ircu2.10.12 pk910 fork

[ircu2.10.12-pk.git] / doc / api / events.txt

The IRC server is built around an event loop.  Until the u2.10.11
release, this event loop has been rather ad-hoc; timed events are
hard-coded in, signals are handled inside the signal handler, etc.
All of this has changed with u2.10.11.  A new subsystem, the events
subsystem, has been introduced; the new subsystem contains a
generalization of the concept of an event.  An event is a signal, the
expiration of a timer, or some form of activity on a network socket.
This new subsystem has the potential to vastly simplify the code that
is arguably the core of any network program, and makes it much simpler
to support more exotic forms of network activity monitoring than the
conventional select() and poll() calls.

The primary concepts that the events subsystem works with are the
"event," represented by a struct Event, and the "generator."  There
are three types of generators: sockets, represented by struct Socket;
signals, represented by struct Signal; and timers, represented by
struct Timer.  Each of these generators will be described in turn.

Signals

The signal is perhaps the simplest generator in the entire events
subsystem.  Basically, instead of setting a signal handler, the
function signal_add() is called, specifying a function to be called
when a given signal is detected.  Most importantly, that call-back
function is called _outside_ the context of a signal handler,
permitting the call-back to use more exotic functions that are
anathema within a signal handler, such as MyMalloc().  Once a
call-back for a signal has been established, it cannot be deleted;
this design decision was driven by the fact that ircd never changes
its signal handlers.

Whenever a signal is received, an event of type ET_SIGNAL is
generated, and that event is passed to the event call-back function
specified in the signal_add() call.

Timers

Execution of the call-back functions for a timer occur when that timer
_expires_; when a timer expires depends on the type of timer and the
expiration time that was used for that timer.  A TT_ABSOLUTE timer,
for instance, expires at exactly the time given as the expiration
time.  This time is a standard UNIX time_t value, measuring seconds
since the UNIX epoch.  The TT_ABSOLUTE timer type is complemented by
the TT_RELATIVE timer; the time passed as its expiration time is
relative to the current time.  If a TT_RELATIVE timer is given an
expiration time of 5, for instance, it will expire 5 seconds after the
present time.  Internally, TT_RELATIVE timers are converted into
TT_ABSOLUTE timers, with the expiration time adjusted by addition of
the current time.

Those two types of timers, TT_ABSOLUTE and TT_RELATIVE, are
single-shot timers.  Once they expire, they are removed from the timer
list unless re-added by the event call-back or through some other
mechanism.  There is another type of timer, however, the TT_PERIODIC
timer, that is not removed from the timer list.  TT_PERIODIC timers
are similar to TT_RELATIVE timers, in that one passes in the expire
time as a relative number of seconds, but when they expire, they are
re-added to the timer list with the same relative expire time.  This
means that a TT_PERIODIC timer with an expire time of 5 seconds that
is set at 11:50:00 will have its call-back called at 11:50:05,
11:50:10, 11:50:15, and so on.

Timers have to be run by the event engines explicitly by calling
timer_run() on the generator list passed to the engine event loop.
In addition, engines may determine the next (absolute) time that a
timer needs to be run by calling the timer_next() macro; this may be
used to set a timeout on the engine's network activity monitoring
function.  Engines are described in detail below.

When a timer expires, an event of ET_EXPIRE is generated, and the
call-back function is called.  When a timer is destroyed, either as
the result of an expiration or as a result of an explicit timer_del()
call, an event of ET_DESTROY is generated, notifying the call-back
that the struct Timer can be deallocated.

Sockets

Perhaps the most complicated event generator in all of the event
system is the socket, as described by struct Socket.  This single
classification covers datagram sockets and stream sockets.  To
differentiate the different kinds of sockets, there is a socket state
associated with each socket.  The available states are SS_CONNECTING,
which indicates that a particular socket is in the process of
completing a non-blocking connect(); SS_LISTENING, which indicates
that a particular socket is a listening socket; SS_CONNECTED, which is
the state of every other stream socket; SS_DATAGRAM, which is an
ordinary datagram socket, and SS_CONNECTDG, which describes a
connected datagram socket.  (The SS_NOTSOCK state is for the internal
use of the events system and will not be described here.)

In addition to the socket states, there's also an event mask for each
socket; this set of flags is used to tell the events subsystem what
events the application is interested in for the socket.  For
SS_CONNECTING and SS_LISTENING sockets, this events mask has no
meaning, but on the other socket states, the event mask is used to
determine if the application is interested in readable
(SOCK_EVENT_READABLE) or writable (SOCK_EVENT_WRITABLE) indications.

Most of the defined event types have to do with socket generators.
When a socket turns up readable, for instance, an event of type
ET_READ is generated.  Similarly, ET_WRITE is generated when a socket
can be written to.  The ET_ACCEPT event is generated when a listening
socket indicates that there is a connection to be accepted; ET_CONNECT
is generated when a non-blocking connect is completed.  Finally, if an
end-of-file indication is detected, ET_EOF is generated, whereas if an
error has occurred on the socket, ET_ERROR is generated.  Of course,
when a socket has been deleted by the socket_del() function, an event
of ET_DESTROY is generated when it is safe for the memory used by the
struct Socket to be reclaimed.

Events

An event, represented by a struct Event, describes in detail all of
the particulars of an event.  Each event has a type, and an optional
integer piece of data may be passed with some events--in particular,
ET_SIGNAL events pass the signal number, and ET_ERROR events pass the
errno value.  The struct Event also contains a pointer to the
structure describing the generated event--although it should be noted
that the only way to disambiguate which type of generator is contained
within the struct Event is by which call-back function has been
called.

All generators have a void pointer which can be used to pass important
information to the call-back, such as a pointer to a struct Client.
Additionally, generators have a reference count, and a union of a void
pointer and an integer that should only be utilized by the event
engine.  Finally, there is also a field for flags, although the only
flag of concern to the application (or the engine) is the active flag,
which may be tested using the test macros described below.

Whatever the generator, the call-back function is a function returning
nothing (void) and taking as its sole argument a pointer to struct
Event.  This call-back function may be implemented as a single switch
statement that calls out to appropriate external functions as needed.

Engines

Engines implement the actual socket event loop, and may also have some
means of receiving signal events.  Each engine has a name, which
should describe what its core function is; for instance, the engine
based on the standard select() function is named, simply, "select()."
Each engine must implement several call-backs which are used to
initialize the engine, notify the engine of sockets the application is
interested in, etc.  All of this data is described by a single struct
Engine, which should be the only non-static variable or function in
the engine's source file.

The engine's event loop, pointed to by the eng_loop field of the
struct Engine, must consist of a single while loop predicated on the
global variable _running_.  Additionally, this loop's final statement
must be a call to timer_run(), to execute all timers that have become
due.  Ideally, this construction should be pulled out of each engine's
eng_loop and put in the event_loop() function of the events
subsystem.

Reference Counts

As mentioned previously, all generators keep a reference count.
Should timer_del() or socket_del() be called on a generator with a
non-zero reference count, for whatever reason, the actual destruction
of the generator will be delayed until the reference count again
reaches zero.  This is used by the event loop to keep sockets that it
is currently referencing from being deallocated before it is done
checking all pending events on them.  To increment the reference count
by one, call gen_ref_inc() on the generator; the corresponding macro
gen_ref_dec() decrements the reference counts, and will automatically
destroy the generator if the appropriate conditions are met.

Debugging Functions

It can be difficult to debug an engines if, say, a socket state can
only be expressed as a meaningless number.  Therefore, when DEBUGMODE
is #define'd, five number-to-name functions are also defined to make
the debugging data more meaningful.  These functions must only be
called when DEBUGMODE is #define'd.  Calling them from within Debug()
macro calls is safe; calling them from log_write() calls is not.

<typedef>
typedef void (*EventCallBack)(struct Event*);

The _EventCallBack_ type is used to simplify declaration of event
call-back functions.  It is used in timer_add(), signal_add(), and
socket_add().  The event call-back should process the event, taking
whatever actions are necessary.  The function should be declared as
returning void.
</typedef>

<typedef>
typedef int (*EngineInit)(int);

The _EngineInit_ function takes an integer specifying the maximum
number of sockets the event system is expecting to handle.  This
number may be used by the engine initialization function for memory
allocation computations.  If initialization succeeds, this function
must return 1.  If initialization fails, the function should clean up
after itself and return 0.  The events subsystem has the ability to
fall back upon another engine, should an engine initialization fail.
Needless to say, the engines based upon poll() and select() should
never fail in this way.
</typedef>

<typedef>
typedef void (*EngineSignal)(struct Signal*);

If an engine has the capability to directly detect signals, it should
set the eng_signal field of struct Engine non-zero.  When the
application indicates interest in a particular signal, the
_EngineSignal_ function will be called with the filled-in struct
Signal, in order to register interest in that signal with the engine.
</typedef>

<typedef>
typedef int (*EngineAdd)(struct Socket*);

All engines must define an _EngineAdd_ function, which is used to
inform the engine of the application's interest in the socket.  If the
new socket cannot be accommodated by the engine for whatever reason,
this function must return 0.  Otherwise, the function must return 1,
informing the events subsystem that the interest has been noted.
</typedef>

<typedef>
typedef void (*EngineState)(struct Socket*, enum SocketState new_state);

Sockets can change state.  SS_CONNECTING sockets, for instance, can
become SS_CONNECTED.  Whenever a socket state changes, the engine is
informed, since some states require different notification procedures
than others.  This is accomplished by calling the _EngineState_
function with the new state.  The struct Socket passed to the engine
will still have the old state, if the engine must reference that.
</typedef>

<typedef>
typedef void (*EngineEvents)(struct Socket*, unsigned int new_events);

Applications may only be interested in given events on a socket for a
limited time.  When the application's interest shifts, a new events
mask is set for the socket.  The engine is informed of this change by
a call to its _EngineEvents_ function.
</typedef>

<typedef>
typedef void (*EngineDelete)(struct Socket*);

Eventually, an application will close all the sockets it has opened.
When a socket is closed, and the corresponding struct Socket deleted
with a call to socket_del(), the _EngineDelete_ function will be
called to notify the engine of the change.
</typedef>

<typedef>
typedef void (*EngineLoop)(struct Generators*);

The workhorse of the entire events subsystem is the event loop,
implemented by each engine as the _EngineLoop_ function.  This
function is called with a single argument that may be passed to
timer_next() to calculate the next time a timer will expire.
</typedef>

<enum>
enum SocketState {
  SS_CONNECTING,        /* Connection in progress on socket */
  SS_LISTENING,         /* Socket is a listening socket */
  SS_CONNECTED,         /* Socket is a connected socket */
  SS_DATAGRAM,          /* Socket is a datagram socket */
  SS_CONNECTDG,         /* Socket is a connected datagram socket */
  SS_NOTSOCK            /* Socket isn't a socket at all */
};

This enumeration contains a list of all possible states a socket can
be in.  Applications should not use SS_NOTSOCK; engines should treat
it as a special socket state for non-sockets.  The only event that
should be watched for on a struct Socket in the SS_NOTSOCK state is
readability.  This socket state is used to implement the fall-back
signal event generation.
</enum>

<enum>
enum TimerType {
  TT_ABSOLUTE,          /* timer that runs at a specific time */
  TT_RELATIVE,          /* timer that runs so many seconds in the future */
  TT_PERIODIC           /* timer that runs periodically */
};

The three possible timer types are defined by the TimerType
enumeration.  More details can be found in the "Timers" sub-section.
</enum>

<enum>
enum EventType {
  ET_READ,              /* Readable event detected */
  ET_WRITE,             /* Writable event detected */
  ET_ACCEPT,            /* Connection can be accepted */
  ET_CONNECT,           /* Connection completed */
  ET_EOF,               /* End-of-file on connection */
  ET_ERROR,             /* Error condition detected */
  ET_SIGNAL,            /* A signal was received */
  ET_EXPIRE,            /* A timer expired */
  ET_DESTROY            /* The generator is being destroyed */
};

This enumeration contains all the types of events that can be
generated by the events subsystem.  The first 6 are generated by
socket generators, the next by signal generators, and the next by
timer generators.  ET_DESTROY is generated by both socket and timer
generators when the events subsystem is finished with the memory
allocated by both.
</enum>

<struct>
struct Socket;

This structure describes everything the events subsystem knows about a
given socket.  All of its fields may be accessed through the s_*
macros described below.
</struct>

<struct>
struct Timer;

The struct Timer structure describes everything the events subsystem
knows about a given timer.  Again, all of its fields may be accessed
through the t_* macros described below.
</struct>

<struct>
struct Signal;

Signal generators are described by a struct Signal.  All of the fields
of a struct Signal may be accessed by the sig_* macros described
below.
</struct>

<struct>
struct Event;

Each event is described by a struct Event.  Its fields may be examined
using the ev_* macros described below.
</struct>

<struct>
struct Generators;

Each engine is passed a list of all generators when the engine's
_EngineLoop_ function is called.  The only valid way to access this
structure is via the timer_next() function described below.
</struct>

<struct>
struct Engine {
  const char*   eng_name;       /* a name for the engine */
  EngineInit    eng_init;       /* initialize engine */
  EngineSignal  eng_signal;     /* express interest in a signal */
  EngineAdd     eng_add;        /* express interest in a socket */
  EngineState   eng_state;      /* mention a change in state to engine */
  EngineEvents  eng_events;     /* express interest in socket events */
  EngineDelete  eng_closing;    /* socket is being closed */
  EngineLoop    eng_loop;       /* actual event loop */
};

Each engine is described by the struct Engine structure.  Each engine
must define all of the functions described above except for the
_EngineSignal_ function, which is optional.
</struct>

<macro>
#define SOCK_EVENT_READABLE     0x0001  /* interested in readable */

The SOCK_EVENT_READABLE flag indicates to the engine that the
application is interested in readability on this particular socket.
</macro>

<macro>
#define SOCK_EVENT_WRITABLE     0x0002  /* interested in writable */

The SOCK_EVENT_WRITABLE flag indicates to the engine that the
application is interested in this socket being writable.
</macro>

<macro>
#define SOCK_EVENT_MASK         (SOCK_EVENT_READABLE | SOCK_EVENT_WRITABLE)

SOCK_EVENT_MASK may be used to extract only the event interest flags
from an event interest set.
</macro>

<macro>
#define SOCK_ACTION_SET         0x0000  /* set interest set as follows */

When socket_events() is called with a set of event interest flags and
SOCK_ACTION_SET, the socket's event interest flags are set to those
passed into socket_events().
</macro>

<macro>
#define SOCK_ACTION_ADD         0x1000  /* add to interest set */

When SOCK_ACTION_ADD is used in a call to socket_events(), the event
interest flags passed in are added to the existing event interest
flags for the socket.
</macro>

<macro>
#define SOCK_ACTION_DEL         0x2000  /* remove from interest set */

When SOCK_ACTION_DEL is used in a call to socket_events(), the event
interest flags passed in are removed from the existing event interest
flags for the socket.
</macro>

<macro>
#define SOCK_ACTION_MASK        0x3000  /* mask out the actions */

SOCK_ACTION_MASK is used to isolate the socket action desired.
</macro>

<function>
enum SocketState s_state(struct Socket* sock);

This macro returns the state of the given socket.
</function>

<function>
unsigned int s_events(struct Socket* sock);

This macro returns the current event interest mask for a given
socket.  Note that if the socket is in the SS_CONNECTING or
SS_LISTENING states, this mask has no meaning.
</function>

<function>
int s_fd(struct Socket* sock);

This macro simply returns the file descriptor for the given socket.
</function>

<function>
void* s_data(struct Socket* sock);

When a struct Socket is initialized, data that the call-back function
may find useful, such as a pointer to a struct Connection, is stored
in the struct Socket.  This macro returns that pointer.
</function>

<function>
int s_ed_int(struct Socket* sock);

Engines may find it convenient to associate an integer with a struct
Socket.  This macro may be used to retrieve that integer or, when used
as an lvalue, to assign a value to it.  Engine data must be either an
int or a void*; use of both is prohibited.
</function>

<function>
void* s_ed_ptr(struct Socket* sock);

Engines may find it convenient to associate a void* pointer with a
struct Socket.  This macro may be used to retrieve that pointer or,
when used as an lvalue, to assign a value to it.  Engine data must be
either an int or a void*; use of both is prohibited.
</function>

<function>
int s_active(struct Socket* sock);

A socket's active flag is set when initialized by socket_add(), and is
cleared immediately prior to generating an event of type ET_DESTROY.
This may be used by the application to determine whether or not the
socket is still in use by the events subsystem.  If it is, s_active()
returns a non-zero value; otherwise, its value is 0.
</function>

<function>
int socket_add(struct Socket* sock, EventCallBack call, void* data,
               enum SocketState state, unsigned int events, int fd);

This function is called to add a socket to the list of sockets to be
monitored.  The _sock_ parameter is a pointer to a struct Socket that
is allocated by the application.  The _call_ parameter is a pointer to
a function to process any events on the socket.  The _data_ parameter
is for use of the socket call-back and may be zero.  The _state_
parameter must be one of the valid socket states.  The _events_
parameter must be a valid events interest mask--0, or the binary OR of
SOCK_EVENT_READABLE or SOCK_EVENT_WRITABLE.  Finally, the _fd_
parameter specifies the socket's file descriptor.  This function
returns 1 if successful or 0 otherwise.
</function>

<function>
void socket_del(struct Socket* sock);

When the application is no longer interested in a particular socket,
it should call the socket_del() function.  This function must be
called no later than when the socket has been closed, to avoid
attempting to call select() or similar functions on closed sockets.
</function>

<function>
void socket_state(struct Socket* sock, enum SocketState state);

Occasionally, a socket's state will change.  This function is used to
inform the events subsystem of that change.  Only certain state
transitions are valid--a socket in the SS_LISTENING or SS_CONNECTED
states cannot change states, nor can an SS_CONNECTING socket change to
some state other than SS_CONNECTED.  Of course, SS_DATAGRAM sockets
may change state only to SS_CONNECTDG, and SS_CONNECTDG sockets may
only change states to SS_DATAGRAM.
</function>

<function>
void socket_events(struct Socket* sock, unsigned int events);

When the application changes the events it is interested in, it uses
socket_events() to notify the events subsystem of that change.  The
_events_ parameter is the binary OR of one of SOCK_ACTION_SET,
SOCK_ACTION_ADD, or SOCK_ACTION_DEL with an events mask.  See the
documentation for the SOCK_* macros for more information.
</function>

<function>
const char* state_to_name(enum SocketState state);

This function is defined only when DEBUGMODE is #define'd.  It takes
the given _state_ and returns a string giving that state's name.  This
function may safely be called from Debug() macros.
</function>

<function>
const char* sock_flags(unsigned int flags);

This function is defined only when DEBUGMODE is #define'd.  It takes
the given event interest flags and returns a string naming each of
those flags.  This function may safely be called from Debug() macros,
but may only be called once, since it uses function static storage to
store the flag strings.
</function>

<function>
int sig_signal(struct Signal* sig);

This macro returns the signal number for the given struct Signal.
</function>

<function>
void* sig_data(struct Signal* sig);

When a struct Signal is initialized, data that the call-back function
may find useful is stored in the struct Signal.  This macro returns
that pointer.
</function>

<function>
int sig_ed_int(struct Signal* sig);

Engines may find it convenient to associate an integer with a struct
Signal.  This macro may be used to retrieve that integer or, when used
as an lvalue, to assign a value to it.  Engine data must be either an
int or a void*; use of both is prohibited.
</function>

<function>
void* sig_ed_ptr(struct Signal* sig);

Engines may find it convenient to associate a void* pointer with a
struct Signal.  This macro may be used to retrieve that pointer or,
when used as an lvalue, to assign a value to it.  Engine data must be
either an int or a void*; use of both is prohibited.
</function>

<function>
int sig_active(struct Signal* sig);

A signal's active flag is set when initialized by signal_add().  This
may be used by the application to determine whether or not the signal
has been initialized yet.  If it is, sig_active() returns a non-zero
value; otherwise, its value is 0.
</function>

<function>
void signal_add(struct Signal* signal, EventCallBack call, void* data,
                int sig);

This function is called to add a signal to the list of signals to be
monitored.  The _signal_ parameter is a pointer is a pointer to a
struct Signal that is allocated by the application.  The _call_
parameter is a pointer to a function to process any signal events.
The _data_ parameter is for use of the signal call-back and may be
zero.  The _sig_ parameter is the integer value of the signal to be
monitored.
</function>

<function>
enum TimerType t_type(struct Timer* tim);

This macro returns the type of the given timer.
</function>

<function>
time_t t_value(struct Timer* tim);

This macro returns the value that was used when the given timer was
initialized by the events subsystem.  It will contain an absolute time
if the timer type is TT_ABSOLUTE, and a relative time otherwise.
</function>

<function>
time_t t_expire(struct Timer* tim);

This macro returns the absolute time at which the timer will next
expire.
</function>

<function>
void* t_data(struct Timer* tim);

When a struct Timer is initialized, data that the call-back function
may find useful is stored in the struct Socket.  This macro returns
that pointer.
</function>

<function>
int t_ed_int(struct Timer *tim);

Engines may find it convenient to associate an integer with a struct
Timer.  This macro may be used to retrieve that integer or, when used
as an lvalue, to assign a value to it.  Engine data must be either an
int or a void*; use of both is prohibited.
</function>

<function>
void* t_ed_ptr(struct Timer *tim);

Engines may find it convenient to associate a void* pointer with a
struct Timer.  This macro may be used to retrieve that pointer or,
when used as an lvalue, to assign a value to it.  Engine data must be
either an int or a void*; use of both is prohibited.
</function>

<function>
int t_active(struct Timer *tim);

A timer's active flag is set when initialized by timer_add(), and is
cleared immediately prior to generating an event of type ET_DESTROY.
This may be used by the application to determine whether or not the
timer is still in use by the events subsystem.  If it is, s_active()
returns a non-zero value; otherwise, its value is 0.
</function>

<function>
void timer_add(struct Timer* timer, EventCallBack call, void* data,
               enum TimerType type, time_t value);

This function is called to initialize and queue a timer.  The _timer_
parameter is a pointer to a struct Timer that is allocated by the
application.  The _call_ parameter is a pointer to a function to
process the timer's expiration.  The _data_ parameter is for use of
the timer call-back and may be zero.  The _type_ parameter must be one
of the valid timer types--TT_ABSOLUTE, TT_RELATIVE, or TT_PERIODIC.
Finally, _value_ is the value for the timer's expiration.
</function>

<function>
void timer_del(struct Timer* timer);

When the application no longer needs a TT_PERIODIC timer, or when it
wishes to stop a TT_ABSOLUTE or TT_RELATIVE timer before its
expiration, it should call the timer_del() function.
</function>

<function>
void timer_chg(struct Timer* timer, enum TimerType type, time_t value);

Occasionally, an application may wish to delay an existing TT_ABSOLUTE
or TT_RELATIVE timer; this may be done with the timer_chg() function.
The _type_ parameter must be one of TT_ABSOLUTE or
TT_RELATIVE--changing the values of TT_PERIODIC timers is not
supported.  The _value_ parameter is the same as would be given to
timer_add() for that particular type of timer.
</function>

<function>
void timer_run(void);

When an engine has finished processing the results of its socket and
signal checks--just before it loops around to test for more events--it
should call the timer_run() function to expire any waiting timers.
</function>

<function>
time_t timer_next(struct Generators* gen);

Most engines will use a blocking call with a timeout to check for
socket activity.  To determine when the next timer needs to be run,
and thus to calculate how long the call should block, the engine
should call timer_next() with the _gen_ parameter passed to the
_EngineLoop_ function.  The timer_next() function returns an absolute
time, which may have to be massaged into a relative time before the
engine may use it.
</function>

<function>
const char* timer_to_name(enum TimerType type);

This function is defined only when DEBUGMODE is #define'd.  It takes
the given _type_ and returns a string giving that type's name.  This
function may safely be called from Debug() macros.
</function>

<function>
enum EventType ev_type(struct Event* ev);

This macro simply returns the type of the event _ev_.
</function>

<function>
int ev_data(struct Event* ev);

When an event is generated, a single integer can be passed along as a
piece of extra information.  This can be used, for instance, to carry
an errno value when an ET_ERROR is generated.  This macro simply
returns that integer.
</function>

<function>
struct Socket* ev_socket(struct Event* ev);

If the event was generated by a socket, this macro returns a pointer
to the struct Socket that generated the event.  The results are
undefined if the event was not generated by a socket.
</function>

<function>
struct Signal* ev_signal(struct Event* ev);

If the event was generated by a signal, this macro returns a pointer
to the struct Signal that generated the event.  The results are
undefined if the event was not generated by a signal.
</function>

<function>
struct Timer* ev_timer(struct Event* ev);

If the event was generated by a timer, this macro returns a pointer to
the struct Timer that generated the event.  The results are undefined
if the event was not generated by a timer.
</function>

<function>
void event_init(int max_sockets);

Before any of the functions or macros described here can be called,
the events subsystem must be initialized by calling event_init().  The
_max_sockets_ parameter specifies to the events subsystem how many
sockets it must be able to support; this figure may be used for memory
allocation by the engines.
</function>

<function>
void event_loop(void);

Once the initial sockets are open, signals added, and timers queued,
the application must call the event_loop() function in order to
actually begin monitoring those sockets, signals, and timers.
</function>

<function>
void event_generate(enum EventType type, void* arg, int data);

This is the function called by the events subsystem to generate
particular events.  The _type_ parameter specifies the type of event
to generate, and the _arg_ parameter must be a pointer to the event's
generator.  The _data_ parameter may be used for passing such things
as signal numbers or errno values.
</function>

<function>
const char* event_to_name(enum EventType type);

This function is defined only when DEBUGMODE is #define'd.  It takes
the given _type_ and returns a string giving that event type's name.
This function may safely be called from Debug() macros.
</function>

<function>
const char* engine_name(void);

This function is used to retrieve the name of the engine presently
being used by the events subsystem.
</function>

<function>
void gen_ref_inc(void* gen);

This macro increments the reference count of the generator _gen_,
preventing it from simply disappearing without warning.
</function>

<function>
void gen_ref_dec(void* gen);

This macro decrements the reference count of the generator _gen_, and
releases the memory associated with it by generating at ET_DESTROY
event if the reference count falls to zero and the generator is marked
for destruction.  No references should be made to the generator after
calling this macro.
</function>

<authors>
Kev <klmitch@mit.edu>
</authors>

<changelog>
[2001-6-14 Kev] Finished initial description of the events subsystem.

[2001-6-13 Kev] Initial description of the events subsystem.
</changelog>