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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 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:

     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 descrip-
            tor 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 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 sim-
            ple code to detect peer shutdown when using Edge Triggered moni-
            toring.)

     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.

     EPOLLHUP
            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 outstanding data in the channel has
            been consumed.

     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.

     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.

     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  multiple  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 EPOLLEXCLU-
            SIVE 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.
            Attempts  to  specify  other  values  in  events yield an error.
            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 spec-
            ifies  the  target  file descriptor fd as an epoll instance will
            likewise fail.  The error in all of these cases is EINVAL.

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 inter-
            face.

     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.

     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.  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.

CONFORMING TO

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

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  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.