Re: read /dev/iio:device0 return -1 (Invalid argument)

[Lars-Peter Clausen <lars@metafoo.de>]

On 01/04/2016 12:34 PM, Jonathan Cameron wrote:
> On 04/01/16 04:59, Julio Cruz wrote:
>> Hi Jonathan,
>>
>> Previously, you help me about an issue related with data loss. You suggest
>> me to debug deep in the core elements. I will try to summarize the results
>> below for future reference.
>>
>> When there is not data available in the buffer (kfifo), and the application
>> try to read data (using "read" function), it return zero (0).
>>
>> If libiio will be used to read the data, there is a problem (detailed at
>> https://github.com/analogdevicesinc/libiio/issues/23). In brief, Paul
>> (pcercuei) suggest me that this issue must be manage by the driver, in this
>> case, return -EAGAIN when there is not data available [Resource temporarily
>> unavailable (POSIX.1)].
>>
>> After review the core elements as suggested, I changed the line (in
>> function iio_read_first_n_kfifo of kfifo_buf.c) as below:
>>
>> - return copied;
>> + return copied == 0 ? -EAGAIN: copied;
>>
>> Do you think will be OK like this?
> Hmm.. This is an interesting one (thanks for tracking it down)
> 
> The man page for read indeed allows for this to occur.
> 
>        When attempting to read a file (other than a pipe or  FIFO)  that  sup‐
>        ports non-blocking reads and has no data currently available:
> 
>         *  If  O_NONBLOCK  is  set,  read()  shall  return −1 and set errno to
>            [EAGAIN].
> 
> 
> However the issue here is that this is an ABI change and there may
> unfortunately be code out there relying on it returning 0.

We never propagate 0 to userspace though. The referenced function is
iio_read_first_n_kfifo() which is an internal function. The function that
handles the userspace ABI is iio_buffer_read_first_n_outer() and here, as
Daniel pointed out, there are two things that can happen.

We are in non-blocking mode and iio_read_first_n_kfifo() returns 0. In that
case we'll return -EAGAIN as mandated by the specification.

We are in blocking mode and iio_read_first_n_kfifo() returns 0. In that case
we'll go back to waiting for more data and we'll only return if either data
was received or the application was interrupted by a signal. In the former
case we'll return the number of received bytes in the later case -ERESTARTSYS.

So either way we should never return 0, something else must be going on.


Btw. letting iio_read_first_n_kfifo() return -EAGAIN will break blocking mode.

- Lars