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NAME
epoll - I/O event notification facility
SYNOPSIS
#include <sys/epoll.h>
DESCRIPTION
The epoll API performs a similar task to poll(2): monitoring multiple file descriptors to
see if I/O is possible on any of them. The epoll API can be used either as an edge-
triggered or a level-triggered interface and scales well to large numbers of watched file
descriptors.
The central concept of the epoll API is the epoll instance, an in-kernel data structure
which, from a user-space perspective, can be considered as a container for two lists:
• The interest list (sometimes also called the epoll set): the set of file descriptors
that the process has registered an interest in monitoring.
• The ready list: the set of file descriptors that are "ready" for I/O. The ready list is
a subset of (or, more precisely, a set of references to) the file descriptors in the
interest list. The ready list is dynamically populated by the kernel as a result of I/O
activity on those file descriptors.
The following system calls are provided to create and manage an epoll instance:
• epoll_create(2) creates a new epoll instance and returns a file descriptor referring to
that instance. (The more recent epoll_create1(2) extends the functionality of
epoll_create(2).)
• Interest in particular file descriptors is then registered via epoll_ctl(2), which adds
items to the interest list of the epoll instance.
• epoll_wait(2) waits for I/O events, blocking the calling thread if no events are
currently available. (This system call can be thought of as fetching items from the
ready list of the epoll instance.)
Level-triggered and edge-triggered
The epoll event distribution interface is able to behave both as edge-triggered (ET) and
as 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 registered on the
epoll instance.
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 (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 delivers events only 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 nonblocking 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:
a) with nonblocking file descriptors; and
b) by waiting for an event only after read(2) or write(2) return EAGAIN.
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 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.
If multiple threads (or processes, if child processes have inherited the epoll file
descriptor across fork(2)) are blocked in epoll_wait(2) waiting on the same epoll file
descriptor and a file descriptor in the interest list that is marked for edge-triggered
(EPOLLET) notification becomes ready, just one of the threads (or processes) is awoken
from epoll_wait(2). This provides a useful optimization for avoiding "thundering herd"
wake-ups in some scenarios.
Interaction with autosleep
If the system is in autosleep mode via /sys/power/autosleep and an event happens which
wakes the device from sleep, the device driver will keep the device awake only until that
event is queued. To keep the device awake until the event has been processed, it is
necessary to use the epoll_ctl(2) EPOLLWAKEUP flag.
When the EPOLLWAKEUP flag is set in the events field for a struct epoll_event, the system
will be kept awake from the moment the event is queued, through the epoll_wait(2) call
which returns the event until the subsequent epoll_wait(2) call. If the event should keep
the system awake beyond that time, then a separate wake_lock should be taken before the
second epoll_wait(2) call.
/proc interfaces
The following interfaces can be used to limit the amount of kernel memory consumed by
epoll:
/proc/sys/fs/epoll/max_user_watches (since Linux 2.6.28)
This specifies a limit on the total number of file descriptors that a user can
register across all epoll instances on the system. The limit is per real user ID.
Each registered file descriptor costs roughly 90 bytes on a 32-bit kernel, and
roughly 160 bytes on a 64-bit kernel. Currently, the default value for
max_user_watches is 1/25 (4%) of the available low memory, divided by the
registration cost in bytes.
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 nonblocking 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.
#define MAX_EVENTS 10
struct epoll_event ev, events[MAX_EVENTS];
int listen_sock, conn_sock, nfds, epollfd;
/* Code to set up listening socket, 'listen_sock',
(socket(), bind(), listen()) omitted */
epollfd = epoll_create1(0);
if (epollfd == -1) {
perror("epoll_create1");
exit(EXIT_FAILURE);
}
ev.events = EPOLLIN;
ev.data.fd = listen_sock;
if (epoll_ctl(epollfd, EPOLL_CTL_ADD, listen_sock, &ev) == -1) {
perror("epoll_ctl: listen_sock");
exit(EXIT_FAILURE);
}
for (;;) {
nfds = epoll_wait(epollfd, events, MAX_EVENTS, -1);
if (nfds == -1) {
perror("epoll_wait");
exit(EXIT_FAILURE);
}
for (n = 0; n < nfds; ++n) {
if (events[n].data.fd == listen_sock) {
conn_sock = accept(listen_sock,
(struct sockaddr *) &addr, &addrlen);
if (conn_sock == -1) {
perror("accept");
exit(EXIT_FAILURE);
}
setnonblocking(conn_sock);
ev.events = EPOLLIN | EPOLLET;
ev.data.fd = conn_sock;
if (epoll_ctl(epollfd, EPOLL_CTL_ADD, conn_sock,
&ev) == -1) {
perror("epoll_ctl: conn_sock");
exit(EXIT_FAILURE);
}
} 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
0. What is the key used to distinguish the file descriptors registered in an interest
list?
The 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).
1. What happens if you register the same file descriptor on an epoll instance twice?
You will probably get EEXIST. However, it is possible to add a duplicate (dup(2),
dup2(2), fcntl(2) F_DUPFD) file descriptor to the same epoll instance. This can be a
useful technique for filtering events, if the duplicate file descriptors are
registered with different events masks.
2. Can two epoll instances wait for the same file descriptor? If so, are events reported
to both epoll file descriptors?
Yes, and events would be reported to both. However, careful programming may be needed
to do this correctly.
3. Is the epoll file descriptor itself poll/epoll/selectable?
Yes. If an epoll file descriptor has events waiting, then it will indicate as being
readable.
4. What happens if one attempts to put an epoll file descriptor into its own file
descriptor set?
The epoll_ctl(2) call fails (EINVAL). However, you can add an epoll file descriptor
inside another epoll file descriptor set.
5. Can I send an epoll file descriptor over a UNIX domain socket to another process?
Yes, but it does not make sense to do this, since the receiving process would not have
copies of the file descriptors in the interest list.
6. Will closing a file descriptor cause it to be removed from all epoll interest lists?
Yes, but be aware of the following point. A file descriptor is a reference to an open
file description (see open(2)). Whenever a file 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 interest list only after all the file descriptors
referring to the underlying open file description have been closed. This means that
even after a file descriptor that is part of an interest list has been closed, events
may be reported for that file descriptor if other file descriptors referring to the
same underlying file description remain open. To prevent this happening, the file
descriptor must be explicitly removed from the interest list (using epoll_ctl(2)
EPOLL_CTL_DEL) before it is duplicated. Alternatively, the application must ensure
that all file descriptors are closed (which may be difficult if file descriptors were
duplicated behind the scenes by library functions that used dup(2) or fork(2)).
7. If more than one event occurs between epoll_wait(2) calls, are they combined or
reported separately?
They will be combined.
8. Does an operation on a file descriptor affect the already collected but not yet
reported events?
You can do two operations on an existing file descriptor. Remove would be meaningless
for this case. Modify will reread available I/O.
9. Do I need to continuously read/write a file descriptor until EAGAIN when using the
EPOLLET flag (edge-triggered behavior)?
Receiving 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
(nonblocking) 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.)
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
The epoll API was introduced in Linux kernel 2.5.44. Support was added to glibc in
version 2.3.2.
CONFORMING TO
The epoll API is Linux-specific. Some other systems provide similar mechanisms, for
example, FreeBSD has kqueue, and Solaris has /dev/poll.
NOTES
The set of file descriptors that is being monitored via an epoll file descriptor can be
viewed via the entry for the epoll file descriptor in the process's /proc/[pid]/fdinfo
directory. See proc(5) for further details.
The kcmp(2) KCMP_EPOLL_TFD operation can be used to test whether a file descriptor is
present in an epoll instance.
SEE ALSO
epoll_create(2), epoll_create1(2), epoll_ctl(2), epoll_wait(2), poll(2), select(2)
COLOPHON
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