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 nec-
              essary 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 architec-
              tures.

       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 argu-
       ment  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 3.25 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                                                      2009-01-17                                               EPOLL_CTL(2)