EPOLL_CTL(2)



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

NAME
       epoll_ctl - control interface for an epoll file descriptor

SYNOPSIS
       #include <sys/epoll.h>

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

DESCRIPTION
       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:

       EPOLL_CTL_ADD
              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.

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

       EPOLL_CTL_DEL
              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  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 of the following available event types:

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

       EPOLLOUT
              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-
              toring.)

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

       EPOLLERR
              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  nec-
              essary to set it in events when calling epoll_ctl().

       EPOLLHUP
              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 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 outstanding data in the channel has
              been consumed.

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

              This flag is an input flag for the event.events field when call-
              ing epoll_ctl(); it is never returned by epoll_wait(2).

       EPOLLONESHOT (since Linux 2.6.2)
              Requests one-shot notification for the associated file  descrip-
              tor.   This  means that after an event notified for the file de-
              scriptor 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.

              This flag is an input flag for the event.events field when call-
              ing epoll_ctl(); it is never returned by epoll_wait(2).

       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.

              This flag is an input flag for the event.events field when call-
              ing epoll_ctl(); it is never returned by epoll_wait(2).

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

              The EPOLLEXCLUSIVE flag is an input flag  for  the  event.events
              field   when  calling  epoll_ctl();  it  is  never  returned  by
              epoll_wait(2).

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

ERRORS
       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-
              face.

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

       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.

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

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

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

BUGS
       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.

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

COLOPHON
       This  page  is  part of release 5.07 of the Linux man-pages project.  A
       description of the project, information about reporting bugs,  and  the
       latest     version     of     this    page,    can    be    found    at
       https://www.kernel.org/doc/man-pages/.

Linux                             2020-04-11                      EPOLL_CTL(2)

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