Re: AIO support

Hi Denis,


On 9/19/21 6:00 PM, Denis Kenzior wrote:
> Hi Simon,
> 
> On 9/19/21 12:12 PM, Simon Maurer wrote:
>> Hi
>> Awesome library! Thank you very much. I'm using it for an embedded linux 
> 
> Glad you like it!
> 
>> application with not so strict real time constraints. Problem is, I 
>> need non blocking file access, because the memory card on my system is 
>> quite slow. It turns out that this whole non-blocking access doesn't 
>> work for files, so now I'm 
> 
> Can you elaborate?  I always thought
> fcntl(fd, F_SETFL, O_NONBLOCK);
> 
> would enable non-blocking behavior on files just fine?  Or is this an 
> issue with a particular driver?
> 

I tested it and never got EAGAIN. I found this in the man page of open 
for the flag O_NONBLOCK or O_NDELAY:

"Note  that this flag has no effect for regular files and block devices; 
that is, I/O operations will (briefly) block when device activity is 
required, regardless of whether O_NONBLOCK is set.  Since O_NONBLOCK 
semantics might eventually be implemented, applications should not 
depend  upon  blocking  behavior  when specifying this flag for regular 
files and block devices."

An alternative could be preadv2 with the flag RWF_NOWAIT. But it is only 
available since linux 4.14 and we are using an older kernel. Also epoll 
doesn't wait for preadv2.


>> using the POSIX AIO interface and I've integrated it into ELL:
>> https://github.com/mausys/ell/blob/master/ell/aio.c
>> (But needs to be linked against rt)
>> This is just a draft and it also works on top of ELL, but maybe other 
>> applications need this too.
>>
> 
> Thanks, I'll take a peek later during the week.
> 
> Regards,
> -Denis

Best regards,
Simon