epoll_ctl(2) — Linux manual page


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

NAME         top

       epoll_ctl - control interface for an epoll file descriptor

SYNOPSIS         top

       #include <sys/epoll.h>

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

DESCRIPTION         top

       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 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

              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 writ‐
              ing simple code to detect peer shutdown when using edge-trig‐
              gered 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 nec‐
              essary 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.

              Requests edge-triggered notification for the associated file
              descriptor.  The default behavior for epoll is level-trig‐
              gered.  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
              calling epoll_ctl(); it is never returned by epoll_wait(2).

       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 dis‐
              abled 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
              calling 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)
              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 clear‐
              ing 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
              calling 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 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 avoid‐
              ing 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 subse‐
              quent 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.

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

RETURN VALUE         top

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

ERRORS         top

       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 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
              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

VERSIONS         top

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

CONFORMING TO         top

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

NOTES         top

       The epoll interface supports all file descriptors that support

BUGS         top

       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.

SEE ALSO         top

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

COLOPHON         top

       This page is part of release 5.08 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

Linux                            2020-04-11                     EPOLL_CTL(2)

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