Re: AIO support

[Denis Kenzior <denkenz at gmail.com>]

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Hi Simon,

>> 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."
> 

Ah, indeed.  I think I even knew that, sorry for the noise.

> 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.

Right, epoll will just simply return immediately on regular files.  Have you 
tried using a timer or l_idle to drive the reading?  Is there ioctl(fd, 
FIONREAD, ...) or equivalent that could be used not to block?

Regards,
-Denis