epoll

EPOLL(7) Linux Programmer’s Manual EPOLL(7)

NAME
epoll - I/O event notification facility

SYNOPSIS
#include

           #define MAX_EVENTS 10           struct epoll_event ev, events[MAX_EVENTS];           int listen_sock, conn_sock, nfds, epollfd;           /* Set up listening socket, 'listen_sock' (socket(),              bind(), listen()) */           epollfd = epoll_create(10);           if (epollfd == -1) {               perror("epoll_create");               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_pwait");                   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  repre‐       sentation 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  con‐       tinue 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 descrip‐   tor 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  associ‐   ated 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)

COLOPHON
This page is part of release 3.74 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of
this page, can be found at http://www.kernel.org/doc/man-pages/.

Linux 2014-07-08 EPOLL(7)