EPOLL_CREATE(2) Linux Programmer's Manual EPOLL_CREATE(2)
NAME
epoll_create, epoll_create1 - open an epoll file descriptor
EPOLL(7) Linux Programmer's 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 interface and scales well to large numbers of watched file descriptors. The following system
calls are provided to create and manage an epoll instance:
* epoll_create(2) creates an 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). The set of file descriptors currently registered on an epoll instance
is sometimes called an epoll set.
* epoll_wait(2) waits for I/O events, blocking the calling thread if no events are currently available.
水平触发和边缘触发
边缘触发和水平触发机制有以下不同，假设以下事件发生：
The epoll event distribution interface is able to behave both as edge-triggered (ET) and as level-triggered (LT). The difference between the two mecha‐
nisms 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.
setup1:管道读端rfd注册到epoll
setup2:写2KB数据到管道
setup3:调用epoll_wait，返回就绪的rfd。
setup4:管道通过rfd读取1KB数据。
setup5:调用epoll_wait
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.
如果rfd使用边缘触发方式添加到epoll,在setup5调用epoll_wait时，尽管有有效的数据在输入缓冲区，但是epoll_wait仍可能会挂起。同时远端可能期望得到一个基于之前已经发送的数据的应答。这个原因是因为边缘触发模式仅在监控的文件描述符发生变化时才会产生事件。因此，在setup5，调用者又可能最终成为等待一些已经在输入缓冲区中的数据。上面的例子中，rfd上的事件产生时因为在setup2有写入完成并且在setup3事件被处理。因为在setup4的读操作没有读取完整个的缓冲数据，所以在setup5的epoll_wait可能会一直阻塞.
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 han‐
dling multiple file descriptors. The suggested way to use epoll as an edge-triggered (EPOLLET) interface is as follows:
i with nonblocking file descriptors; and
ii by waiting for an event only after read(2) or write(2) return EAGAIN.
采用边缘触发的应用程序应该使用非阻塞的文件描述符，以避免有阻塞的读写造成处理多个文件描述符的任务饿死。使用边缘触发的epoll 的建议如下：
使用非阻塞的文件描述符号，并且仅在 read/write返回EAGAIN时，在等待事件。
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.
通过对比，当使用水平触发接口，epoll就是一个简单快速的poll,并且可以应用在任何poll应用的地方，因为他们具有一直的语义。
因为边缘触发模式的epoll，一收到多块数据据就会产生多个事件。可以通过选项EPOLLONESHOT来调整。
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(7) EPOLL‐
WAKEUP 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. Cur‐
rently, 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 applica‐
tion 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 *) &local, &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
Q0 What is the key used to distinguish the file descriptors registered in an epoll set?
A0 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).
Q1 What happens if you register the same file descriptor on an epoll instance twice?
A1 You 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 instance.
This can be a useful technique for filtering events, if the duplicate file descriptors are registered with different events masks.
Q2 Can two epoll instances 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, careful programming may be needed to do this correctly.
Q3 Is the epoll file descriptor itself poll/epoll/selectable?
A3 Yes. If an epoll file descriptor has events waiting, then it will indicate as being readable.
Q4 What happens if one attempts to put an epoll file descriptor into its own file descriptor set?
A4 The epoll_ctl(2) call will fail (EINVAL). However, you can add an epoll file descriptor inside another epoll file descriptor set.
Q5 Can I send an epoll file descriptor over a UNIX domain socket to another process?
A5 Yes, 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.
Q6 Will closing a file descriptor cause it to be removed from all epoll sets automatically?
A6 Yes, 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 before if the descriptor is explicitly
removed using epoll_ctl(2) 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.
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 reread 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 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 check‐
ing 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-ori‐
ented 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.
SEE ALSO
epoll_create(2), epoll_create1(2), epoll_ctl(2), epoll_wait(2), poll(2), select(2)
COLOPHON
This page is part of release 4.04 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest ver‐
sion of this page, can be found at http://www.kernel.org/doc/man-pages/.
Linux 2015-04-19 EPOLL(7)
SYNOPSIS
#include <sys/epoll.h>
int epoll_create(int size);
int epoll_create1(int flags);
DESCRIPTION
epoll_create() creates an epoll(7) instance. Since Linux 2.6.8, the size argument is ignored, but must be greater than zero; see NOTES below.
epoll_create() returns a file descriptor referring to the new epoll instance. This file descriptor is used for all the subsequent calls to the epoll
interface. When no longer required, the file descriptor returned by epoll_create() should be closed by using close(2). When all file descriptors
referring to an epoll instance have been closed, the kernel destroys the instance and releases the associated resources for reuse.
epoll_create1()
If flags is 0, then, other than the fact that the obsolete size argument is dropped, epoll_create1() is the same as epoll_create(). The following value
can be included in flags to obtain different behavior:
EPOLL_CLOEXEC
Set the close-on-exec (FD_CLOEXEC) flag on the new file descriptor. See the description of the O_CLOEXEC flag in open(2) for reasons why this
may be useful.
RETURN VALUE
On success, these system calls return a nonnegative file descriptor. On error, -1 is returned, and errno is set to indicate the error.
ERRORS
EINVAL size is not positive.
EINVAL (epoll_create1()) Invalid value specified in flags.
EMFILE The per-user limit on the number of epoll instances imposed by /proc/sys/fs/epoll/max_user_instances was encountered. See epoll(7) for further
details.
EMFILE The per-process limit on the number of open file descriptors has been reached.
ENFILE The system-wide limit on the total number of open files has been reached.
ENOMEM There was insufficient memory to create the kernel object.
VERSIONS
epoll_create() was added to the kernel in version 2.6. Library support is provided in glibc starting with version 2.3.2.
epoll_create1() was added to the kernel in version 2.6.27. Library support is provided in glibc starting with version 2.9.
CONFORMING TO
epoll_create() is Linux-specific.
NOTES
In the initial epoll_create() implementation, the size argument informed the kernel of the number of file descriptors that the caller expected to add to
the epoll instance. The kernel used this information as a hint for the amount of space to initially allocate in internal data structures describing
events. (If necessary, the kernel would allocate more space if the caller's usage exceeded the hint given in size.) Nowadays, this hint is no longer
required (the kernel dynamically sizes the required data structures without needing the hint), but size must still be greater than zero, in order to
ensure backward compatibility when new epoll applications are run on older kernels.
SEE ALSO
close(2), epoll_ctl(2), epoll_wait(2), epoll(7)
COLOPHON
This page is part of release 4.04 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest ver‐
sion of this page, can be found at http://www.kernel.org/doc/man-pages/.
Linux 2015-12-28 EPOLL_CREATE(2)