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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 rearm 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.

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.

SEE ALSO

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

COLOPHON

       This  page  is  part of release 3.35 of the Linux man-pages project.  A description of the
       project, and information about reporting bugs, can be found at  http://man7.org/linux/man-
       pages/.