epoll_ctl(2)                                            System Calls Manual                                            epoll_ctl(2)

       epoll_ctl - control interface for an epoll file descriptor

       Standard C library (libc, -lc)

       #include <sys/epoll.h>

       int epoll_ctl(int epfd, int op, int fd, struct epoll_event *_Nullable 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 an entry to the interest list of the epoll file descriptor, epfd.  The entry includes the file descriptor, fd,  a
              reference to the corresponding open file description (see epoll(7) and open(2)), and the settings specified in event.

              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  described  in

       The  data member of the epoll_event structure specifies data that the kernel should save and then return (via epoll_wait(2))
       when this file descriptor becomes ready.

       The events member of the epoll_event structure is a bit mask composed by ORing together zero or more event  types,  returned
       by epoll_wait(2), and input flags, which affect its behaviour, but aren't returned.  The available event types are:

              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 events when calling epoll_ctl().

              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 when calling epoll_ctl().

              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  out‐
              standing data in the channel has been consumed.

       And the available input flags are:

              Requests  edge-triggered  notification  for the associated file descriptor.  The default behavior for epoll is level-
              triggered.  See epoll(7) for more detailed information about edge-triggered and level-triggered notification.

       EPOLLONESHOT (since Linux 2.6.2)
              Requests one-shot notification for the associated file descriptor.  This means that after an event notified  for  the
              file  descriptor  by  epoll_wait(2), the file descriptor is disabled in the interest list 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 EPOLLEXCLU‐
              SIVE,  one  or more of the epoll file descriptors will receive an event with epoll_wait(2).  The default in this sce‐
              nario (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 EPOL‐
              LET.  EPOLLHUP and EPOLLERR can also be specified, but this is not required: as usual, these events  are  always  re‐
              ported 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  er‐
              ror.   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  de‐
              scriptor 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 to indicate the

       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 or a nesting depth of epoll instances greater than 5.

       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 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

       epoll_ctl() was added to in Linux 2.6.  Library support is provided in glibc 2.3.2.

       epoll_ctl() is Linux-specific.

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

       Before Linux 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 ker‐
       nels before Linux 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 ar‐
       gument 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)

Linux man-pages 6.03                                         2022-12-04                                                epoll_ctl(2)