epoll — I/O event notification facility
#include <sys/epoll.h>
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).
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:
The file descriptor that represents the read side
of a pipe (rfd) is added inside
the epoll device.
A pipe writer writes 2 kB of data on the write side of the pipe.
A call to epoll_wait(2) is
done that will return rfd as a ready file
descriptor.
The pipe reader reads 1 kB of data from rfd.
A call to epoll_wait(2) is done.
If the rfd
file descriptor has been added to the epoll interface using the EPOLLET (edge-triggered) 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 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:
By contrast, when used as a level-triggered interface
(the default, when EPOLLET is
not specified), 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.
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) == −1) { 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.
Q0What is the key used to distinguish the file
descriptors in an epoll set?
A0The key is the combination of the file descriptor number and the open file description (also known as an "open file handle", the kernel's internal representation of an open file).
Q1What happens if you add the same file descriptor
to an epoll set
twice?
A1You will probably get EEXIST. However, it is possible to
add a duplicate (dup(2), dup2(2), fcntl(2)
F_DUPFD) descriptor to
the same epoll set.
This can be a useful technique for filtering events,
if the duplicate file descriptors are registered with
different events masks.
Q2Can two epoll sets
wait for the same file descriptor? If so, are events
reported to both epoll file descriptors?
A2Yes, and events would be reported to both. However, it is not recommended.
Q3Is the epoll file
descriptor itself poll/epoll/selectable?
A3Yes. If an epoll
file descriptor has events waiting then it will
indicate as being readable.
Q4What happens if the epoll file descriptor is put into
its own file descriptor set?
A4The epoll_ctl(2) call
will fail (EINVAL).
However, you can add an epoll file descriptor inside
another epoll file
descriptor set.
Q5Can I send an epoll file descriptor over a Unix
domain socket to another process?
A5Yes, but it does not make sense to do this, since
the receiving process would not have copies of the
file descriptors in the epoll set.
Q6Will closing a file descriptor cause it to be
removed from all epoll sets automatically?
A6Yes, but be aware of the following point. A file
descriptor is a reference to an open file description
(see open(2)). Whenever
a descriptor is duplicated via dup(2), dup2(2), fcntl(2)
F_DUPFD, or fork(2), a new file
descriptor referring to the same open file
description is created. An open file description
continues to exist until all file descriptors
referring to it have been closed. A file descriptor
is removed from an epoll set only after all the file
descriptors referring to the underlying open file
description have been closed (or after if the
descriptor is explicitly removed using epoll_ctl() EPOLL_CTL_DEL). This means that
even after a file descriptor that is part of an
epoll set has been
closed, events may be reported for that file
descriptor if other file descriptors referring to the
same underlying file description remain open.
Q7If more than one event occurs between epoll_wait(2) calls, are they combined or reported separately?
A7They will be combined.
Q8Does an operation on a file descriptor affect the already collected but not yet reported events?
A8You can do two operations on an existing file descriptor. Remove would be meaningless for this case. Modify will re-read available I/O.
Q9Do I need to continuously read/write a file
descriptor until EAGAIN when using the EPOLLET flag (edge-triggered
behavior) ?
A9Receiving an event from epoll_wait(2) should suggest to you that such file descriptor is ready for the requested I/O operation. You must consider it ready until the next (non-blocking) read/write yields EAGAIN. When and how you will use the file descriptor is entirely up to you.
For packet/token-oriented files (e.g., datagram socket, terminal in canonical mode), the only way to detect the end of the read/write I/O space is to continue to read/write until EAGAIN.
For stream-oriented files (e.g., pipe, FIFO, stream socket), the condition that the read/write I/O space is exhausted can also 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 the file descriptor. The same is true when writing using write(2). (Avoid this latter technique if you cannot guarantee that the monitored file descriptor always refers to a stream-oriented file.)
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.
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.
The epoll API was
introduced in Linux kernel 2.5.44. Its interface should be
finalized in Linux kernel 2.5.66.
The epoll API is
Linux-specific. Some other systems provide similar
mechanisms, for example, FreeBSD has kqueue, and Solaris has
/dev/poll.
This page is part of release 2.79 of the Linux man-pages project. A
description of the project, and information about reporting
bugs, can be found at
http://www.kernel.org/doc/man-pages/.
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epoll by Davide Libenzi ( efficient event notification retrieval ) Copyright (C) 2003 Davide Libenzi This program is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version. This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with this program; if not, write to the Free Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA Davide Libenzi <davidelxmailserver.org> |