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  11 .TH EPOLL 5 "Apr 17, 2014"
  12 .SH NAME
  13 epoll \- Linux-compatible I/O event notification facility
  14 .SH SYNOPSIS
  15 
  16 .LP
  17 .nf
  18 #include <sys/epoll.h>
  19 .fi
  20 
  21 .SH DESCRIPTION
  22 .LP
  23 
  24 \fBepoll\fR is a facility for efficient event-oriented I/O that has a
  25 similar model to \fBpoll\fR(2), but does not necessitate rescanning a
  26 set of file descriptors to wait for an event.  \fBepoll\fR is of Linux
  27 origins, and this facility is designed to be binary-compatible with
  28 the Linux facility, including the following interfaces:
  29 
  30 .RS +4
  31 .TP
  32 .ie t \(bu
  33 .el o
  34 \fBepoll_create\fR(3C) creates an \fBepoll\fR instance, returning a file
  35 descriptor. It contains a size arugment which is meaningful only in as
  36 much as it cannot be 0.
  37 .RE
  38 .RS +4
  39 .TP
  40 .ie t \(bu
  41 .el o
  42 \fBepoll_create1\fR(3C) also creates an \fBepoll\fR instance, but eliminates
  43 the meaningless size argument -- replacing it instead with a flags
  44 argument.
  45 .RE
  46 .RS +4
  47 .TP
  48 .ie t \(bu
  49 .el o
  50 \fBepoll_ctl\fR(3C) allows file descriptors to be added
  51 (via \fBEPOLL_CTL_ADD\fR), deleted (via \fBEPOLL_CTL_DEL\fR) or
  52 modified (via \fBEPOLL_CTL_MOD\fR) with respect to the \fBepoll\fR'd set
  53 of file descriptors.
  54 .RE
  55 .RS +4
  56 .TP
  57 .ie t \(bu
  58 .el o
  59 \fBepoll_wait\fR(3C) fetches pending events for file descriptors added
  60 via \fBepoll_ctl\fR(3C), blocking the caller if no such events are pending.
  61 .RE
  62 .RS +4
  63 .TP
  64 .ie t \(bu
  65 .el o
  66 \fBepoll_pwait\fR(3C) opeates in a similar manner to \fBepoll_wait\fR(3C), but
  67 allows the caller to specify a signal mask to be set atomically with respect
  68 to waiting for events.
  69 .RE
  70 
  71 .sp
  72 .SH NOTES
  73 .LP
  74 
  75 The \fBepoll\fR facility is implemented
  76 for purposes of offering compatibility to and portability of Linux-borne
  77 applications; native applications should continue to prefer using event ports
  78 via the \fBport_create\fR(3C),
  79 \fBport_associate\fR(3C) and \fBport_getn\fR(3C) interfaces.
  80 In particular, use of \fBepoll\fR in a multithreaded environment is fraught
  81 with peril; even when using \fBEPOLLONESHOT\fR for one-shot events,
  82 there are race conditions with respect to \fBclose\fR(2) that are unresolvable.
  83 (For more details, see the aborted effort in Linux to resolve this via the
  84 proposed
  85 \fBEPOLL_CTL_DISABLE\fR operation.)
  86 The event port facility -- like the BSD kqueue facility that inspired it --
  87 is designed to deal with such issues via explicit event source dissociation.
  88 
  89 While a best effort has been made to mimic the Linux semantics, there
  90 are some semantics that are too peculiar or ill-conceived to merit
  91 accommodation.  In particular, the Linux \fBepoll\fR facility will -- by
  92 design -- continue to generate events for closed file descriptors where/when
  93 the underlying file description remains open.  For example, if one were
  94 to \fBfork\fR(2) and subsequently close an actively \fBepoll\fR'd file
  95 descriptor in the parent,
  96 any events generated in the child on the implicitly duplicated file descriptor
  97 will continue to be delivered to the parent -- despite the fact that the
  98 parent itself no longer has any notion of the file description!
  99 This \fBepoll\fR facility refuses to honor
 100 these semantics; closing the \fBEPOLL_CTL_ADD\fR'd file descriptor
 101 will always result in no further
 102 events being generated for that event description.
 103 
 104 .SH SEE ALSO
 105 .LP
 106 \fBepoll_create\fR(3C), \fBepoll_create1\fR(3C), \fBepoll_ctl\fR(3C),
 107 \fBepoll_wait\fR(3C), \fBepoll_pwait\fR(3C),
 108 \fBport_create\fR(3C), \fBport_associate\fR(3C), \fBport_dissociate\fR(3C),
 109 \fBport_get\fR(3C),
 110 \fBpselect\fR(3C)