NAME
epoll - I/O event notification facility
SYNOPSIS
#include <sys/epoll.h>
DESCRIPTION
epoll is a variant of poll(2) that can be used either as Edge or Level Triggered interface and scales well to
large numbers of watched fds. Three system calls are provided to set up and control an epoll set: epoll_cre-
ate(2), epoll_ctl(2), epoll_wait(2).
An epoll set is connected to a file descriptor created by epoll_create(2). Interest for certain file descrip-
tors is then registered via epoll_ctl(2). Finally, the actual wait is started by epoll_wait(2).
NOTES
The epoll event distribution interface is able to behave both as Edge Triggered ( ET ) and Level Triggered (
LT ). The difference between ET and LT event distribution mechanism can be described as follows. Suppose that
this scenario happens :
1 The file descriptor that represents the read side of a pipe ( RFD ) is added inside the epoll device.
2 Pipe writer writes 2Kb of data on the write side of the pipe.
3 A call to epoll_wait(2) is done that will return RFD as ready file descriptor.
4 The pipe reader reads 1Kb of data from RFD.
5 A call to epoll_wait(2) is done.
If the RFD file descriptor has been added to the epoll interface using the EPOLLET flag, the call to
epoll_wait(2) done in step 5 will probably hang because of the available data still present in the file input
buffers and the remote peer might be expecting a response based on the data it already sent. The reason for
this is that Edge Triggered event distribution delivers events only when events happens on the monitored file.
So, in step 5 the caller might end up waiting for some data that is already present inside the input buffer.
In the above example, an event on RFD will be generated because of the write done in 2 , and the event is con-
sumed in 3. Since the read operation done in 4 does not consume the whole buffer data, the call to
epoll_wait(2) done in step 5 might lock indefinitely. The epoll interface, when used with the EPOLLET flag (
Edge Triggered ) should use non-blocking file descriptors to avoid having a blocking read or write starve the
task that is handling multiple file descriptors. The suggested way to use epoll as an Edge Triggered ( EPOL-
LET ) interface is below, and possible pitfalls to avoid follow.
i with non-blocking file descriptors
ii by going to wait for an event only after read(2) or write(2) return EAGAIN
On the contrary, when used as a Level Triggered interface, epoll is by all means a faster poll(2), and can be
used wherever the latter is used since it shares the same semantics. Since even with the Edge Triggered epoll
multiple events can be generated up on receival of multiple chunks of data, the caller has the option to spec-
ify the EPOLLONESHOT flag, to tell epoll to disable the associated file descriptor after the receival of an
event with epoll_wait(2). When the EPOLLONESHOT flag is specified, it is caller responsibility to rearm the
file descriptor using epoll_ctl(2) with EPOLL_CTL_MOD.
EXAMPLE FOR SUGGESTED USAGE
While the usage of epoll when employed like a Level Triggered interface does have the same semantics of
poll(2), an Edge Triggered usage requires more clarifiction to avoid stalls in the application event loop. In
this example, listener is a non-blocking socket on which listen(2) has been called. The function do_use_fd()
uses the new ready file descriptor until EAGAIN is returned by either read(2) or write(2). An event driven
state machine application should, after having received EAGAIN, record its current state so that at the next
call to do_use_fd() it will continue to read(2) or write(2) from where it stopped before.
struct epoll_event ev, *events;
for(;;) {
nfds = epoll_wait(kdpfd, events, maxevents, -1);
for(n = 0; n < nfds; ++n) {
if(events[n].data.fd == listener) {
client = accept(listener, (struct sockaddr *) &local,
&addrlen);
if(client < 0){
perror("accept");
continue;
}
setnonblocking(client);
ev.events = EPOLLIN | EPOLLET;
ev.data.fd = client;
if (epoll_ctl(kdpfd, EPOLL_CTL_ADD, client, &ev) < 0) {
fprintf(stderr, "epoll set insertion error: fd=%d0,
client);
return -1;
}
}
else
do_use_fd(events[n].data.fd);
}
}
When used as an Edge triggered interface, for performance reasons, it is possible to add the file descriptor
inside the epoll interface ( EPOLL_CTL_ADD ) once by specifying ( EPOLLIN|EPOLLOUT ). This allows you to avoid
continuously switching between EPOLLIN and EPOLLOUT calling epoll_ctl(2) with EPOLL_CTL_MOD.