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       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 monitoring.)

              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

              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 EPOLLEXCLUSIVE:  EPOLLIN,
              EPOLLOUT,  EPOLLWAKEUP,  and EPOLLET.  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 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 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 interface.

       EINVAL An invalid event type was specified along with EPOLLEXCLUSIVE in events.

       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 instance.

       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

       ENOMEM There was insufficient memory to handle the requested op control operation.

       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 details.

       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


       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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