Annotation of embedaddon/libevent/event.3, revision 1.1

1.1     ! misho       1: .\"    $OpenBSD: event.3,v 1.4 2002/07/12 18:50:48 provos Exp $
        !             2: .\"
        !             3: .\" Copyright (c) 2000 Artur Grabowski <art@openbsd.org>
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        !            29: .Dd August 8, 2000
        !            30: .Dt EVENT 3
        !            31: .Os
        !            32: .Sh NAME
        !            33: .Nm event_init ,
        !            34: .Nm event_dispatch ,
        !            35: .Nm event_loop ,
        !            36: .Nm event_loopexit ,
        !            37: .Nm event_loopbreak ,
        !            38: .Nm event_set ,
        !            39: .Nm event_base_dispatch ,
        !            40: .Nm event_base_loop ,
        !            41: .Nm event_base_loopexit ,
        !            42: .Nm event_base_loopbreak ,
        !            43: .Nm event_base_set ,
        !            44: .Nm event_base_free ,
        !            45: .Nm event_add ,
        !            46: .Nm event_del ,
        !            47: .Nm event_once ,
        !            48: .Nm event_base_once ,
        !            49: .Nm event_pending ,
        !            50: .Nm event_initialized ,
        !            51: .Nm event_priority_init ,
        !            52: .Nm event_priority_set ,
        !            53: .Nm evtimer_set ,
        !            54: .Nm evtimer_add ,
        !            55: .Nm evtimer_del ,
        !            56: .Nm evtimer_pending ,
        !            57: .Nm evtimer_initialized ,
        !            58: .Nm signal_set ,
        !            59: .Nm signal_add ,
        !            60: .Nm signal_del ,
        !            61: .Nm signal_pending ,
        !            62: .Nm signal_initialized ,
        !            63: .Nm bufferevent_new ,
        !            64: .Nm bufferevent_free ,
        !            65: .Nm bufferevent_write ,
        !            66: .Nm bufferevent_write_buffer ,
        !            67: .Nm bufferevent_read ,
        !            68: .Nm bufferevent_enable ,
        !            69: .Nm bufferevent_disable ,
        !            70: .Nm bufferevent_settimeout ,
        !            71: .Nm bufferevent_base_set ,
        !            72: .Nm evbuffer_new ,
        !            73: .Nm evbuffer_free ,
        !            74: .Nm evbuffer_add ,
        !            75: .Nm evbuffer_add_buffer ,
        !            76: .Nm evbuffer_add_printf ,
        !            77: .Nm evbuffer_add_vprintf ,
        !            78: .Nm evbuffer_drain ,
        !            79: .Nm evbuffer_write ,
        !            80: .Nm evbuffer_read ,
        !            81: .Nm evbuffer_find ,
        !            82: .Nm evbuffer_readline ,
        !            83: .Nm evhttp_new ,
        !            84: .Nm evhttp_bind_socket ,
        !            85: .Nm evhttp_free
        !            86: .Nd execute a function when a specific event occurs
        !            87: .Sh SYNOPSIS
        !            88: .Fd #include <sys/time.h>
        !            89: .Fd #include <event.h>
        !            90: .Ft "struct event_base *"
        !            91: .Fn "event_init" "void"
        !            92: .Ft int
        !            93: .Fn "event_dispatch" "void"
        !            94: .Ft int
        !            95: .Fn "event_loop" "int flags"
        !            96: .Ft int
        !            97: .Fn "event_loopexit" "struct timeval *tv"
        !            98: .Ft int
        !            99: .Fn "event_loopbreak" "void"
        !           100: .Ft void
        !           101: .Fn "event_set" "struct event *ev" "int fd" "short event" "void (*fn)(int, short, void *)" "void *arg"
        !           102: .Ft int
        !           103: .Fn "event_base_dispatch" "struct event_base *base"
        !           104: .Ft int
        !           105: .Fn "event_base_loop" "struct event_base *base" "int flags"
        !           106: .Ft int
        !           107: .Fn "event_base_loopexit" "struct event_base *base" "struct timeval *tv"
        !           108: .Ft int
        !           109: .Fn "event_base_loopbreak" "struct event_base *base"
        !           110: .Ft int
        !           111: .Fn "event_base_set" "struct event_base *base" "struct event *"
        !           112: .Ft void
        !           113: .Fn "event_base_free" "struct event_base *base"
        !           114: .Ft int
        !           115: .Fn "event_add" "struct event *ev" "struct timeval *tv"
        !           116: .Ft int
        !           117: .Fn "event_del" "struct event *ev"
        !           118: .Ft int
        !           119: .Fn "event_once" "int fd" "short event" "void (*fn)(int, short, void *)" "void *arg" "struct timeval *tv"
        !           120: .Ft int
        !           121: .Fn "event_base_once" "struct event_base *base" "int fd" "short event" "void (*fn)(int, short, void *)" "void *arg" "struct timeval *tv"
        !           122: .Ft int
        !           123: .Fn "event_pending" "struct event *ev" "short event" "struct timeval *tv"
        !           124: .Ft int
        !           125: .Fn "event_initialized" "struct event *ev"
        !           126: .Ft int
        !           127: .Fn "event_priority_init" "int npriorities"
        !           128: .Ft int
        !           129: .Fn "event_priority_set" "struct event *ev" "int priority"
        !           130: .Ft void
        !           131: .Fn "evtimer_set" "struct event *ev" "void (*fn)(int, short, void *)" "void *arg"
        !           132: .Ft void
        !           133: .Fn "evtimer_add" "struct event *ev" "struct timeval *"
        !           134: .Ft void
        !           135: .Fn "evtimer_del" "struct event *ev"
        !           136: .Ft int
        !           137: .Fn "evtimer_pending" "struct event *ev" "struct timeval *tv"
        !           138: .Ft int
        !           139: .Fn "evtimer_initialized" "struct event *ev"
        !           140: .Ft void
        !           141: .Fn "signal_set" "struct event *ev" "int signal" "void (*fn)(int, short, void *)" "void *arg"
        !           142: .Ft void
        !           143: .Fn "signal_add" "struct event *ev" "struct timeval *"
        !           144: .Ft void
        !           145: .Fn "signal_del" "struct event *ev"
        !           146: .Ft int
        !           147: .Fn "signal_pending" "struct event *ev" "struct timeval *tv"
        !           148: .Ft int
        !           149: .Fn "signal_initialized" "struct event *ev"
        !           150: .Ft "struct bufferevent *"
        !           151: .Fn "bufferevent_new" "int fd" "evbuffercb readcb" "evbuffercb writecb" "everrorcb" "void *cbarg"
        !           152: .Ft void
        !           153: .Fn "bufferevent_free" "struct bufferevent *bufev"
        !           154: .Ft int
        !           155: .Fn "bufferevent_write" "struct bufferevent *bufev" "void *data" "size_t size"
        !           156: .Ft int
        !           157: .Fn "bufferevent_write_buffer" "struct bufferevent *bufev" "struct evbuffer *buf"
        !           158: .Ft size_t
        !           159: .Fn "bufferevent_read" "struct bufferevent *bufev" "void *data" "size_t size"
        !           160: .Ft int
        !           161: .Fn "bufferevent_enable" "struct bufferevent *bufev" "short event"
        !           162: .Ft int
        !           163: .Fn "bufferevent_disable" "struct bufferevent *bufev" "short event"
        !           164: .Ft void
        !           165: .Fn "bufferevent_settimeout" "struct bufferevent *bufev" "int timeout_read" "int timeout_write"
        !           166: .Ft int
        !           167: .Fn "bufferevent_base_set" "struct event_base *base" "struct bufferevent *bufev"
        !           168: .Ft "struct evbuffer *"
        !           169: .Fn "evbuffer_new" "void"
        !           170: .Ft void
        !           171: .Fn "evbuffer_free" "struct evbuffer *buf"
        !           172: .Ft int
        !           173: .Fn "evbuffer_add" "struct evbuffer *buf" "const void *data" "size_t size"
        !           174: .Ft int
        !           175: .Fn "evbuffer_add_buffer" "struct evbuffer *dst" "struct evbuffer *src"
        !           176: .Ft int
        !           177: .Fn "evbuffer_add_printf" "struct evbuffer *buf" "const char *fmt" "..."
        !           178: .Ft int
        !           179: .Fn "evbuffer_add_vprintf" "struct evbuffer *buf" "const char *fmt" "va_list ap"
        !           180: .Ft void
        !           181: .Fn "evbuffer_drain" "struct evbuffer *buf" "size_t size"
        !           182: .Ft int
        !           183: .Fn "evbuffer_write" "struct evbuffer *buf" "int fd"
        !           184: .Ft int
        !           185: .Fn "evbuffer_read" "struct evbuffer *buf" "int fd" "int size"
        !           186: .Ft "u_char *"
        !           187: .Fn "evbuffer_find" "struct evbuffer *buf" "const u_char *data" "size_t size"
        !           188: .Ft "char *"
        !           189: .Fn "evbuffer_readline" "struct evbuffer *buf"
        !           190: .Ft "struct evhttp *"
        !           191: .Fn "evhttp_new" "struct event_base *base"
        !           192: .Ft int
        !           193: .Fn "evhttp_bind_socket" "struct evhttp *http" "const char *address" "u_short port"
        !           194: .Ft "void"
        !           195: .Fn "evhttp_free" "struct evhttp *http"
        !           196: .Ft int
        !           197: .Fa (*event_sigcb)(void) ;
        !           198: .Ft volatile sig_atomic_t
        !           199: .Fa event_gotsig ;
        !           200: .Sh DESCRIPTION
        !           201: The
        !           202: .Nm event
        !           203: API provides a mechanism to execute a function when a specific event
        !           204: on a file descriptor occurs or after a given time has passed.
        !           205: .Pp
        !           206: The
        !           207: .Nm event
        !           208: API needs to be initialized with
        !           209: .Fn event_init
        !           210: before it can be used.
        !           211: .Pp
        !           212: In order to process events, an application needs to call
        !           213: .Fn event_dispatch .
        !           214: This function only returns on error, and should replace the event core
        !           215: of the application program.
        !           216: .Pp
        !           217: The function
        !           218: .Fn event_set
        !           219: prepares the event structure
        !           220: .Fa ev
        !           221: to be used in future calls to
        !           222: .Fn event_add
        !           223: and
        !           224: .Fn event_del .
        !           225: The event will be prepared to call the function specified by the
        !           226: .Fa fn
        !           227: argument with an
        !           228: .Fa int
        !           229: argument indicating the file descriptor, a
        !           230: .Fa short
        !           231: argument indicating the type of event, and a
        !           232: .Fa void *
        !           233: argument given in the
        !           234: .Fa arg
        !           235: argument.
        !           236: The
        !           237: .Fa fd
        !           238: indicates the file descriptor that should be monitored for events.
        !           239: The events can be either
        !           240: .Va EV_READ ,
        !           241: .Va EV_WRITE ,
        !           242: or both,
        !           243: indicating that an application can read or write from the file descriptor
        !           244: respectively without blocking.
        !           245: .Pp
        !           246: The function
        !           247: .Fa fn
        !           248: will be called with the file descriptor that triggered the event and
        !           249: the type of event which will be either
        !           250: .Va EV_TIMEOUT ,
        !           251: .Va EV_SIGNAL ,
        !           252: .Va EV_READ ,
        !           253: or
        !           254: .Va EV_WRITE .
        !           255: Additionally, an event which has registered interest in more than one of the
        !           256: preceeding events, via bitwise-OR to
        !           257: .Fn event_set ,
        !           258: can provide its callback function with a bitwise-OR of more than one triggered
        !           259: event.
        !           260: The additional flag
        !           261: .Va EV_PERSIST
        !           262: makes an
        !           263: .Fn event_add
        !           264: persistent until
        !           265: .Fn event_del
        !           266: has been called.
        !           267: .Pp
        !           268: Once initialized, the
        !           269: .Fa ev
        !           270: structure can be used repeatedly with
        !           271: .Fn event_add
        !           272: and
        !           273: .Fn event_del
        !           274: and does not need to be reinitialized unless the function called and/or
        !           275: the argument to it are to be changed.
        !           276: However, when an
        !           277: .Fa ev
        !           278: structure has been added to libevent using
        !           279: .Fn event_add
        !           280: the structure must persist until the event occurs (assuming
        !           281: .Fa EV_PERSIST
        !           282: is not set) or is removed
        !           283: using
        !           284: .Fn event_del .
        !           285: You may not reuse the same
        !           286: .Fa ev
        !           287: structure for multiple monitored descriptors; each descriptor
        !           288: needs its own
        !           289: .Fa ev .
        !           290: .Pp
        !           291: The function
        !           292: .Fn event_add
        !           293: schedules the execution of the
        !           294: .Fa ev
        !           295: event when the event specified in
        !           296: .Fn event_set
        !           297: occurs or in at least the time specified in the
        !           298: .Fa tv .
        !           299: If
        !           300: .Fa tv
        !           301: is
        !           302: .Dv NULL ,
        !           303: no timeout occurs and the function will only be called
        !           304: if a matching event occurs on the file descriptor.
        !           305: The event in the
        !           306: .Fa ev
        !           307: argument must be already initialized by
        !           308: .Fn event_set
        !           309: and may not be used in calls to
        !           310: .Fn event_set
        !           311: until it has timed out or been removed with
        !           312: .Fn event_del .
        !           313: If the event in the
        !           314: .Fa ev
        !           315: argument already has a scheduled timeout, the old timeout will be
        !           316: replaced by the new one.
        !           317: .Pp
        !           318: The function
        !           319: .Fn event_del
        !           320: will cancel the event in the argument
        !           321: .Fa ev .
        !           322: If the event has already executed or has never been added
        !           323: the call will have no effect.
        !           324: .Pp
        !           325: The functions
        !           326: .Fn evtimer_set ,
        !           327: .Fn evtimer_add ,
        !           328: .Fn evtimer_del ,
        !           329: .Fn evtimer_initialized ,
        !           330: and
        !           331: .Fn evtimer_pending
        !           332: are abbreviations for common situations where only a timeout is required.
        !           333: The file descriptor passed will be \-1, and the event type will be
        !           334: .Va EV_TIMEOUT .
        !           335: .Pp
        !           336: The functions
        !           337: .Fn signal_set ,
        !           338: .Fn signal_add ,
        !           339: .Fn signal_del ,
        !           340: .Fn signal_initialized ,
        !           341: and
        !           342: .Fn signal_pending
        !           343: are abbreviations.
        !           344: The event type will be a persistent
        !           345: .Va EV_SIGNAL .
        !           346: That means
        !           347: .Fn signal_set
        !           348: adds
        !           349: .Va EV_PERSIST .
        !           350: .Pp
        !           351: In order to avoid races in signal handlers, the
        !           352: .Nm event
        !           353: API provides two variables:
        !           354: .Va event_sigcb
        !           355: and
        !           356: .Va event_gotsig .
        !           357: A signal handler
        !           358: sets
        !           359: .Va event_gotsig
        !           360: to indicate that a signal has been received.
        !           361: The application sets
        !           362: .Va event_sigcb
        !           363: to a callback function.
        !           364: After the signal handler sets
        !           365: .Va event_gotsig ,
        !           366: .Nm event_dispatch
        !           367: will execute the callback function to process received signals.
        !           368: The callback returns 1 when no events are registered any more.
        !           369: It can return \-1 to indicate an error to the
        !           370: .Nm event
        !           371: library, causing
        !           372: .Fn event_dispatch
        !           373: to terminate with
        !           374: .Va errno
        !           375: set to
        !           376: .Er EINTR .
        !           377: .Pp
        !           378: The function
        !           379: .Fn event_once
        !           380: is similar to
        !           381: .Fn event_set .
        !           382: However, it schedules a callback to be called exactly once and does not
        !           383: require the caller to prepare an
        !           384: .Fa event
        !           385: structure.
        !           386: This function supports
        !           387: .Fa EV_TIMEOUT ,
        !           388: .Fa EV_READ ,
        !           389: and
        !           390: .Fa EV_WRITE .
        !           391: .Pp
        !           392: The
        !           393: .Fn event_pending
        !           394: function can be used to check if the event specified by
        !           395: .Fa event
        !           396: is pending to run.
        !           397: If
        !           398: .Va EV_TIMEOUT
        !           399: was specified and
        !           400: .Fa tv
        !           401: is not
        !           402: .Dv NULL ,
        !           403: the expiration time of the event will be returned in
        !           404: .Fa tv .
        !           405: .Pp
        !           406: The
        !           407: .Fn event_initialized
        !           408: macro can be used to check if an event has been initialized.
        !           409: .Pp
        !           410: The
        !           411: .Nm event_loop
        !           412: function provides an interface for single pass execution of pending
        !           413: events.
        !           414: The flags
        !           415: .Va EVLOOP_ONCE
        !           416: and
        !           417: .Va EVLOOP_NONBLOCK
        !           418: are recognized.
        !           419: The
        !           420: .Nm event_loopexit
        !           421: function exits from the event loop. The next
        !           422: .Fn event_loop
        !           423: iteration after the
        !           424: given timer expires will complete normally (handling all queued events) then
        !           425: exit without blocking for events again. Subsequent invocations of
        !           426: .Fn event_loop
        !           427: will proceed normally.
        !           428: The
        !           429: .Nm event_loopbreak
        !           430: function exits from the event loop immediately.
        !           431: .Fn event_loop
        !           432: will abort after the next event is completed;
        !           433: .Fn event_loopbreak
        !           434: is typically invoked from this event's callback. This behavior is analogous
        !           435: to the "break;" statement. Subsequent invocations of
        !           436: .Fn event_loop
        !           437: will proceed normally.
        !           438: .Pp
        !           439: It is the responsibility of the caller to provide these functions with
        !           440: pre-allocated event structures.
        !           441: .Pp
        !           442: .Sh EVENT PRIORITIES
        !           443: By default
        !           444: .Nm libevent
        !           445: schedules all active events with the same priority.
        !           446: However, sometimes it is desirable to process some events with a higher
        !           447: priority than others.
        !           448: For that reason,
        !           449: .Nm libevent
        !           450: supports strict priority queues.
        !           451: Active events with a lower priority are always processed before events
        !           452: with a higher priority.
        !           453: .Pp
        !           454: The number of different priorities can be set initially with the
        !           455: .Fn event_priority_init
        !           456: function.
        !           457: This function should be called before the first call to
        !           458: .Fn event_dispatch .
        !           459: The
        !           460: .Fn event_priority_set
        !           461: function can be used to assign a priority to an event.
        !           462: By default,
        !           463: .Nm libevent
        !           464: assigns the middle priority to all events unless their priority
        !           465: is explicitly set.
        !           466: .Sh THREAD SAFE EVENTS
        !           467: .Nm Libevent
        !           468: has experimental support for thread-safe events.
        !           469: When initializing the library via
        !           470: .Fn event_init ,
        !           471: an event base is returned.
        !           472: This event base can be used in conjunction with calls to
        !           473: .Fn event_base_set ,
        !           474: .Fn event_base_dispatch ,
        !           475: .Fn event_base_loop ,
        !           476: .Fn event_base_loopexit ,
        !           477: .Fn bufferevent_base_set
        !           478: and
        !           479: .Fn event_base_free .
        !           480: .Fn event_base_set
        !           481: should be called after preparing an event with
        !           482: .Fn event_set ,
        !           483: as
        !           484: .Fn event_set
        !           485: assigns the provided event to the most recently created event base.
        !           486: .Fn bufferevent_base_set
        !           487: should be called after preparing a bufferevent with
        !           488: .Fn bufferevent_new .
        !           489: .Fn event_base_free
        !           490: should be used to free memory associated with the event base
        !           491: when it is no longer needed.
        !           492: .Sh BUFFERED EVENTS
        !           493: .Nm libevent
        !           494: provides an abstraction on top of the regular event callbacks.
        !           495: This abstraction is called a
        !           496: .Va "buffered event" .
        !           497: A buffered event provides input and output buffers that get filled
        !           498: and drained automatically.
        !           499: The user of a buffered event no longer deals directly with the IO,
        !           500: but instead is reading from input and writing to output buffers.
        !           501: .Pp
        !           502: A new bufferevent is created by
        !           503: .Fn bufferevent_new .
        !           504: The parameter
        !           505: .Fa fd
        !           506: specifies the file descriptor from which data is read and written to.
        !           507: This file descriptor is not allowed to be a
        !           508: .Xr pipe 2 .
        !           509: The next three parameters are callbacks.
        !           510: The read and write callback have the following form:
        !           511: .Ft void
        !           512: .Fn "(*cb)" "struct bufferevent *bufev" "void *arg" .
        !           513: The error callback has the following form:
        !           514: .Ft void
        !           515: .Fn "(*cb)" "struct bufferevent *bufev" "short what" "void *arg" .
        !           516: The argument is specified by the fourth parameter
        !           517: .Fa "cbarg" .
        !           518: A
        !           519: .Fa bufferevent struct
        !           520: pointer is returned on success, NULL on error.
        !           521: Both the read and the write callback may be NULL.
        !           522: The error callback has to be always provided.
        !           523: .Pp
        !           524: Once initialized, the bufferevent structure can be used repeatedly with
        !           525: bufferevent_enable() and bufferevent_disable().
        !           526: The flags parameter can be a combination of
        !           527: .Va EV_READ
        !           528: and
        !           529: .Va EV_WRITE .
        !           530: When read enabled the bufferevent will try to read from the file
        !           531: descriptor and call the read callback.
        !           532: The write callback is executed
        !           533: whenever the output buffer is drained below the write low watermark,
        !           534: which is
        !           535: .Va 0
        !           536: by default.
        !           537: .Pp
        !           538: The
        !           539: .Fn bufferevent_write
        !           540: function can be used to write data to the file descriptor.
        !           541: The data is appended to the output buffer and written to the descriptor
        !           542: automatically as it becomes available for writing.
        !           543: .Fn bufferevent_write
        !           544: returns 0 on success or \-1 on failure.
        !           545: The
        !           546: .Fn bufferevent_read
        !           547: function is used to read data from the input buffer,
        !           548: returning the amount of data read.
        !           549: .Pp
        !           550: If multiple bases are in use, bufferevent_base_set() must be called before
        !           551: enabling the bufferevent for the first time.
        !           552: .Sh NON-BLOCKING HTTP SUPPORT
        !           553: .Nm libevent
        !           554: provides a very thin HTTP layer that can be used both to host an HTTP
        !           555: server and also to make HTTP requests.
        !           556: An HTTP server can be created by calling
        !           557: .Fn evhttp_new .
        !           558: It can be bound to any port and address with the
        !           559: .Fn evhttp_bind_socket
        !           560: function.
        !           561: When the HTTP server is no longer used, it can be freed via
        !           562: .Fn evhttp_free .
        !           563: .Pp
        !           564: To be notified of HTTP requests, a user needs to register callbacks with the
        !           565: HTTP server.
        !           566: This can be done by calling
        !           567: .Fn evhttp_set_cb .
        !           568: The second argument is the URI for which a callback is being registered.
        !           569: The corresponding callback will receive an
        !           570: .Va struct evhttp_request
        !           571: object that contains all information about the request.
        !           572: .Pp
        !           573: This section does not document all the possible function calls; please
        !           574: check
        !           575: .Va event.h
        !           576: for the public interfaces.
        !           577: .Sh ADDITIONAL NOTES
        !           578: It is possible to disable support for
        !           579: .Va epoll , kqueue , devpoll , poll
        !           580: or
        !           581: .Va select
        !           582: by setting the environment variable
        !           583: .Va EVENT_NOEPOLL , EVENT_NOKQUEUE , EVENT_NODEVPOLL , EVENT_NOPOLL
        !           584: or
        !           585: .Va EVENT_NOSELECT ,
        !           586: respectively.
        !           587: By setting the environment variable
        !           588: .Va EVENT_SHOW_METHOD ,
        !           589: .Nm libevent
        !           590: displays the kernel notification method that it uses.
        !           591: .Sh RETURN VALUES
        !           592: Upon successful completion
        !           593: .Fn event_add
        !           594: and
        !           595: .Fn event_del
        !           596: return 0.
        !           597: Otherwise, \-1 is returned and the global variable errno is
        !           598: set to indicate the error.
        !           599: .Sh SEE ALSO
        !           600: .Xr kqueue 2 ,
        !           601: .Xr poll 2 ,
        !           602: .Xr select 2 ,
        !           603: .Xr evdns 3 ,
        !           604: .Xr timeout 9
        !           605: .Sh HISTORY
        !           606: The
        !           607: .Nm event
        !           608: API manpage is based on the
        !           609: .Xr timeout 9
        !           610: manpage by Artur Grabowski.
        !           611: The port of
        !           612: .Nm libevent
        !           613: to Windows is due to Michael A. Davis.
        !           614: Support for real-time signals is due to Taral.
        !           615: .Sh AUTHORS
        !           616: The
        !           617: .Nm event
        !           618: library was written by Niels Provos.
        !           619: .Sh BUGS
        !           620: This documentation is neither complete nor authoritative.
        !           621: If you are in doubt about the usage of this API then
        !           622: check the source code to find out how it works, write
        !           623: up the missing piece of documentation and send it to
        !           624: me for inclusion in this man page.

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