path: root/eio.pod

=head1 NAME

libeio - truly asynchronous POSIX I/O

=head1 SYNOPSIS

  #include <eio.h>

=head1 DESCRIPTION

The newest version of this document is also available as an html-formatted
web page you might find easier to navigate when reading it for the first
time: L<http://pod.tst.eu/http://cvs.schmorp.de/libeio/eio.pod>.

Note that this library is a by-product of the C<IO::AIO> perl
module, and many of the subtler points regarding requets lifetime
and so on are only documented in its documentation at the
moment: L<http://pod.tst.eu/http://cvs.schmorp.de/IO-AIO/AIO.pm>.

=head2 FEATURES

This library provides fully asynchronous versions of most POSIX functions
dealign with I/O. Unlike most asynchronous libraries, this not only
includes C<read> and C<write>, but also C<open>, C<stat>, C<unlink> and
similar functions, as well as less rarely ones such as C<mknod>, C<futime>
or C<readlink>.

It also offers wrappers around C<sendfile> (Solaris, Linux, HP-UX and
FreeBSD, with emulation on other platforms) and C<readahead> (Linux, with
emulation elsewhere>).

The goal is to enbale you to write fully non-blocking programs. For
example, in a game server, you would not want to freeze for a few seconds
just because the server is running a backup and you happen to call
C<readdir>.

=head2 TIME REPRESENTATION

Libeio represents time as a single floating point number, representing the
(fractional) number of seconds since the (POSIX) epoch (somewhere near
the beginning of 1970, details are complicated, don't ask). This type is
called C<eio_tstamp>, but it is guarenteed to be of type C<double> (or
better), so you can freely use C<double> yourself.

Unlike the name component C<stamp> might indicate, it is also used for
time differences throughout libeio.

=head2 FORK SUPPORT

Calling C<fork ()> is fully supported by this module. It is implemented in these steps:

   1. wait till all requests in "execute" state have been handled
      (basically requests that are already handed over to the kernel).
   2. fork
   3. in the parent, continue business as usual, done
   4. in the child, destroy all ready and pending requests and free the
      memory used by the worker threads. This gives you a fully empty
      libeio queue.

=head1 INITIALISATION/INTEGRATION

Before you can call any eio functions you first have to initialise the
library. The library integrates into any event loop, but can also be used
without one, including in polling mode.

You have to provide the necessary glue yourself, however.

=over 4

=item int eio_init (void (*want_poll)(void), void (*done_poll)(void))

This function initialises the library. On success it returns C<0>, on
failure it returns C<-1> and sets C<errno> appropriately.

It accepts two function pointers specifying callbacks as argument, both of
which can be C<0>, in which case the callback isn't called.

=item want_poll callback

The C<want_poll> callback is invoked whenever libeio wants attention (i.e.
it wants to be polled by calling C<eio_poll>). It is "edge-triggered",
that is, it will only be called once when eio wants attention, until all
pending requests have been handled.

This callback is called while locks are being held, so I<you must
not call any libeio functions inside this callback>. That includes
C<eio_poll>. What you should do is notify some other thread, or wake up
your event loop, and then call C<eio_poll>.

=item done_poll callback

This callback is invoked when libeio detects that all pending requests
have been handled. It is "edge-triggered", that is, it will only be
called once after C<want_poll>. To put it differently, C<want_poll> and
C<done_poll> are invoked in pairs: after C<want_poll> you have to call
C<eio_poll ()> until either C<eio_poll> indicates that everything has been
handled or C<done_poll> has been called, which signals the same.

Note that C<eio_poll> might return after C<done_poll> and C<want_poll>
have been called again, so watch out for races in your code.

As with C<want_poll>, this callback is called while lcoks are being held,
so you I<must not call any libeio functions form within this callback>.

=item int eio_poll ()

This function has to be called whenever there are pending requests that
need finishing. You usually call this after C<want_poll> has indicated
that you should do so, but you can also call this function regularly to
poll for new results.

If any request invocation returns a non-zero value, then C<eio_poll ()>
immediately returns with that value as return value.

Otherwise, if all requests could be handled, it returns C<0>. If for some
reason not all requests have been handled, i.e. some are still pending, it
returns C<-1>.

=back

For libev, you would typically use an C<ev_async> watcher: the
C<want_poll> callback would invoke C<ev_async_send> to wake up the event
loop. Inside the callback set for the watcher, one would call C<eio_poll
()> (followed by C<ev_async_send> again if C<eio_poll> indicates that not
all requests have been handled yet). The race is taken care of because
libev resets/rearms the async watcher before calling your callback,
and therefore, before calling C<eio_poll>. This might result in (some)
spurious wake-ups, but is generally harmless.

For most other event loops, you would typically use a pipe - the event
loop should be told to wait for read readyness on the read end. In
C<want_poll> you would write a single byte, in C<done_poll> you would try
to read that byte, and in the callback for the read end, you would call
C<eio_poll>. The race is avoided here because the event loop should invoke
your callback again and again until the byte has been read (as the pipe
read callback does not read it, only C<done_poll>).

=head2 CONFIGURATION

The functions in this section can sometimes be useful, but the default
configuration will do in most case, so you should skip this section on
first reading.

=over 4

=item eio_set_max_poll_time (eio_tstamp nseconds)

This causes C<eio_poll ()> to return after it has detected that it was
running for C<nsecond> seconds or longer (this number can be fractional).

This can be used to limit the amount of time spent handling eio requests,
for example, in interactive programs, you might want to limit this time to
C<0.01> seconds or so.

Note that:

a) libeio doesn't know how long your request callbacks take, so the time
spent in C<eio_poll> is up to one callback invocation longer then this
interval.

b) this is implemented by calling C<gettimeofday> after each request,
which can be costly.

c) at least one request will be handled.

=item eio_set_max_poll_reqs (unsigned int nreqs)

When C<nreqs> is non-zero, then C<eio_poll> will not handle more than
C<nreqs> requests per invocation. This is a less costly way to limit the
amount of work done by C<eio_poll> then setting a time limit.

If you know your callbacks are generally fast, you could use this to
encourage interactiveness in your programs by setting it to C<10>, C<100>
or even C<1000>.

=item eio_set_min_parallel (unsigned int nthreads)

Make sure libeio can handle at least this many requests in parallel. It
might be able handle more.

=item eio_set_max_parallel (unsigned int nthreads)

Set the maximum number of threads that libeio will spawn.

=item eio_set_max_idle (unsigned int nthreads)

Libeio uses threads internally to handle most requests, and will start and stop threads on demand.

This call can be used to limit the number of idle threads (threads without
work to do): libeio will keep some threads idle in preperation for more
requests, but never longer than C<nthreads> threads.

In addition to this, libeio will also stop threads when they are idle for
a few seconds, regardless of this setting.

=item unsigned int eio_nthreads ()

Return the number of worker threads currently running.

=item unsigned int eio_nreqs ()

Return the number of requests currently handled by libeio. This is the
total number of requests that have been submitted to libeio, but not yet
destroyed.

=item unsigned int eio_nready ()

Returns the number of ready requests, i.e. requests that have been
submitted but have not yet entered the execution phase.

=item unsigned int eio_npending ()

Returns the number of pending requests, i.e. requests that have been
executed and have results, but have not been finished yet by a call to
C<eio_poll>).

=back


=head1 ANATOMY OF AN EIO REQUEST

#TODO


=head1 HIGH LEVEL REQUEST API

#TODO

=back


=head1 LOW LEVEL REQUEST API

#TODO

=head1 EMBEDDING

Libeio can be embedded directly into programs. This functionality is not
documented and not (yet) officially supported.

If you need to know how, check the C<IO::AIO> perl module, which does
exactly that.


=head1 PORTABILITY REQUIREMENTS

In addition to a working ISO-C implementation, libeio relies on a few
additional extensions:

=over 4

=item POSIX threads

To be portable, this module uses threads, specifically, the POSIX threads
library must be available (and working, which partially excludes many xBSD
systems, where C<fork ()> is buggy).

=item POSIX-compatible filesystem API

This is actually a harder portability requirement: The libeio API is quite
demanding regarding POSIX API calls (symlinks, user/group management
etc.).

=item C<double> must hold a time value in seconds with enough accuracy

The type C<double> is used to represent timestamps. It is required to
have at least 51 bits of mantissa (and 9 bits of exponent), which is good
enough for at least into the year 4000. This requirement is fulfilled by
implementations implementing IEEE 754 (basically all existing ones).

=back

If you know of other additional requirements drop me a note.


=head1 AUTHOR

Marc Lehmann <libeio@schmorp.de>.