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 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.

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 3.27 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/.

Linux								    2010-08-29							      EPOLL_CTL(2)