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       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 operation 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

              Change the event event associated with the target file descriptor 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 simple code  to  detect  peer  shutdown
              when using Edge Triggered monitoring.)

              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 necessary 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 necessary 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 outstanding data  in  the  channel  has  been

              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 interface.

       ENOENT op  was  EPOLL_CTL_MOD  or  EPOLL_CTL_DEL, and fd is not registered with this epoll

       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.  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


       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
       implementation, 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 capability 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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