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



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 dis-
              tribution 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 on with this epoll instance.

       ENOMEM There was insufficient memory to handle the requested op control operation.

       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.   Appli-
       cations 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 3.22 of the Linux man-pages project.  A description of the project, and informa-
       tion about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



Linux                             2009-01-17                      EPOLL_CTL(2)