💾 Archived View for gmi.noulin.net › man › man2 › epoll_ctl.2.gmi captured on 2022-06-12 at 06:28:32. Gemini links have been rewritten to link to archived content

View Raw

More Information

-=-=-=-=-=-=-

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

NAME
       epoll_ctl - control interface for an epoll file descriptor

SYNOPSIS
       #include <sys/epoll.h>

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

DESCRIPTION
       This  system  call  is used to add, modify, or remove entries in the interest list of 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:

       EPOLL_CTL_ADD
              Add an entry to the interest list of the epoll file descriptor, epfd.  The entry includes the file descriptor, fd, a reference to the  corresponding  open
              file description (see epoll(7) and open(2)), and the settings specified in event.

       EPOLL_CTL_MOD
              Change the settings associated with fd in the interest list to the new settings specified in event.

       EPOLL_CTL_DEL
              Remove (deregister) the target file descriptor fd from the interest list.  The event argument 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  data  member  of  the epoll_event structure specifies data that the kernel should save and then return (via epoll_wait(2)) when this file descriptor becomes
       ready.

       The events member of the epoll_event structure is a bit mask composed by ORing together zero or more of 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 an exceptional condition on the file descriptor.  See the discussion of POLLPRI in poll(2).

       EPOLLERR
              Error condition happened on the associated file descriptor.  This event is also reported for the write end of a pipe when the read end has been closed.

              epoll_wait(2) will always report for this event; it is not necessary to set it in events when calling epoll_ctl().

       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 when calling epoll_ctl().

              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.  Subse‐
              quent reads from the channel will return 0 (end of file) only after all outstanding data in the channel has been consumed.

       EPOLLET
              Requests edge-triggered notification for the associated file descriptor.  The default behavior for epoll is level-triggered.  See epoll(7)  for  more  de‐
              tailed information about edge-triggered and level-triggered notification.

              This flag is an input flag for the event.events field when calling epoll_ctl(); it is never returned by epoll_wait(2).

       EPOLLONESHOT (since Linux 2.6.2)
              Requests  one-shot notification for the associated file descriptor.  This means that after an event notified for the file descriptor by epoll_wait(2), the
              file descriptor is disabled in the interest list 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.

              This flag is an input flag for the event.events field when calling epoll_ctl(); it is never returned by epoll_wait(2).

       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.

              This flag is an input flag for the event.events field when calling epoll_ctl(); it is never returned by epoll_wait(2).

       EPOLLEXCLUSIVE (since Linux 4.5)
              Sets an exclusive wakeup mode for the epoll file descriptor that is being attached to the target file descriptor, fd.  When a wakeup event occurs and mul‐
              tiple  epoll  file  descriptors are attached to the same target file using EPOLLEXCLUSIVE, one or more of the epoll file descriptors will receive an event
              with epoll_wait(2).  The default in this scenario (when EPOLLEXCLUSIVE is not set) is for all epoll file descriptors to receive an event.   EPOLLEXCLUSIVE
              is thus useful for avoiding thundering herd problems in certain scenarios.

              If  the  same  file  descriptor is in multiple epoll instances, some with the EPOLLEXCLUSIVE flag, and others without, then events will be provided to all
              epoll instances that did not specify EPOLLEXCLUSIVE, and at least one of the epoll instances that did specify EPOLLEXCLUSIVE.

              The following values may be specified in conjunction with EPOLLEXCLUSIVE: EPOLLIN, EPOLLOUT, EPOLLWAKEUP, and EPOLLET.  EPOLLHUP and EPOLLERR can also  be
              specified,  but  this  is not required: as usual, these events are always reported if they occur, regardless of whether they are specified in events.  At‐
              tempts to specify other values in events yield the error EINVAL.

              EPOLLEXCLUSIVE may be used only in an EPOLL_CTL_ADD operation; attempts to employ it with EPOLL_CTL_MOD yield an error.  If EPOLLEXCLUSIVE  has  been  set
              using  epoll_ctl(),  then  a  subsequent  EPOLL_CTL_MOD on the same epfd, fd pair yields an error.  A call to epoll_ctl() that specifies EPOLLEXCLUSIVE in
              events and specifies the target file descriptor fd as an epoll instance will likewise fail.  The error in all of these cases is EINVAL.

              The EPOLLEXCLUSIVE flag is an input flag for the event.events field when calling epoll_ctl(); it is never returned by epoll_wait(2).

RETURN VALUE
       When successful, epoll_ctl() returns zero.  When an error occurs, epoll_ctl() returns -1 and errno is set to indicate the error.

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.

       EINVAL An invalid event type was specified along with EPOLLEXCLUSIVE in events.

       EINVAL op was EPOLL_CTL_MOD and events included EPOLLEXCLUSIVE.

       EINVAL op was EPOLL_CTL_MOD and the EPOLLEXCLUSIVE flag has previously been applied to this epfd, fd pair.

       EINVAL EPOLLEXCLUSIVE was specified in event and fd refers to an epoll instance.

       ELOOP  fd refers to an epoll instance and this EPOLL_CTL_ADD operation would result in a circular loop of epoll instances monitoring one  another  or  a  nesting
              depth of epoll instances greater than 5.

       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 in‐
              stance.  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.

VERSIONS
       epoll_ctl() was added to the kernel in version 2.6.  Library support is provided in glibc starting with version 2.3.2.

CONFORMING TO
       epoll_ctl() is Linux-specific.

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.

       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  unfor‐
       tunate  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  ap‐
       plication that happened to randomly (and uselessly) specify this bit.  A robust application should therefore double check that it has the CAP_BLOCK_SUSPEND capa‐
       bility if attempting to use the EPOLLWAKEUP flag.

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

Linux                                                                          2021-03-22                                                                   EPOLL_CTL(2)