I've written a usb device driver and a program that sends and receives data over the bulk pipe. The read function sometimes returns -1 when I'm reading an unknown amount of data. However, using a usb tracker I can see that the correct data is being sent.

The error only occurs sometimes. I expect that the read function is told to read more data that it receives it would fail and return -1, however if this was the case then every read call would fail.

Thanks in advance for any help.

Are you trapping the error value (errno) and checking what you're getting?

The read function only returns a -1 if an error occurs, which if you are using non-blocking could include the EAGAIN error meaning no data was available at the time you tried to read

Thanks for the response. The value I get back is errno 5, which is Input/Output error. Also the problem goes away every time I disconnect and reconnect the device.

I also found out that I can make as many read/write calls to the device with out any problems initially. Its when the program closes then opens again that the problem occurs.

I know the data is being sent at the time of the read as I have a usb tracker which is logging every usb packet on a separate machine so I know the data is being sent, the same data each time whether the read command returns an error or not.

do you verify that you are getting a good file descriptor on your open, maybe the device isn't getting released for another open to occur successfully on