POLLRDHUP inconsistency between poll() and epoll

POLLRDHUP inconsistency between poll() and epoll

[Michael Kerrisk]

Davide,

While playing about with the new POLLRDHUP flag, I've noticed
an inconsistency, which may or may not be intentional...

When a POLLRDHUP condition occurs, epoll_wait() tells us about 
the condition, regardless of whether or not we specified 
(E)POLLRDHUPP in the 'events' flag given to epoll_ctl() 
EPOLL_CTL_ADD.  In this respect, POLLRDHUP is treated just like
POLLHUP and POLLERR.  This seems perfectly reasonable.

By contrast, poll() will only tell us that POLLRDHUP occurred
if we specified POLLRDHUP in the file descriptor 'events' mask 
given to the poll() call.  In other words, poll() treats 
POLLRDHUP differently from POLLHUP and POLLERR.  This seems
a little strange.

Is this difference really intended?  If it is, what is the 
reason for the difference?

Cheers,

Michael

-- 
Michael Kerrisk
maintainer of Linux man pages Sections 2, 3, 4, 5, and 7 

Want to help with man page maintenance?  
Grab the latest tarball at
ftp://ftp.win.tue.nl/pub/linux-local/manpages/, 
read the HOWTOHELP file and grep the source 
files for 'FIXME'.

Re: POLLRDHUP inconsistency between poll() and epoll

[Davide Libenzi]

On Tue, 4 Apr 2006, Michael Kerrisk wrote:

> Davide,
>
> While playing about with the new POLLRDHUP flag, I've noticed
> an inconsistency, which may or may not be intentional...
>
> When a POLLRDHUP condition occurs, epoll_wait() tells us about
> the condition, regardless of whether or not we specified
> (E)POLLRDHUPP in the 'events' flag given to epoll_ctl()
> EPOLL_CTL_ADD.  In this respect, POLLRDHUP is treated just like
> POLLHUP and POLLERR.  This seems perfectly reasonable.
>
> By contrast, poll() will only tell us that POLLRDHUP occurred
> if we specified POLLRDHUP in the file descriptor 'events' mask
> given to the poll() call.  In other words, poll() treats
> POLLRDHUP differently from POLLHUP and POLLERR.  This seems
> a little strange.
>
> Is this difference really intended?  If it is, what is the
> reason for the difference?

No. It is definitely better to keep behaviour consistent with poll/select. 
I'll submit a patch ASAP. Thank you spotting this out!



- Davide