EPOLL_CTL(2)               Linux Programmer's Manual              EPOLL_CTL(2)

       epoll_ctl - control interface for an epoll file descriptor

       #include <sys/epoll.h>

       int epoll_ctl(int epfd, int op, int fd, struct epoll_event *event);

       This system call is used to add, modify, or remove entries in the
       interest list of the epoll(7) instance referred to by the file
       descriptor epfd.  It requests that the operation op be performed for
       the target file descriptor, fd.

       Valid values for the op argument are:

              Add fd to the interest list and associate the settings specified
              in event with the internal file linked to fd.

              Change the settings associated with fd in the interest list to
              the new settings specified in event.

              Remove (deregister) the target file descriptor fd from the
              interest list.  The event argument is ignored and can be NULL
              (but see BUGS below).

       The event argument describes the object linked to the file descriptor
       fd.  The struct epoll_event is defined as:

           typedef union epoll_data {
               void        *ptr;
               int          fd;
               uint32_t     u32;
               uint64_t     u64;
           } epoll_data_t;

           struct epoll_event {
               uint32_t     events;      /* Epoll events */
               epoll_data_t data;        /* User data variable */

       The events member is a bit mask composed by ORing together zero or more
       of the following available event types:

              The associated file is available for read(2) operations.

              The associated file is available for write(2) operations.

       EPOLLRDHUP (since Linux 2.6.17)
              Stream socket peer closed connection, or shut down writing half
              of connection.  (This flag is especially useful for writing
              simple code to detect peer shutdown when using Edge Triggered

              There is an exceptional condition on the file descriptor.  See
              the discussion of POLLPRI in poll(2).

              Error condition happened on the associated file descriptor.
              This event is also reported for the write end of a pipe when the
              read end has been closed.  epoll_wait(2) will always report for
              this event; it is not necessary to set it in events.

              Hang up happened on the associated file descriptor.
              epoll_wait(2) will always wait for this event; it is not
              necessary to set it in events.

              Note that when reading from a channel such as a pipe or a stream
              socket, this event merely indicates that the peer closed its end
              of the channel.  Subsequent reads from the channel will return 0
              (end of file) only after all outstanding data in the channel has
              been consumed.

              Sets the Edge Triggered behavior for the associated file
              descriptor.  The default behavior for epoll is Level Triggered.
              See epoll(7) for more detailed information about Edge and Level
              Triggered event distribution architectures.

       EPOLLONESHOT (since Linux 2.6.2)
              Sets the one-shot behavior for the associated file descriptor.
              This means that after an event is pulled out with epoll_wait(2)
              the associated file descriptor is internally disabled and no
              other events will be reported by the epoll interface.  The user
              must call epoll_ctl() with EPOLL_CTL_MOD to rearm the file
              descriptor with a new event mask.

       EPOLLWAKEUP (since Linux 3.5)
              If EPOLLONESHOT and EPOLLET are clear and the process has the
              CAP_BLOCK_SUSPEND capability, ensure that the system does not
              enter "suspend" or "hibernate" while this event is pending or
              being processed.  The event is considered as being "processed"
              from the time when it is returned by a call to epoll_wait(2)
              until the next call to epoll_wait(2) on the same epoll(7) file
              descriptor, the closure of that file descriptor, the removal of
              the event file descriptor with EPOLL_CTL_DEL, or the clearing of
              EPOLLWAKEUP for the event file descriptor with EPOLL_CTL_MOD.
              See also BUGS.

       EPOLLEXCLUSIVE (since Linux 4.5)
              Sets an exclusive wakeup mode for the epoll file descriptor that
              is being attached to the target file descriptor, fd.  When a
              wakeup event occurs and multiple epoll file descriptors are
              attached to the same target file using EPOLLEXCLUSIVE, one or
              more of the epoll file descriptors will receive an event with
              epoll_wait(2).  The default in this scenario (when
              EPOLLEXCLUSIVE is not set) is for all epoll file descriptors to
              receive an event.  EPOLLEXCLUSIVE is thus useful for avoiding
              thundering herd problems in certain scenarios.

              If the same file descriptor is in multiple epoll instances, some
              with the EPOLLEXCLUSIVE flag, and others without, then events
              will be provided to all epoll instances that did not specify
              EPOLLEXCLUSIVE, and at least one of the epoll instances that did
              specify EPOLLEXCLUSIVE.

              The following values may be specified in conjunction with
              EPOLLHUP and EPOLLERR can also be specified, but this is not
              required: as usual, these events are always reported if they
              occur, regardless of whether they are specified in events.
              Attempts to specify other values in events yield the error

              EPOLLEXCLUSIVE may be used only in an EPOLL_CTL_ADD operation;
              attempts to employ it with EPOLL_CTL_MOD yield an error.  If
              EPOLLEXCLUSIVE has been set using epoll_ctl(), then a subsequent
              EPOLL_CTL_MOD on the same epfd, fd pair yields an error.  A call
              to epoll_ctl() that specifies EPOLLEXCLUSIVE in events and
              specifies the target file descriptor fd as an epoll instance
              will likewise fail.  The error in all of these cases is EINVAL.

       When successful, epoll_ctl() returns zero.  When an error occurs,
       epoll_ctl() returns -1 and errno is set appropriately.

       EBADF  epfd or fd is not a valid file descriptor.

       EEXIST op was EPOLL_CTL_ADD, and the supplied file descriptor fd is
              already registered with this epoll instance.

       EINVAL epfd is not an epoll file descriptor, or fd is the same as epfd,
              or the requested operation op is not supported by this

       EINVAL An invalid event type was specified along with EPOLLEXCLUSIVE in

       EINVAL op was EPOLL_CTL_MOD and events included EPOLLEXCLUSIVE.

       EINVAL op was EPOLL_CTL_MOD and the EPOLLEXCLUSIVE flag has previously
              been applied to this epfd, fd pair.

       EINVAL EPOLLEXCLUSIVE was specified in event and fd refers to an epoll

       ELOOP  fd refers to an epoll instance and this EPOLL_CTL_ADD operation
              would result in a circular loop of epoll instances monitoring
              one another.

       ENOENT op was EPOLL_CTL_MOD or EPOLL_CTL_DEL, and fd is not registered
              with this epoll instance.

       ENOMEM There was insufficient memory to handle the requested op control

       ENOSPC The limit imposed by /proc/sys/fs/epoll/max_user_watches was
              encountered while trying to register (EPOLL_CTL_ADD) a new file
              descriptor on an epoll instance.  See epoll(7) for further

       EPERM  The target file fd does not support epoll.  This error can occur
              if fd refers to, for example, a regular file or a directory.

       epoll_ctl() was added to the kernel in version 2.6.

       epoll_ctl() is Linux-specific.  Library support is provided in glibc
       starting with version 2.3.2.

       The epoll interface supports all file descriptors that support poll(2).

       In kernel versions before 2.6.9, the EPOLL_CTL_DEL operation required a
       non-null pointer in event, even though this argument is ignored.  Since
       Linux 2.6.9, event can be specified as NULL when using EPOLL_CTL_DEL.
       Applications that need to be portable to kernels before 2.6.9 should
       specify a non-null pointer in event.

       If EPOLLWAKEUP is specified in flags, but the caller does not have the
       CAP_BLOCK_SUSPEND capability, then the EPOLLWAKEUP flag is silently
       ignored.  This unfortunate behavior is necessary because no validity
       checks were performed on the flags argument in the original
       implementation, and the addition of the EPOLLWAKEUP with a check that
       caused the call to fail if the caller did not have the
       CAP_BLOCK_SUSPEND capability caused a breakage in at least one existing
       user-space application that happened to randomly (and uselessly)
       specify this bit.  A robust application should therefore double check
       that it has the CAP_BLOCK_SUSPEND capability if attempting to use the
       EPOLLWAKEUP flag.

       epoll_create(2), epoll_wait(2), poll(2), epoll(7)

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Linux                             2019-03-06                      EPOLL_CTL(2)