История изменений

1.  What happens if you register the same file descriptor on an
           epoll instance twice?

           You will probably get EEXIST.  However, it is possible to add
           a duplicate (dup(2), dup2(2), fcntl(2) F_DUPFD) file
           descriptor to the same epoll instance.  This can be a useful
           technique for filtering events, if the duplicate file
           descriptors are registered with different events masks.
6.  Will closing a file descriptor cause it to be removed from
           all epoll interest lists?

           Yes, but be aware of the following point.  A file descriptor
           is a reference to an open file description (see open(2)).
           Whenever a file descriptor is duplicated via dup(2), dup2(2),
           fcntl(2) F_DUPFD, or fork(2), a new file descriptor referring
           to the same open file description is created.  An open file
           description continues to exist until all file descriptors
           referring to it have been closed.

           A file descriptor is removed from an interest list only after
           all the file descriptors referring to the underlying open
           file description have been closed.  This means that even
           after a file descriptor that is part of an interest list has
           been closed, events may be reported for that file descriptor
           if other file descriptors referring to the same underlying
           file description remain open.  To prevent this happening, the
           file descriptor must be explicitly removed from the interest
           list (using epoll_ctl(2) EPOLL_CTL_DEL) before it is
           duplicated.  Alternatively, the application must ensure that
           all file descriptors are closed (which may be difficult if
           file descriptors were duplicated behind the scenes by library
           functions that used dup(2) or fork(2)).

1.  What happens if you register the same file descriptor on an
           epoll instance twice?

           You will probably get EEXIST.  However, it is possible to add
           a duplicate (dup(2), dup2(2), fcntl(2) F_DUPFD) file
           descriptor to the same epoll instance.  This can be a useful
           technique for filtering events, if the duplicate file
           descriptors are registered with different events masks.
6.  Will closing a file descriptor cause it to be removed from
           all epoll interest lists?

           Yes, but be aware of the following point.  A file descriptor
           is a reference to an open file description (see open(2)).
           Whenever a file descriptor is duplicated via dup(2), dup2(2),
           fcntl(2) F_DUPFD, or fork(2), a new file descriptor referring
           to the same open file description is created.  An open file
           description continues to exist until all file descriptors
           referring to it have been closed.

           A file descriptor is removed from an interest list only after
           all the file descriptors referring to the underlying open
           file description have been closed.  This means that even
           after a file descriptor that is part of an interest list has
           been closed, events may be reported for that file descriptor
           if other file descriptors referring to the same underlying
           file description remain open.  To prevent this happening, the
           file descriptor must be explicitly removed from the interest
           list (using epoll_ctl(2) EPOLL_CTL_DEL) before it is
           duplicated.  Alternatively, the application must ensure that
           all file descriptors are closed (which may be difficult if
           file descriptors were duplicated behind the scenes by library
           functions that used dup(2) or fork(2)).