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

       epoll_ctl - control interface for an epoll descriptor

       #include <sys/epoll.h>

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

       This  system  call performs control operations on the epoll(7) instance
       referred to by the file descriptor epfd.  It requests that  the  opera-
       tion op be performed for the target file descriptor, fd.

       Valid values for the op argument are:

              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.

              Change  the event event associated with the target file descrip-
              tor fd.

              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 mask composed using the following available
       event types:

              The associated file is available for read(2) operations.

              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 sim-
              ple code to detect peer shutdown when using Edge Triggered moni-

              There is urgent data available for read(2) operations.

              Error  condition  happened  on  the  associated file descriptor.
              epoll_wait(2) will always wait for this event; it is not  neces-
              sary to set it in events.

              Hang   up   happened   on   the   associated   file  descriptor.
              epoll_wait(2) will always wait for this event; it is not  neces-
              sary to set it in events.  Note that when reading from a channel
              such as a pipe or a stream socket, this event  merely  indicates
              that  the  peer closed its end of the channel.  Subsequent reads
              from the channel will return 0 (end of file) only after all out-
              standing data in the channel has been consumed.

              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.

       EPOLLWAKEUP (since Linux 3.5)
              If  EPOLLONESHOT  and  EPOLLET are clear and the process has the
              CAP_BLOCK_SUSPEND capability, ensure that the  system  does  not
              enter  "suspend"  or  "hibernate" while this event is pending or
              being processed.  The event is considered as  being  "processed"
              from  the  time  when  it is returned by a call to epoll_wait(2)
              until the next call to epoll_wait(2) on the same  epoll(7)  file
              descriptor,  the closure of that file descriptor, the removal of
              the event file descriptor with EPOLL_CTL_DEL, or the clearing of
              EPOLLWAKEUP  for  the  event file descriptor with EPOLL_CTL_MOD.
              See also BUGS.

       When successful, epoll_ctl()  returns  zero.   When  an  error  occurs,
       epoll_ctl() returns -1 and errno is set appropriately.

       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 inter-

       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

       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

       EPERM  The target file fd does not support epoll.  This error can occur
              if fd refers to, for example, a regular file or a directory.

       epoll_ctl() was added to the kernel in version 2.6.

       epoll_ctl() is Linux-specific.  Library support is  provided  in  glibc
       starting with version 2.3.2.

       The epoll interface supports all file descriptors that support poll(2).

       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.

       If  EPOLLWAKEUP is specified in flags, but the caller does not have the
       CAP_BLOCK_SUSPEND capability, then the  EPOLLWAKEUP  flag  is  silently
       ignored.   This  unfortunate  behavior is necessary because no validity
       checks were performed on the flags argument in the original implementa-
       tion,  and the addition of the EPOLLWAKEUP with a check that caused the
       call to fail if the caller did not have the CAP_BLOCK_SUSPEND  capabil-
       ity  caused  a breakage in at least one existing user-space application
       that happened to randomly (and uselessly) specify this bit.   A  robust
       application   should   therefore   double   check   that   it  has  the
       CAP_BLOCK_SUSPEND capability if attempting to use the EPOLLWAKEUP flag.

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

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Linux                             2015-05-07                      EPOLL_CTL(2)
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