epoll(7) - Linux manual page (original) (raw)
epoll(7) Miscellaneous Information Manual epoll(7)
NAME top
epoll - I/O event notification facilitySYNOPSIS top
**#include <sys/epoll.h>**DESCRIPTION top
The **epoll** API performs a similar task to [poll(2)](../man2/poll.2.html): 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)](../man2/epoll%5Fcreate.2.html) creates a new **epoll** instance and returns a file
descriptor referring to that instance. (The more recent
[epoll_create1(2)](../man2/epoll%5Fcreate1.2.html) extends the functionality of [epoll_create(2)](../man2/epoll%5Fcreate.2.html).)
• Interest in particular file descriptors is then registered via
[epoll_ctl(2)](../man2/epoll%5Fctl.2.html), which adds items to the interest list of the
**epoll** instance.
• [epoll_wait(2)](../man2/epoll%5Fwait.2.html) 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)](../man2/epoll%5Fwait.2.html) 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)](../man2/epoll%5Fwait.2.html) 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)](../man2/epoll%5Fwait.2.html)
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, that is, an
event will be generated upon each receipt of a chunk of data. 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)](../man2/epoll%5Fwait.2.html)
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)](../man2/read.2.html) or [write(2)](../man2/write.2.html) 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)](../man2/poll.2.html), 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)](../man2/epoll%5Fwait.2.html). When the **EPOLLONESHOT** flag is
specified, it is the caller's responsibility to rearm the file
descriptor using [epoll_ctl(2)](../man2/epoll%5Fctl.2.html) with **EPOLL_CTL_MOD**.
If multiple threads (or processes, if child processes have
inherited the **epoll** file descriptor across [fork(2)](../man2/fork.2.html)) are blocked in
[epoll_wait(2)](../man2/epoll%5Fwait.2.html) 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)](../man2/epoll%5Fwait.2.html). 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_
_epollevent_, the system will be kept awake from the moment the
event is queued, through the [epoll_wait(2)](../man2/epoll%5Fwait.2.html) call which returns the
event until the subsequent [epoll_wait(2)](../man2/epoll%5Fwait.2.html) call. If the event
should keep the system awake beyond that time, then a separate
_wakelock_ should be taken before the second [epoll_wait(2)](../man2/epoll%5Fwait.2.html) call./proc interfaces The following interfaces can be used to limit the amount of kernel memory consumed by epoll:
_/proc/sys/fs/epoll/maxuserwatches_ (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 _maxuserwatches_ 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 dousefd() 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 dousefd() 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)](../man2/epoll%5Fctl.2.html) 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 kernel'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
duplicate ([dup(2)](../man2/dup.2.html), [dup2(2)](../man2/dup2.2.html), [fcntl(2)](../man2/fcntl.2.html) **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.
• 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.
• 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)](../man2/epoll%5Fctl.2.html) 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 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.
• 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)](../man2/open.2.html)).
Whenever a file descriptor is duplicated via [dup(2)](../man2/dup.2.html), [dup2(2)](../man2/dup2.2.html),
[fcntl(2)](../man2/fcntl.2.html) **F_DUPFD**, or [fork(2)](../man2/fork.2.html), 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)](../man2/epoll%5Fctl.2.html) **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)](../man2/dup.2.html) or [fork(2)](../man2/fork.2.html)).
• If more than one event occurs between [epoll_wait(2)](../man2/epoll%5Fwait.2.html) 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)](../man2/epoll%5Fwait.2.html) 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)](../man2/read.2.html) by asking to read a certain amount of data and
[read(2)](../man2/read.2.html) 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)](../man2/write.2.html). (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 • 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.
• **If using an event cache...**
If you use an event cache or store all the file descriptors
returned from [epoll_wait(2)](../man2/epoll%5Fwait.2.html), 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)](../man2/epoll%5Fwait.2.html), and in event #47 a condition causes event #13 to
be closed. If you remove the structure and [close(2)](../man2/close.2.html) 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)](../man2/close.2.html), 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 top
Some other systems provide similar mechanisms; for example,
FreeBSD has _kqueue_, and Solaris has _/dev/poll_.STANDARDS top
Linux.HISTORY top
Linux 2.5.44. glibc 2.3.2.NOTES top
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)](../man5/proc.5.html) for further details.
The [kcmp(2)](../man2/kcmp.2.html) **KCMP_EPOLL_TFD** operation can be used to test whether a
file descriptor is present in an epoll instance.SEE ALSO top
[epoll_create(2)](../man2/epoll%5Fcreate.2.html), [epoll_create1(2)](../man2/epoll%5Fcreate1.2.html), [epoll_ctl(2)](../man2/epoll%5Fctl.2.html), [epoll_wait(2)](../man2/epoll%5Fwait.2.html),
[ioctl_eventpoll(2)](../man2/ioctl%5Feventpoll.2.html), [poll(2)](../man2/poll.2.html), [select(2)](../man2/select.2.html)COLOPHON top
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man-pages@man7.orgLinux man-pages 6.10 2024-08-21 epoll(7)
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