	 
	On Linux, to get a device list, I first call libusb_initialize
to get a specific context, then I call libusb_get_device_list with that
context. Let's say I get back the following single device in the list:
	 
	vid=0xABCD
	pid=0x1234
	product_string="The Product AAAA"
	 
	Suffice it to say that I can get notifications from the system
for hotplug purposes, so I know when a device from vendor ABCD is
plugged in or is unplugged.
	 
	So if I unplug Product AAAA and now plug in Product BBBB, I get
notified and I call libusb_get_device_list again using the same context
I passed into the initial call to libusb_get_device_list. The info that
comes back is now:
	 
	
	vid=0xABCD
	pid=0x5678
	product_string="The Product BBBB"
	 
	which is correct.
	 
	However, if I do this same process on the Mac, initially I get:
	 
	
	vid=0xABCD
	pid=0x1234
	product_string="The Product AAAA"
	 
	which I should, but after unplugging AAAA and plugging in BBBB,
I get:
	 
	
	vid=0xABCD
	pid=0x1234
	product_string="The Product BBBB"
	 
	The product ID remains the same, regardless of what product I
plug in. Yet the product string is correct (as is the serial number).
	 
	Both devices are bulk devices.
	 
	If on my second call to libusb_get_device_list, I first call
libusb_initialize, getting a different context, and pass that new
context, then I get the correct response. I could conceivably throw away
my initial list and go with the second, but the problem with that is
that I am keeping an internal copy of the device handles around and may
have active processes from the user being done on devices that were not
unplugged or plugged. I can't simply throw them away because that might
disrupt the user.
	 
	I could start keeping a separate context for each device instead
of for a device list, but that would awkward, and I am not sure how
calling libusb_initialize over and over again might also screw things
up.
	 
	I am now looking at the libusb/Darwin backend code to see what I
might be able to change to correct this problem. Any guidance would be
greatly appreciated.
	 
	Thanks.
	 
	-- Phil
	 
	
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