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

NAME         top

       epoll_ctl - control interface for an epoll 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 performs control operations on 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:

              Register the target file descriptor fd on the epoll instance
              referred to by the file descriptor epfd and associate the
              event event with the internal file linked to fd.

              Change the event event associated with the target file
              descriptor fd.

              Remove (deregister) the target file descriptor fd from the
              epoll instance referred to by epfd.  The event 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 set composed using 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 urgent data available for read(2) operations.

              Error condition happened on the associated file descriptor.
              epoll_wait(2) will always wait for this event; it is not
              necessary to set it in events.

              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

              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.

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

       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 4.02 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                            2015-05-07                     EPOLL_CTL(2)