EPOLL(7)

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 an edge-triggered or a level-triggered interface and scales well to large numbers of watched file descriptors. Three system calls are provided to set up and control an epoll set: epoll_create(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 descriptors is then registered via epoll_ctl(2). Finally, the actual wait is started by epoll_wait(2).

  Level-Triggered and Edge-Triggered
The epoll event distribution interface is able to behave both as edge-triggered (ET) and level-triggered (LT). The difference between the two mechanisms 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.  A pipe writer writes 2 kB of data on the write side of the pipe.

3.  A call to epoll_wait(2) is done that will return rfd as a ready file descriptor.

4.  The pipe reader reads 1 kB 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 despite the available data still present in the file input buffer; meanwhile the remote peer might be expecting a response based on the data it already sent. The reason for this is that edge-triggered mode only delivers events when changes occur on the monitored file descriptor. 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 consumed 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 block indefinitely. An application that employs the EPOLLET flag (edge-triggered) should use non-blocking file descriptors to avoid having a blocking read or write starve a task that is handling multiple file descriptors. The suggested way to use epoll as an edge-triggered (EPOLLET) interface is as follows:

    i   with non-blocking file descriptors

    ii  by waiting for an event only after read(2) or write(2) return EAGAIN.

By contrast, when used as a level-triggered interface, epoll is simply 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 upon receipt of multiple chunks of data, the caller has the option to specify the EPOLLONESHOT flag, to tell epoll to disable the associated file descriptor after the receipt of an event with epoll_wait(2). When the EPOLLONESHOT flag is specified, it is the caller's 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 as a level-triggered interface does have the same semantics as poll(2), the edge-triggered usage requires more clarification 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=%d\n",
                        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.

  Questions and Answers

Q1  What happens if you add the same file descriptor to an epoll set twice?

A1  You will probably get EEXIST. However, it is possible that two threads may add the same file descriptor twice. This is a harmless condition.

Q2  Can two epoll sets wait for the same file descriptor? If so, are events reported to both epoll file descriptors?

A2  Yes, and events would be reported to both. However, it is not recommended.

Q3  Is the epoll file descriptor itself poll/epoll/selectable?

A3  Yes.

Q4  What happens if the epoll file descriptor is put into its own file descriptor set?

A4  It will fail. However, you can add an epoll file descriptor inside another epoll file descriptor set.

Q5  Can I send the epoll file descriptor over a unix-socket to another process?

A5  No.

Q6  Will closing a file descriptor cause it to be removed from all epoll sets automatically?

A6  Yes.

Q7  If more than one event occurs between epoll_wait(2) calls, are they combined or reported separately?

A7  They will be combined.

Q8  Does an operation on a file descriptor affect the already collected but not yet reported events?

A8  You can do two operations on an existing file descriptor. Remove would be meaningless for this case. Modify will re-read available I/O.

Q9  Do I need to continuously read/write a file descriptor until EAGAIN when using the EPOLLET flag (edge-triggered behavior) ?

A9  No you don't. Receiving an event from epoll_wait(2) should suggest to you that such file descriptor is ready for the requested I/O operation. You have simply to consider it ready until you will receive the next EAGAIN. When and how you will use such file descriptor is entirely up to you. Also, the condition that the read/write I/O space is exhausted can be detected by checking the amount of data read from / written to the target file descriptor. For example, if you call read(2) by asking to read a certain amount of data and read(2) returns a lower number of bytes, you can be sure of having exhausted the read I/O space for such file descriptor. The same is true when writing using the write(2).

  Possible Pitfalls and Ways to Avoid Them

o Starvation (edge-triggered)

If there is a large amount of I/O space, it is possible that by trying to drain it the other files will not get processed causing starvation. (This problem is not specific to epoll.)

The solution is to maintain a ready list and mark the file descriptor as ready in its associated data structure, thereby allowing the application to remember which files need to be processed but still round robin amongst all the ready files. This also supports ignoring subsequent events you receive for file descriptors that are already ready.

o If using an event cache...

If you use an event cache or store all the file descriptors returned from epoll_wait(2), then make sure to provide a way to mark its closure dynamically (i.e., caused by a previous event's processing). Suppose you receive 100 events from epoll_wait(2), and in event #47 a condition causes event #13 to be closed. If you remove the structure and close(2) the file descriptor for event #13, then your event cache might still say there are events waiting for that file descriptor causing confusion.

One solution for this is to call, during the processing of event 47, epoll_ctl(EPOLL_CTL_DEL) to delete file descriptor 13 and close(2), then mark its associated data structure as removed and link it to a cleanup list. If you find another event for file descriptor 13 in your batch processing, you will discover the file descriptor had been previously removed and there will be no confusion.

VERSIONS
epoll(7) is a new API introduced in Linux kernel 2.5.44. Its interface should be finalized in Linux kernel 2.5.66.

CONFORMING TO
The epoll API is Linux specific. Some other systems provide similar mechanisms, for example, FreeBSD has kqueue, and Solaris has /dev/poll.

SEE ALSO
epoll_create(2), epoll_ctl(2), epoll_wait(2)