epoll_ctl(2) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | CONFORMING TO | NOTES | BUGS | SEE ALSO | COLOPHON

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:

       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
              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 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 simple code to detect peer shutdown when using
              edge-triggered monitoring.)

       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
              necessary 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
              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
              outstanding data in the channel has been consumed.

       EPOLLET
              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.

              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 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
              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 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
              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 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.

              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         top

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

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 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
              directory.

VERSIONS         top

       epoll_ctl() was added to the kernel in version 2.6.  Library
       support is provided in glibc starting with version 2.3.2.

CONFORMING TO         top

       epoll_ctl() is Linux-specific.

NOTES         top

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

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 5.12 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                          2021-03-22                   EPOLL_CTL(2)

Pages that refer to this page: epoll_create(2)epoll_wait(2)signalfd(2)syscalls(2)sd_event_add_io(3)sd_event_get_fd(3)sd_notify(3)proc(5)epoll(7)