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NAME

       epoll_ctl - control interface for an epoll descriptor

SYNOPSIS

       #include <sys/epoll.h>

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

DESCRIPTION

       This  system  call  performs  control  operations on the epoll 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
              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.

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

       EPOLL_CTL_DEL
              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:

       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 urgent data available for read(2) operations.

       EPOLLERR
              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.

       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.

       EPOLLET
              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  re-arm  the  file
              descriptor with a new event mask.

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
              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.

       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.

CONFORMING TO

       epoll_ctl() is Linux-specific, and was introduced in kernel 2.5.44.

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.

SEE ALSO

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

COLOPHON

       This  page  is  part of release 3.23 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.