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 in-
       terest 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  in-
              terest 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 sim-
              ple code to detect peer shutdown when using Edge Triggered moni-

              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  neces-
              sary 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 de-
              scriptor.  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  de-
              scriptor 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) un-
              til the next call to epoll_wait(2) on the same epoll(7) file de-
              scriptor,  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  at-
              tached 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 EPOLLEXCLU-
              SIVE 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 re-
              quired:  as  usual, these events are always reported if they oc-
              cur, regardless of whether they are specified  in  events.   At-
              tempts to specify other values in events yield the error EINVAL.

              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 spec-
              ifies 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 al-
              ready 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  inter-

       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 en-
              countered  while  trying  to register (EPOLL_CTL_ADD) a new file
              descriptor on an epoll instance.  See epoll(7) for  further  de-

       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 ig-
       nored.   This  unfortunate  behavior  is  necessary because no validity
       checks were performed on the flags argument in the original implementa-
       tion,  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  capabil-
       ity  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)
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