# epoll

## 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, 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)
           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
                **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**
           •  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), 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.
    
           •  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) 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)).
              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)).
    
           •  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 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**
           •  **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), 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



           Some other systems provide similar mechanisms; for example,
           FreeBSD has _kqueue_ , and Solaris has _/dev/poll_.
    


## STANDARDS



           Linux.
    


## 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.
    


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