Ensure low-memory renderers retry failed loads correctly.

[chromium-blink-merge.git] / third_party / libevent / event.3

.\"     $OpenBSD: event.3,v 1.4 2002/07/12 18:50:48 provos Exp $
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.\" Copyright (c) 2000 Artur Grabowski <art@openbsd.org>
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.Dd August 8, 2000
.Dt EVENT 3
.Os
.Sh NAME
.Nm event_init ,
.Nm event_dispatch ,
.Nm event_loop ,
.Nm event_loopexit ,
.Nm event_loopbreak ,
.Nm event_set ,
.Nm event_base_dispatch ,
.Nm event_base_loop ,
.Nm event_base_loopexit ,
.Nm event_base_loopbreak ,
.Nm event_base_set ,
.Nm event_base_free ,
.Nm event_add ,
.Nm event_del ,
.Nm event_once ,
.Nm event_base_once ,
.Nm event_pending ,
.Nm event_initialized ,
.Nm event_priority_init ,
.Nm event_priority_set ,
.Nm evtimer_set ,
.Nm evtimer_add ,
.Nm evtimer_del ,
.Nm evtimer_pending ,
.Nm evtimer_initialized ,
.Nm signal_set ,
.Nm signal_add ,
.Nm signal_del ,
.Nm signal_pending ,
.Nm signal_initialized ,
.Nm bufferevent_new ,
.Nm bufferevent_free ,
.Nm bufferevent_write ,
.Nm bufferevent_write_buffer ,
.Nm bufferevent_read ,
.Nm bufferevent_enable ,
.Nm bufferevent_disable ,
.Nm bufferevent_settimeout ,
.Nm bufferevent_base_set ,
.Nm evbuffer_new ,
.Nm evbuffer_free ,
.Nm evbuffer_add ,
.Nm evbuffer_add_buffer ,
.Nm evbuffer_add_printf ,
.Nm evbuffer_add_vprintf ,
.Nm evbuffer_drain ,
.Nm evbuffer_write ,
.Nm evbuffer_read ,
.Nm evbuffer_find ,
.Nm evbuffer_readline ,
.Nm evhttp_new ,
.Nm evhttp_bind_socket ,
.Nm evhttp_free
.Nd execute a function when a specific event occurs
.Sh SYNOPSIS
.Fd #include <sys/time.h>
.Fd #include <event.h>
.Ft "struct event_base *"
.Fn "event_init" "void"
.Ft int
.Fn "event_dispatch" "void"
.Ft int
.Fn "event_loop" "int flags"
.Ft int
.Fn "event_loopexit" "struct timeval *tv"
.Ft int
.Fn "event_loopbreak" "void"
.Ft void
.Fn "event_set" "struct event *ev" "int fd" "short event" "void (*fn)(int, short, void *)" "void *arg"
.Ft int
.Fn "event_base_dispatch" "struct event_base *base"
.Ft int
.Fn "event_base_loop" "struct event_base *base" "int flags"
.Ft int
.Fn "event_base_loopexit" "struct event_base *base" "struct timeval *tv"
.Ft int
.Fn "event_base_loopbreak" "struct event_base *base"
.Ft int
.Fn "event_base_set" "struct event_base *base" "struct event *"
.Ft void
.Fn "event_base_free" "struct event_base *base"
.Ft int
.Fn "event_add" "struct event *ev" "struct timeval *tv"
.Ft int
.Fn "event_del" "struct event *ev"
.Ft int
.Fn "event_once" "int fd" "short event" "void (*fn)(int, short, void *)" "void *arg" "struct timeval *tv"
.Ft int
.Fn "event_base_once" "struct event_base *base" "int fd" "short event" "void (*fn)(int, short, void *)" "void *arg" "struct timeval *tv"
.Ft int
.Fn "event_pending" "struct event *ev" "short event" "struct timeval *tv"
.Ft int
.Fn "event_initialized" "struct event *ev"
.Ft int
.Fn "event_priority_init" "int npriorities"
.Ft int
.Fn "event_priority_set" "struct event *ev" "int priority"
.Ft void
.Fn "evtimer_set" "struct event *ev" "void (*fn)(int, short, void *)" "void *arg"
.Ft void
.Fn "evtimer_add" "struct event *ev" "struct timeval *"
.Ft void
.Fn "evtimer_del" "struct event *ev"
.Ft int
.Fn "evtimer_pending" "struct event *ev" "struct timeval *tv"
.Ft int
.Fn "evtimer_initialized" "struct event *ev"
.Ft void
.Fn "signal_set" "struct event *ev" "int signal" "void (*fn)(int, short, void *)" "void *arg"
.Ft void
.Fn "signal_add" "struct event *ev" "struct timeval *"
.Ft void
.Fn "signal_del" "struct event *ev"
.Ft int
.Fn "signal_pending" "struct event *ev" "struct timeval *tv"
.Ft int
.Fn "signal_initialized" "struct event *ev"
.Ft "struct bufferevent *"
.Fn "bufferevent_new" "int fd" "evbuffercb readcb" "evbuffercb writecb" "everrorcb" "void *cbarg"
.Ft void
.Fn "bufferevent_free" "struct bufferevent *bufev"
.Ft int
.Fn "bufferevent_write" "struct bufferevent *bufev" "void *data" "size_t size"
.Ft int
.Fn "bufferevent_write_buffer" "struct bufferevent *bufev" "struct evbuffer *buf"
.Ft size_t
.Fn "bufferevent_read" "struct bufferevent *bufev" "void *data" "size_t size"
.Ft int
.Fn "bufferevent_enable" "struct bufferevent *bufev" "short event"
.Ft int
.Fn "bufferevent_disable" "struct bufferevent *bufev" "short event"
.Ft void
.Fn "bufferevent_settimeout" "struct bufferevent *bufev" "int timeout_read" "int timeout_write"
.Ft int
.Fn "bufferevent_base_set" "struct event_base *base" "struct bufferevent *bufev"
.Ft "struct evbuffer *"
.Fn "evbuffer_new" "void"
.Ft void
.Fn "evbuffer_free" "struct evbuffer *buf"
.Ft int
.Fn "evbuffer_add" "struct evbuffer *buf" "const void *data" "size_t size"
.Ft int
.Fn "evbuffer_add_buffer" "struct evbuffer *dst" "struct evbuffer *src"
.Ft int
.Fn "evbuffer_add_printf" "struct evbuffer *buf" "const char *fmt" "..."
.Ft int
.Fn "evbuffer_add_vprintf" "struct evbuffer *buf" "const char *fmt" "va_list ap"
.Ft void
.Fn "evbuffer_drain" "struct evbuffer *buf" "size_t size"
.Ft int
.Fn "evbuffer_write" "struct evbuffer *buf" "int fd"
.Ft int
.Fn "evbuffer_read" "struct evbuffer *buf" "int fd" "int size"
.Ft "u_char *"
.Fn "evbuffer_find" "struct evbuffer *buf" "const u_char *data" "size_t size"
.Ft "char *"
.Fn "evbuffer_readline" "struct evbuffer *buf"
.Ft "struct evhttp *"
.Fn "evhttp_new" "struct event_base *base"
.Ft int
.Fn "evhttp_bind_socket" "struct evhttp *http" "const char *address" "u_short port"
.Ft "void"
.Fn "evhttp_free" "struct evhttp *http"
.Ft int
.Fa (*event_sigcb)(void) ;
.Ft volatile sig_atomic_t
.Fa event_gotsig ;
.Sh DESCRIPTION
The
.Nm event
API provides a mechanism to execute a function when a specific event
on a file descriptor occurs or after a given time has passed.
.Pp
The
.Nm event
API needs to be initialized with
.Fn event_init
before it can be used.
.Pp
In order to process events, an application needs to call
.Fn event_dispatch .
This function only returns on error, and should replace the event core
of the application program.
.Pp
The function
.Fn event_set
prepares the event structure
.Fa ev
to be used in future calls to
.Fn event_add
and
.Fn event_del .
The event will be prepared to call the function specified by the
.Fa fn
argument with an
.Fa int
argument indicating the file descriptor, a
.Fa short
argument indicating the type of event, and a
.Fa void *
argument given in the
.Fa arg
argument.
The
.Fa fd
indicates the file descriptor that should be monitored for events.
The events can be either
.Va EV_READ ,
.Va EV_WRITE ,
or both,
indicating that an application can read or write from the file descriptor
respectively without blocking.
.Pp
The function
.Fa fn
will be called with the file descriptor that triggered the event and
the type of event which will be either
.Va EV_TIMEOUT ,
.Va EV_SIGNAL ,
.Va EV_READ ,
or
.Va EV_WRITE .
Additionally, an event which has registered interest in more than one of the
preceeding events, via bitwise-OR to
.Fn event_set ,
can provide its callback function with a bitwise-OR of more than one triggered
event.
The additional flag
.Va EV_PERSIST
makes an
.Fn event_add
persistent until
.Fn event_del
has been called.
.Pp
Once initialized, the
.Fa ev
structure can be used repeatedly with
.Fn event_add
and
.Fn event_del
and does not need to be reinitialized unless the function called and/or
the argument to it are to be changed.
However, when an
.Fa ev
structure has been added to libevent using
.Fn event_add
the structure must persist until the event occurs (assuming
.Fa EV_PERSIST
is not set) or is removed
using
.Fn event_del .
You may not reuse the same
.Fa ev
structure for multiple monitored descriptors; each descriptor
needs its own
.Fa ev .
.Pp
The function
.Fn event_add
schedules the execution of the
.Fa ev
event when the event specified in
.Fn event_set
occurs or in at least the time specified in the
.Fa tv .
If
.Fa tv
is
.Dv NULL ,
no timeout occurs and the function will only be called
if a matching event occurs on the file descriptor.
The event in the
.Fa ev
argument must be already initialized by
.Fn event_set
and may not be used in calls to
.Fn event_set
until it has timed out or been removed with
.Fn event_del .
If the event in the
.Fa ev
argument already has a scheduled timeout, the old timeout will be
replaced by the new one.
.Pp
The function
.Fn event_del
will cancel the event in the argument
.Fa ev .
If the event has already executed or has never been added
the call will have no effect.
.Pp
The functions
.Fn evtimer_set ,
.Fn evtimer_add ,
.Fn evtimer_del ,
.Fn evtimer_initialized ,
and
.Fn evtimer_pending
are abbreviations for common situations where only a timeout is required.
The file descriptor passed will be \-1, and the event type will be
.Va EV_TIMEOUT .
.Pp
The functions
.Fn signal_set ,
.Fn signal_add ,
.Fn signal_del ,
.Fn signal_initialized ,
and
.Fn signal_pending
are abbreviations.
The event type will be a persistent
.Va EV_SIGNAL .
That means
.Fn signal_set
adds
.Va EV_PERSIST .
.Pp
In order to avoid races in signal handlers, the
.Nm event
API provides two variables:
.Va event_sigcb
and
.Va event_gotsig .
A signal handler
sets
.Va event_gotsig
to indicate that a signal has been received.
The application sets
.Va event_sigcb
to a callback function.
After the signal handler sets
.Va event_gotsig ,
.Nm event_dispatch
will execute the callback function to process received signals.
The callback returns 1 when no events are registered any more.
It can return \-1 to indicate an error to the
.Nm event
library, causing
.Fn event_dispatch
to terminate with
.Va errno
set to
.Er EINTR .
.Pp
The function
.Fn event_once
is similar to
.Fn event_set .
However, it schedules a callback to be called exactly once and does not
require the caller to prepare an
.Fa event
structure.
This function supports
.Fa EV_TIMEOUT ,
.Fa EV_READ ,
and
.Fa EV_WRITE .
.Pp
The
.Fn event_pending
function can be used to check if the event specified by
.Fa event
is pending to run.
If
.Va EV_TIMEOUT
was specified and
.Fa tv
is not
.Dv NULL ,
the expiration time of the event will be returned in
.Fa tv .
.Pp
The
.Fn event_initialized
macro can be used to check if an event has been initialized.
.Pp
The
.Nm event_loop
function provides an interface for single pass execution of pending
events.
The flags
.Va EVLOOP_ONCE
and
.Va EVLOOP_NONBLOCK
are recognized.
The
.Nm event_loopexit
function exits from the event loop. The next
.Fn event_loop
iteration after the
given timer expires will complete normally (handling all queued events) then
exit without blocking for events again. Subsequent invocations of
.Fn event_loop
will proceed normally.
The
.Nm event_loopbreak
function exits from the event loop immediately.
.Fn event_loop
will abort after the next event is completed;
.Fn event_loopbreak
is typically invoked from this event's callback. This behavior is analogous
to the "break;" statement. Subsequent invocations of
.Fn event_loop
will proceed normally.
.Pp
It is the responsibility of the caller to provide these functions with
pre-allocated event structures.
.Pp
.Sh EVENT PRIORITIES
By default
.Nm libevent
schedules all active events with the same priority.
However, sometimes it is desirable to process some events with a higher
priority than others.
For that reason,
.Nm libevent
supports strict priority queues.
Active events with a lower priority are always processed before events
with a higher priority.
.Pp
The number of different priorities can be set initially with the
.Fn event_priority_init
function.
This function should be called before the first call to
.Fn event_dispatch .
The
.Fn event_priority_set
function can be used to assign a priority to an event.
By default,
.Nm libevent
assigns the middle priority to all events unless their priority
is explicitly set.
.Sh THREAD SAFE EVENTS
.Nm Libevent
has experimental support for thread-safe events.
When initializing the library via
.Fn event_init ,
an event base is returned.
This event base can be used in conjunction with calls to
.Fn event_base_set ,
.Fn event_base_dispatch ,
.Fn event_base_loop ,
.Fn event_base_loopexit ,
.Fn bufferevent_base_set
and
.Fn event_base_free .
.Fn event_base_set
should be called after preparing an event with
.Fn event_set ,
as
.Fn event_set
assigns the provided event to the most recently created event base.
.Fn bufferevent_base_set
should be called after preparing a bufferevent with
.Fn bufferevent_new .
.Fn event_base_free
should be used to free memory associated with the event base
when it is no longer needed.
.Sh BUFFERED EVENTS
.Nm libevent
provides an abstraction on top of the regular event callbacks.
This abstraction is called a
.Va "buffered event" .
A buffered event provides input and output buffers that get filled
and drained automatically.
The user of a buffered event no longer deals directly with the IO,
but instead is reading from input and writing to output buffers.
.Pp
A new bufferevent is created by
.Fn bufferevent_new .
The parameter
.Fa fd
specifies the file descriptor from which data is read and written to.
This file descriptor is not allowed to be a
.Xr pipe 2 .
The next three parameters are callbacks.
The read and write callback have the following form:
.Ft void
.Fn "(*cb)" "struct bufferevent *bufev" "void *arg" .
The error callback has the following form:
.Ft void
.Fn "(*cb)" "struct bufferevent *bufev" "short what" "void *arg" .
The argument is specified by the fourth parameter
.Fa "cbarg" .
A
.Fa bufferevent struct
pointer is returned on success, NULL on error.
Both the read and the write callback may be NULL.
The error callback has to be always provided.
.Pp
Once initialized, the bufferevent structure can be used repeatedly with
bufferevent_enable() and bufferevent_disable().
The flags parameter can be a combination of
.Va EV_READ
and
.Va EV_WRITE .
When read enabled the bufferevent will try to read from the file
descriptor and call the read callback.
The write callback is executed
whenever the output buffer is drained below the write low watermark,
which is
.Va 0
by default.
.Pp
The
.Fn bufferevent_write
function can be used to write data to the file descriptor.
The data is appended to the output buffer and written to the descriptor
automatically as it becomes available for writing.
.Fn bufferevent_write
returns 0 on success or \-1 on failure.
The
.Fn bufferevent_read
function is used to read data from the input buffer,
returning the amount of data read.
.Pp
If multiple bases are in use, bufferevent_base_set() must be called before
enabling the bufferevent for the first time.
.Sh NON-BLOCKING HTTP SUPPORT
.Nm libevent
provides a very thin HTTP layer that can be used both to host an HTTP
server and also to make HTTP requests.
An HTTP server can be created by calling
.Fn evhttp_new .
It can be bound to any port and address with the
.Fn evhttp_bind_socket
function.
When the HTTP server is no longer used, it can be freed via
.Fn evhttp_free .
.Pp
To be notified of HTTP requests, a user needs to register callbacks with the
HTTP server.
This can be done by calling
.Fn evhttp_set_cb .
The second argument is the URI for which a callback is being registered.
The corresponding callback will receive an
.Va struct evhttp_request
object that contains all information about the request.
.Pp
This section does not document all the possible function calls; please
check
.Va event.h
for the public interfaces.
.Sh ADDITIONAL NOTES
It is possible to disable support for
.Va epoll , kqueue , devpoll , poll
or
.Va select
by setting the environment variable
.Va EVENT_NOEPOLL , EVENT_NOKQUEUE , EVENT_NODEVPOLL , EVENT_NOPOLL
or
.Va EVENT_NOSELECT ,
respectively.
By setting the environment variable
.Va EVENT_SHOW_METHOD ,
.Nm libevent
displays the kernel notification method that it uses.
.Sh RETURN VALUES
Upon successful completion
.Fn event_add
and
.Fn event_del
return 0.
Otherwise, \-1 is returned and the global variable errno is
set to indicate the error.
.Sh SEE ALSO
.Xr kqueue 2 ,
.Xr poll 2 ,
.Xr select 2 ,
.Xr evdns 3 ,
.Xr timeout 9
.Sh HISTORY
The
.Nm event
API manpage is based on the
.Xr timeout 9
manpage by Artur Grabowski.
The port of
.Nm libevent
to Windows is due to Michael A. Davis.
Support for real-time signals is due to Taral.
.Sh AUTHORS
The
.Nm event
library was written by Niels Provos.
.Sh BUGS
This documentation is neither complete nor authoritative.
If you are in doubt about the usage of this API then
check the source code to find out how it works, write
up the missing piece of documentation and send it to
me for inclusion in this man page.