epoll(7) Miscellaneous Information Manual epoll(7)
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 inter-
face and scales well to large numbers of watched file descriptors.
The central concept of the epoll API is the epoll instance, an in-ker-
nel data structure which, from a user-space perspective, can be consid-
ered 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 mon-
itoring.
• 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 ac-
tivity 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 de-
scriptor referring to that instance. (The more recent epoll_cre-
ate1(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 in-
stance.
• 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 wait-
ing 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 opera-
tion 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:
(1) with nonblocking file descriptors; and
(2) by waiting for an event only after read(2) or write(2) return EA-
GAIN.
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 seman-
tics.
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 "thun-
dering 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 de-
vice 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 be-
fore the second epoll_wait(2) call.
/proc interfaces
The following interfaces can be used to limit the amount of kernel mem-
ory 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 sys-
tem. The limit is per real user ID. Each registered file de-
scriptor 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, di-
vided 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 re-
quires more clarification to avoid stalls in the application event
loop. In this example, listener is a nonblocking socket on which lis-
ten(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
• 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 ker-
nel's internal representation of an open file).
• 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 du-
plicate (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 dif-
ferent events masks.
• 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 pro-
gramming may be needed to do this correctly.
• 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.
• 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.
• Can I send an epoll file descriptor over a UNIX domain socket to an-
other 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 inter-
est list.
• 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 descrip-
tion have been closed. This means that even after a file descriptor
that is part of an interest list has been closed, events may be re-
ported 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 descrip-
tors were duplicated behind the scenes by library functions that
used dup(2) or fork(2)).
• If more than one event occurs between epoll_wait(2) calls, are they
combined or reported separately?
They will be combined.
• 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.
• 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 en-
tirely 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 de-
tected 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 moni-
tored file descriptor always refers to a stream-oriented file.)
Possible pitfalls and ways to avoid them
• Starvation (edge-triggered)
If there is a large amount of I/O space, it is possible that by try-
ing to drain it the other files will not get processed causing star-
vation. (This problem is not specific to epoll.)
The solution is to maintain a ready list and mark the file descrip-
tor 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 ignor-
ing subsequent events you receive for file descriptors that are al-
ready ready.
• 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 clo-
sure 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 struc-
ture and close(2) the file descriptor for event #13, then your event
cache might still say there are events waiting for that file de-
scriptor 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
in glibc 2.3.2.
STANDARDS
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 de-
tails.
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)
Linux man-pages 6.03 2023-02-05 epoll(7)
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