epoll - Linux-compatible I/O event notification facility
epoll is a facility for efficient event-oriented I/O that has a similar
model to poll(2), but does not necessitate rescanning a set of file
descriptors to wait for an event. epoll is of Linux origins, and this
facility is designed to be binary-compatible with the Linux facility,
including the following interfaces:
- o
- epoll_create(3C) creates an epoll instance, returning a file
descriptor. It contains a size arugment which is meaningful only in as
much as it cannot be 0.
- o
- epoll_create1(3C) also creates an epoll instance, but
eliminates the meaningless size argument -- replacing it instead with a
flags argument.
- o
- epoll_ctl(3C) allows file descriptors to be added (via
EPOLL_CTL_ADD), deleted (via EPOLL_CTL_DEL) or modified (via
EPOLL_CTL_MOD) with respect to the epoll'd set of file
descriptors.
- o
- epoll_wait(3C) fetches pending events for file descriptors added
via epoll_ctl(3C), blocking the caller if no such events are
pending.
- o
- epoll_pwait(3C) opeates in a similar manner to
epoll_wait(3C), but allows the caller to specify a signal mask to
be set atomically with respect to waiting for events.
The epoll facility is implemented for purposes of offering compatibility
to and portability of Linux-borne applications; native applications should
continue to prefer using event ports via the port_create(3C),
port_associate(3C) and port_getn(3C) interfaces. In particular,
use of epoll in a multithreaded environment is fraught with peril; even
when using EPOLLONESHOT for one-shot events, there are race conditions
with respect to close(2) that are unresolvable. (For more details, see
the aborted effort in Linux to resolve this via the proposed
EPOLL_CTL_DISABLE operation.) The event port facility -- like the BSD
kqueue facility that inspired it -- is designed to deal with such issues via
explicit event source dissociation.
While a best effort has been made to mimic the Linux semantics,
there are some semantics that are too peculiar or ill-conceived to merit
accommodation. In particular, the Linux epoll facility will -- by
design -- continue to generate events for closed file descriptors where/when
the underlying file description remains open. For example, if one were to
fork(2) and subsequently close an actively epoll'd file
descriptor in the parent, any events generated in the child on the
implicitly duplicated file descriptor will continue to be delivered to the
parent -- despite the fact that the parent itself no longer has any notion
of the file description! This epoll facility refuses to honor these
semantics; closing the EPOLL_CTL_ADD'd file descriptor will always
result in no further events being generated for that event description.
epoll_create(3C), epoll_create1(3C), epoll_ctl(3C),
epoll_wait(3C), epoll_pwait(3C), port_create(3C),
port_associate(3C), port_dissociate(3C), port_get(3C),
pselect(3C)