EPoll issue

[Andreas Roth]

Hi,

i wrote a little program an found a strange effect in the epoll
implementation. The following program simple tests these epoll api. It
create a socket and a epoll instance. If epoll_wait waits for the socket
and only EPOLLOUT is requested, epoll_wait returns and have the EPOLLHUP
flag set in the returned event structure.
I think if i request only the EPOLLOUT event epoll_wait only give me all
EPOLLOUT event and NOT the EPOLLHUP events.

My system:
SuSE Linux 9.0
Kernel 2.6.4 (self compiled)

Does anybody have the same issue?

Thanks in advance,
A. Roth

#include <stdio.h>
#include <sys/epoll.h>
#include <sys/socket.h>

int main(int argc,char ** argv)
{
int sock;
int epfd;
int i;
int res;
struct epoll_event events[8];
struct epoll_event ev;

sock = socket(AF_INET,SOCK_STREAM,0);
printf("Socket descriptor %i\n",sock);

epfd = epoll_create(8);
printf("Epoll descriptor %i\n",epfd);

ev.data.u32 = 1;
ev.events = EPOLLOUT;
res = epoll_ctl(epfd,EPOLL_CTL_ADD,sock,&ev);
printf("epoll_ctl returned %i\n",res);

for(i = 0; i < 5; i++)
{
res = epoll_wait(epfd,events,8,-1);
printf("epoll_wait returned %i\n",res);
for(;res>0;res--)
{
printf("event %X id %i\n",events[res-1].events,events[res-1].data.u32);
}

}

close(epfd);
close(sock);
}

[Michael Kerrisk]

On Sun, 14 Mar 2004 16:57:51 +0100, Andreas Roth
<ar...@arsoft-online.com> wrote:

>Hi,
>
>i wrote a little program an found a strange effect in the epoll
>implementation. The following program simple tests these epoll api. It
>create a socket and a epoll instance. If epoll_wait waits for the socket
>and only EPOLLOUT is requested, epoll_wait returns and have the EPOLLHUP
>flag set in the returned event structure.
>I think if i request only the EPOLLOUT event epoll_wait only give me all
>EPOLLOUT event and NOT the EPOLLHUP events.
>
>My system:
>SuSE Linux 9.0
>Kernel 2.6.4 (self compiled)
>
>Does anybody have the same issue?

This is expected behaviour. Precisely the same occurs with poll().
EPOLLHUP and EPOLLERR are only ever set in the returned events mask --
you don't set them in the epoll_ctl() call(s). EPOLLHUP means the
other end of a communication was closed. In this particular case, it
is returned for an unconnected stream socket.

Cheers,

Michael

[Andreas Roth]

If this is the 'by design' behaviour it's ok. But how can i wait on a
socket until the socket has connected? If i can epoll_wait i always
returns EPOLLHUP until the socket is connected (so i get a busy loop).
Is there any possiblity to wait for a connect without epoll?

Andreas

[Frederik Hertzum]

Have you considered having the socket reading/write procedures running in a
thread out side the rest of the programme?

[David Schwartz]

"Andreas Roth" <ar...@arsoft-online.com> wrote in message
news:pan.2004.03.15....@arsoft-online.com...

> If this is the 'by design' behaviour it's ok. But how can i wait on a
> socket until the socket has connected? If i can epoll_wait i always
> returns EPOLLHUP until the socket is connected (so i get a busy loop).
> Is there any possiblity to wait for a connect without epoll?

To wait until a socket has connected, wait only for writability.

DS

[Michael Kerrisk]

On Mon, 15 Mar 2004 16:40:20 +0100, Andreas Roth
<ar...@arsoft-online.com> wrote:

This is a somewhat different question than you origianlly asked...
Note the following:

If you do a normal connect(), then it blocks until the connection has
been completed. There's thus no need for poll()/epoll()/select().

If you make the socket non-blocking before you do the connect() (you
don't say this, but I suspect it's the scenario you are thinking of),
then

a) the three-way handshake (assuming TCP) will be initiated.

b) connect() will return immediately with the error EINPROGRESS (may
vary by platform).

c) a later select()/poll()/epoll() with a non-zero timeout will block
until the connection completes, and then indicate the socket as being
writable (writefds / POLLOUT / EPOLLOUT).

Cheers,

Michael