IO-AIO/AIO.pm

=head1 NAME

IO::AIO - Asynchronous Input/Output

=head1 SYNOPSIS

 use IO::AIO;

 aio_open "/etc/passwd", O_RDONLY, 0, sub {
    my ($fh) = @_;
    ...
 };

 aio_unlink "/tmp/file", sub { };

 aio_read $fh, 30000, 1024, $buffer, 0, sub {
    $_[0] > 0 or die "read error: $!";
 };

 # AnyEvent
 open my $fh, "<&=" . IO::AIO::poll_fileno or die "$!";
 my $w = AnyEvent->io (fh => $fh, poll => 'r', cb => sub { IO::AIO::poll_cb });

 # Event
 Event->io (fd => IO::AIO::poll_fileno,
            poll => 'r',
            cb => \&IO::AIO::poll_cb);

 # Glib/Gtk2
 add_watch Glib::IO IO::AIO::poll_fileno,
           in => sub { IO::AIO::poll_cb; 1 };

 # Tk
 Tk::Event::IO->fileevent (IO::AIO::poll_fileno, "",
                           readable => \&IO::AIO::poll_cb);

 # Danga::Socket
 Danga::Socket->AddOtherFds (IO::AIO::poll_fileno =>
                             \&IO::AIO::poll_cb);


=head1 DESCRIPTION

This module implements asynchronous I/O using whatever means your
operating system supports.

Currently, a number of threads are started that execute your read/writes
and signal their completion. You don't need thread support in your libc or
perl, and the threads created by this module will not be visible to the
pthreads library. In the future, this module might make use of the native
aio functions available on many operating systems. However, they are often
not well-supported (Linux doesn't allow them on normal files currently,
for example), and they would only support aio_read and aio_write, so the
remaining functionality would have to be implemented using threads anyway.

Although the module will work with in the presence of other threads, it is
currently not reentrant, so use appropriate locking yourself, always call
C<poll_cb> from within the same thread, or never call C<poll_cb> (or other
C<aio_> functions) recursively.

=cut

package IO::AIO;

no warnings;

use base 'Exporter';

use Fcntl ();

BEGIN {
   $VERSION = '1.73';

   @EXPORT = qw(aio_sendfile aio_read aio_write aio_open aio_close aio_stat
                aio_lstat aio_unlink aio_rmdir aio_readdir aio_scandir aio_symlink
                aio_fsync aio_fdatasync aio_readahead);
   @EXPORT_OK = qw(poll_fileno poll_cb min_parallel max_parallel
                   max_outstanding nreqs);

   require XSLoader;
   XSLoader::load IO::AIO, $VERSION;
}

=head1 FUNCTIONS

=head2 AIO FUNCTIONS

All the C<aio_*> calls are more or less thin wrappers around the syscall
with the same name (sans C<aio_>). The arguments are similar or identical,
and they all accept an additional (and optional) C<$callback> argument
which must be a code reference. This code reference will get called with
the syscall return code (e.g. most syscalls return C<-1> on error, unlike
perl, which usually delivers "false") as it's sole argument when the given
syscall has been executed asynchronously.

All functions expecting a filehandle keep a copy of the filehandle
internally until the request has finished.

The pathnames you pass to these routines I<must> be absolute and
encoded in byte form. The reason for the former is that at the time the
request is being executed, the current working directory could have
changed. Alternatively, you can make sure that you never change the
current working directory.

To encode pathnames to byte form, either make sure you either: a)
always pass in filenames you got from outside (command line, readdir
etc.), b) are ASCII or ISO 8859-1, c) use the Encode module and encode
your pathnames to the locale (or other) encoding in effect in the user
environment, d) use Glib::filename_from_unicode on unicode filenames or e)
use something else.

=over 4

=item aio_open $pathname, $flags, $mode, $callback->($fh)

Asynchronously open or create a file and call the callback with a newly
created filehandle for the file.

The pathname passed to C<aio_open> must be absolute. See API NOTES, above,
for an explanation.

The C<$flags> argument is a bitmask. See the C<Fcntl> module for a
list. They are the same as used by C<sysopen>.

Likewise, C<$mode> specifies the mode of the newly created file, if it
didn't exist and C<O_CREAT> has been given, just like perl's C<sysopen>,
except that it is mandatory (i.e. use C<0> if you don't create new files,
and C<0666> or C<0777> if you do).

Example:

   aio_open "/etc/passwd", O_RDONLY, 0, sub {
      if ($_[0]) {
         print "open successful, fh is $_[0]\n";
         ...
      } else {
         die "open failed: $!\n";
      }
   };

=item aio_close $fh, $callback->($status)

Asynchronously close a file and call the callback with the result
code. I<WARNING:> although accepted, you should not pass in a perl
filehandle here, as perl will likely close the file descriptor another
time when the filehandle is destroyed. Normally, you can safely call perls
C<close> or just let filehandles go out of scope.

This is supposed to be a bug in the API, so that might change. It's
therefore best to avoid this function.

=item aio_read  $fh,$offset,$length, $data,$dataoffset, $callback->($retval)

=item aio_write $fh,$offset,$length, $data,$dataoffset, $callback->($retval)

Reads or writes C<length> bytes from the specified C<fh> and C<offset>
into the scalar given by C<data> and offset C<dataoffset> and calls the
callback without the actual number of bytes read (or -1 on error, just
like the syscall).

The C<$data> scalar I<MUST NOT> be modified in any way while the request
is outstanding. Modifying it can result in segfaults or WW3 (if the
necessary/optional hardware is installed).

Example: Read 15 bytes at offset 7 into scalar C<$buffer>, starting at
offset C<0> within the scalar:

   aio_read $fh, 7, 15, $buffer, 0, sub {
      $_[0] > 0 or die "read error: $!";
      print "read $_[0] bytes: <$buffer>\n";
   };

=item aio_sendfile $out_fh, $in_fh, $in_offset, $length, $callback->($retval)

Tries to copy C<$length> bytes from C<$in_fh> to C<$out_fh>. It starts
reading at byte offset C<$in_offset>, and starts writing at the current
file offset of C<$out_fh>. Because of that, it is not safe to issue more
than one C<aio_sendfile> per C<$out_fh>, as they will interfere with each
other.

This call tries to make use of a native C<sendfile> syscall to provide
zero-copy operation. For this to work, C<$out_fh> should refer to a
socket, and C<$in_fh> should refer to mmap'able file.

If the native sendfile call fails or is not implemented, it will be
emulated, so you can call C<aio_sendfile> on any type of filehandle
regardless of the limitations of the operating system.

Please note, however, that C<aio_sendfile> can read more bytes from
C<$in_fh> than are written, and there is no way to find out how many
bytes have been read from C<aio_sendfile> alone, as C<aio_sendfile> only
provides the number of bytes written to C<$out_fh>. Only if the result
value equals C<$length> one can assume that C<$length> bytes have been
read.

=item aio_readahead $fh,$offset,$length, $callback->($retval)

C<aio_readahead> populates the page cache with data from a file so that
subsequent reads from that file will not block on disk I/O. The C<$offset>
argument specifies the starting point from which data is to be read and
C<$length> specifies the number of bytes to be read. I/O is performed in
whole pages, so that offset is effectively rounded down to a page boundary
and bytes are read up to the next page boundary greater than or equal to
(off-set+length). C<aio_readahead> does not read beyond the end of the
file. The current file offset of the file is left unchanged.

If that syscall doesn't exist (likely if your OS isn't Linux) it will be
emulated by simply reading the data, which would have a similar effect.

=item aio_stat  $fh_or_path, $callback->($status)

=item aio_lstat $fh, $callback->($status)

Works like perl's C<stat> or C<lstat> in void context. The callback will
be called after the stat and the results will be available using C<stat _>
or C<-s _> etc...

The pathname passed to C<aio_stat> must be absolute. See API NOTES, above,
for an explanation.

Currently, the stats are always 64-bit-stats, i.e. instead of returning an
error when stat'ing a large file, the results will be silently truncated
unless perl itself is compiled with large file support.

Example: Print the length of F</etc/passwd>:

   aio_stat "/etc/passwd", sub {
      $_[0] and die "stat failed: $!";
      print "size is ", -s _, "\n";
   };

=item aio_unlink $pathname, $callback->($status)

Asynchronously unlink (delete) a file and call the callback with the
result code.

=item aio_rmdir $pathname, $callback->($status)

Asynchronously rmdir (delete) a directory and call the callback with the
result code.

=item aio_readdir $pathname, $callback->($entries)

Unlike the POSIX call of the same name, C<aio_readdir> reads an entire
directory (i.e. opendir + readdir + closedir). The entries will not be
sorted, and will B<NOT> include the C<.> and C<..> entries.

The callback a single argument which is either C<undef> or an array-ref
with the filenames.

=item aio_scandir $path, $maxreq, $callback->($dirs, $nondirs)

Scans a directory (similar to C<aio_readdir>) and tries to separate the
entries of directory C<$path> into two sets of names, ones you can recurse
into (directories), and ones you cannot recurse into (everything else).

C<aio_scandir> is a composite request that consists of many
aio-primitives. C<$maxreq> specifies the maximum number of outstanding
aio requests that this function generates. If it is C<< <= 0 >>, then a
suitable default will be chosen (currently 8).

On error, the callback is called without arguments, otherwise it receives
two array-refs with path-relative entry names.

Example:

   aio_scandir $dir, 0, sub {
      my ($dirs, $nondirs) = @_;
      print "real directories: @$dirs\n";
      print "everything else: @$nondirs\n";
   };

Implementation notes.

The C<aio_readdir> cannot be avoided, but C<stat()>'ing every entry can.

After reading the directory, the modification time, size etc. of the
directory before and after the readdir is checked, and if they match, the
link count will be used to decide how many entries are directories (if
>= 2). Otherwise, no knowledge of the number of subdirectories will be
assumed.

Then entires will be sorted into likely directories (everything without a
non-initial dot) and likely non-directories (everything else).  Then every
entry + C</.> will be C<stat>'ed, likely directories first. This is often
faster because filesystems might detect the type of the entry without
reading the inode data (e.g. ext2fs filetype feature). If that succeeds,
it assumes that the entry is a directory or a symlink to directory (which
will be checked seperately).

If the known number of directories has been reached, the rest of the
entries is assumed to be non-directories.

=cut

sub aio_scandir($$$) {
   my ($path, $maxreq, $cb) = @_;

   $maxreq = 8 if $maxreq <= 0;

   # stat once
   aio_stat $path, sub {
      return $cb->() if $_[0];
      my $hash1 = join ":", (stat _)[0,1,3,7,9];

      # read the directory entries
      aio_readdir $path, sub {
         my $entries = shift
            or return $cb->();

         # stat the dir another time
         aio_stat $path, sub {
            my $hash2 = join ":", (stat _)[0,1,3,7,9];

            my $ndirs;

            # take the slow route if anything looks fishy
            if ($hash1 ne $hash2) {
               $ndirs = -1;
            } else {
               # if nlink == 2, we are finished
               # on non-posix-fs's, we rely on nlink < 2
               $ndirs = (stat _)[3] - 2
                  or return $cb->([], $entries);
            }

            # sort into likely dirs and likely nondirs
            # dirs == files without ".", short entries first
            $entries = [map $_->[0],
                           sort { $b->[1] cmp $a->[1] }
                              map [$_, sprintf "%s%04d", (/.\./ ? "1" : "0"), length],
                                 @$entries];

            my (@dirs, @nondirs);

            my ($statcb, $schedcb);
            my $nreq = 0;

            $schedcb = sub {
               if (@$entries) {
                  if ($nreq < $maxreq) {
                     my $ent = pop @$entries;
                     $nreq++;
                     aio_stat "$path/$ent/.", sub { $statcb->($_[0], $ent) };
                  }
               } elsif (!$nreq) {
                  # finished
                  undef $statcb;
                  undef $schedcb;
                  $cb->(\@dirs, \@nondirs) if $cb;
                  undef $cb;
               }
            };
            $statcb = sub {
               my ($status, $entry) = @_;

               if ($status < 0) {
                  $nreq--;
                  push @nondirs, $entry;
                  &$schedcb;
               } else {
                  # need to check for real directory
                  aio_lstat "$path/$entry", sub {
                     $nreq--;

                     if (-d _) {
                        push @dirs, $entry;

                        if (!--$ndirs) {
                           push @nondirs, @$entries;
                           $entries = [];
                        }
                     } else {
                        push @nondirs, $entry;
                     }

                     &$schedcb;
                  }
               }
            };

            &$schedcb while @$entries && $nreq < $maxreq;
         };
      };
   };
}

=item aio_fsync $fh, $callback->($status)

Asynchronously call fsync on the given filehandle and call the callback
with the fsync result code.

=item aio_fdatasync $fh, $callback->($status)

Asynchronously call fdatasync on the given filehandle and call the
callback with the fdatasync result code.

If this call isn't available because your OS lacks it or it couldn't be
detected, it will be emulated by calling C<fsync> instead.

=back

=head2 SUPPORT FUNCTIONS

=over 4

=item $fileno = IO::AIO::poll_fileno

Return the I<request result pipe file descriptor>. This filehandle must be
polled for reading by some mechanism outside this module (e.g. Event or
select, see below or the SYNOPSIS). If the pipe becomes readable you have
to call C<poll_cb> to check the results.

See C<poll_cb> for an example.

=item IO::AIO::poll_cb

Process all outstanding events on the result pipe. You have to call this
regularly. Returns the number of events processed. Returns immediately
when no events are outstanding.

Example: Install an Event watcher that automatically calls
IO::AIO::poll_cb with high priority:

   Event->io (fd => IO::AIO::poll_fileno,
              poll => 'r', async => 1,
              cb => \&IO::AIO::poll_cb);

=item IO::AIO::poll_wait

Wait till the result filehandle becomes ready for reading (simply does a
C<select> on the filehandle. This is useful if you want to synchronously wait
for some requests to finish).

See C<nreqs> for an example.

=item IO::AIO::nreqs

Returns the number of requests currently outstanding (i.e. for which their
callback has not been invoked yet).

Example: wait till there are no outstanding requests anymore:

   IO::AIO::poll_wait, IO::AIO::poll_cb
      while IO::AIO::nreqs;

=item IO::AIO::flush

Wait till all outstanding AIO requests have been handled.

Strictly equivalent to:

   IO::AIO::poll_wait, IO::AIO::poll_cb
      while IO::AIO::nreqs;

=item IO::AIO::poll

Waits until some requests have been handled.

Strictly equivalent to:

   IO::AIO::poll_wait, IO::AIO::poll_cb
      if IO::AIO::nreqs;

=item IO::AIO::min_parallel $nthreads

Set the minimum number of AIO threads to C<$nthreads>. The current default
is C<4>, which means four asynchronous operations can be done at one time
(the number of outstanding operations, however, is unlimited).

IO::AIO starts threads only on demand, when an AIO request is queued and
no free thread exists.

It is recommended to keep the number of threads low, as some Linux
kernel versions will scale negatively with the number of threads (higher
parallelity => MUCH higher latency). With current Linux 2.6 versions, 4-32
threads should be fine.

Under most circumstances you don't need to call this function, as the
module selects a default that is suitable for low to moderate load.

=item IO::AIO::max_parallel $nthreads

Sets the maximum number of AIO threads to C<$nthreads>. If more than the
specified number of threads are currently running, this function kills
them. This function blocks until the limit is reached.

While C<$nthreads> are zero, aio requests get queued but not executed
until the number of threads has been increased again.

This module automatically runs C<max_parallel 0> at program end, to ensure
that all threads are killed and that there are no outstanding requests.

Under normal circumstances you don't need to call this function.

=item $oldnreqs = IO::AIO::max_outstanding $nreqs

Sets the maximum number of outstanding requests to C<$nreqs>. If you
try to queue up more than this number of requests, the caller will block until
some requests have been handled.

The default is very large, so normally there is no practical limit. If you
queue up many requests in a loop it often improves speed if you set
this to a relatively low number, such as C<100>.

Under normal circumstances you don't need to call this function.

=back

=cut

# support function to convert a fd into a perl filehandle
sub _fd2fh {
   return undef if $_[0] < 0;

   # try to generate nice filehandles
   my $sym = "IO::AIO::fd#$_[0]";
   local *$sym;

   open *$sym, "+<&=$_[0]"      # usually works under any unix
      or open *$sym, "<&=$_[0]" # cygwin needs this
      or open *$sym, ">&=$_[0]" # or this
      or return undef;

   *$sym
}

min_parallel 4;

END {
   max_parallel 0;
}

1;

=head2 FORK BEHAVIOUR

Before the fork, IO::AIO enters a quiescent state where no requests
can be added in other threads and no results will be processed. After
the fork the parent simply leaves the quiescent state and continues
request/result processing, while the child clears the request/result
queue (so the requests started before the fork will only be handled in
the parent). Threats will be started on demand until the limit ste in the
parent process has been reached again.

=head1 SEE ALSO

L<Coro>, L<Linux::AIO>.

=head1 AUTHOR

 Marc Lehmann <schmorp@schmorp.de>
 http://home.schmorp.de/

=cut

Revision:	1.49
Committed:	Wed Mar 1 23:56:54 2006 UTC (18 years, 3 months ago) by root
Branch:	MAIN
CVS Tags:	rel-1_73
Changes since 1.48:	+1 -1 lines
Log Message:	* empty log message *
#	Content
1	=head1 NAME
2
3	IO::AIO - Asynchronous Input/Output
4
5	=head1 SYNOPSIS
6
7	use IO::AIO;
8
9	aio_open "/etc/passwd", O_RDONLY, 0, sub {
10	my ($fh) = @_;
11	...
12	};
13
14	aio_unlink "/tmp/file", sub { };
15
16	aio_read $fh, 30000, 1024, $buffer, 0, sub {
17	$_[0] > 0 or die "read error: $!";
18	};
19
20	# AnyEvent
21	open my $fh, "<&=" . IO::AIO::poll_fileno or die "$!";
22	my $w = AnyEvent->io (fh => $fh, poll => 'r', cb => sub { IO::AIO::poll_cb });
23
24	# Event
25	Event->io (fd => IO::AIO::poll_fileno,
26	poll => 'r',
27	cb => \&IO::AIO::poll_cb);
28
29	# Glib/Gtk2
30	add_watch Glib::IO IO::AIO::poll_fileno,
31	in => sub { IO::AIO::poll_cb; 1 };
32
33	# Tk
34	Tk::Event::IO->fileevent (IO::AIO::poll_fileno, "",
35	readable => \&IO::AIO::poll_cb);
36
37	# Danga::Socket
38	Danga::Socket->AddOtherFds (IO::AIO::poll_fileno =>
39	\&IO::AIO::poll_cb);
40
41
42	=head1 DESCRIPTION
43
44	This module implements asynchronous I/O using whatever means your
45	operating system supports.
46
47	Currently, a number of threads are started that execute your read/writes
48	and signal their completion. You don't need thread support in your libc or
49	perl, and the threads created by this module will not be visible to the
50	pthreads library. In the future, this module might make use of the native
51	aio functions available on many operating systems. However, they are often
52	not well-supported (Linux doesn't allow them on normal files currently,
53	for example), and they would only support aio_read and aio_write, so the
54	remaining functionality would have to be implemented using threads anyway.
55
56	Although the module will work with in the presence of other threads, it is
57	currently not reentrant, so use appropriate locking yourself, always call
58	C<poll_cb> from within the same thread, or never call C<poll_cb> (or other
59	C<aio_> functions) recursively.
60
61	=cut
62
63	package IO::AIO;
64
65	no warnings;
66
67	use base 'Exporter';
68
69	use Fcntl ();
70
71	BEGIN {
72	$VERSION = '1.73';
73
74	@EXPORT = qw(aio_sendfile aio_read aio_write aio_open aio_close aio_stat
75	aio_lstat aio_unlink aio_rmdir aio_readdir aio_scandir aio_symlink
76	aio_fsync aio_fdatasync aio_readahead);
77	@EXPORT_OK = qw(poll_fileno poll_cb min_parallel max_parallel
78	max_outstanding nreqs);
79
80	require XSLoader;
81	XSLoader::load IO::AIO, $VERSION;
82	}
83
84	=head1 FUNCTIONS
85
86	=head2 AIO FUNCTIONS
87
88	All the C<aio_*> calls are more or less thin wrappers around the syscall
89	with the same name (sans C<aio_>). The arguments are similar or identical,
90	and they all accept an additional (and optional) C<$callback> argument
91	which must be a code reference. This code reference will get called with
92	the syscall return code (e.g. most syscalls return C<-1> on error, unlike
93	perl, which usually delivers "false") as it's sole argument when the given
94	syscall has been executed asynchronously.
95
96	All functions expecting a filehandle keep a copy of the filehandle
97	internally until the request has finished.
98
99	The pathnames you pass to these routines I<must> be absolute and
100	encoded in byte form. The reason for the former is that at the time the
101	request is being executed, the current working directory could have
102	changed. Alternatively, you can make sure that you never change the
103	current working directory.
104
105	To encode pathnames to byte form, either make sure you either: a)
106	always pass in filenames you got from outside (command line, readdir
107	etc.), b) are ASCII or ISO 8859-1, c) use the Encode module and encode
108	your pathnames to the locale (or other) encoding in effect in the user
109	environment, d) use Glib::filename_from_unicode on unicode filenames or e)
110	use something else.
111
112	=over 4
113
114	=item aio_open $pathname, $flags, $mode, $callback->($fh)
115
116	Asynchronously open or create a file and call the callback with a newly
117	created filehandle for the file.
118
119	The pathname passed to C<aio_open> must be absolute. See API NOTES, above,
120	for an explanation.
121
122	The C<$flags> argument is a bitmask. See the C<Fcntl> module for a
123	list. They are the same as used by C<sysopen>.
124
125	Likewise, C<$mode> specifies the mode of the newly created file, if it
126	didn't exist and C<O_CREAT> has been given, just like perl's C<sysopen>,
127	except that it is mandatory (i.e. use C<0> if you don't create new files,
128	and C<0666> or C<0777> if you do).
129
130	Example:
131
132	aio_open "/etc/passwd", O_RDONLY, 0, sub {
133	if ($_[0]) {
134	print "open successful, fh is $_[0]\n";
135	...
136	} else {
137	die "open failed: $!\n";
138	}
139	};
140
141	=item aio_close $fh, $callback->($status)
142
143	Asynchronously close a file and call the callback with the result
144	code. I<WARNING:> although accepted, you should not pass in a perl
145	filehandle here, as perl will likely close the file descriptor another
146	time when the filehandle is destroyed. Normally, you can safely call perls
147	C<close> or just let filehandles go out of scope.
148
149	This is supposed to be a bug in the API, so that might change. It's
150	therefore best to avoid this function.
151
152	=item aio_read $fh,$offset,$length, $data,$dataoffset, $callback->($retval)
153
154	=item aio_write $fh,$offset,$length, $data,$dataoffset, $callback->($retval)
155
156	Reads or writes C<length> bytes from the specified C<fh> and C<offset>
157	into the scalar given by C<data> and offset C<dataoffset> and calls the
158	callback without the actual number of bytes read (or -1 on error, just
159	like the syscall).
160
161	The C<$data> scalar I<MUST NOT> be modified in any way while the request
162	is outstanding. Modifying it can result in segfaults or WW3 (if the
163	necessary/optional hardware is installed).
164
165	Example: Read 15 bytes at offset 7 into scalar C<$buffer>, starting at
166	offset C<0> within the scalar:
167
168	aio_read $fh, 7, 15, $buffer, 0, sub {
169	$_[0] > 0 or die "read error: $!";
170	print "read $_[0] bytes: <$buffer>\n";
171	};
172
173	=item aio_sendfile $out_fh, $in_fh, $in_offset, $length, $callback->($retval)
174
175	Tries to copy C<$length> bytes from C<$in_fh> to C<$out_fh>. It starts
176	reading at byte offset C<$in_offset>, and starts writing at the current
177	file offset of C<$out_fh>. Because of that, it is not safe to issue more
178	than one C<aio_sendfile> per C<$out_fh>, as they will interfere with each
179	other.
180
181	This call tries to make use of a native C<sendfile> syscall to provide
182	zero-copy operation. For this to work, C<$out_fh> should refer to a
183	socket, and C<$in_fh> should refer to mmap'able file.
184
185	If the native sendfile call fails or is not implemented, it will be
186	emulated, so you can call C<aio_sendfile> on any type of filehandle
187	regardless of the limitations of the operating system.
188
189	Please note, however, that C<aio_sendfile> can read more bytes from
190	C<$in_fh> than are written, and there is no way to find out how many
191	bytes have been read from C<aio_sendfile> alone, as C<aio_sendfile> only
192	provides the number of bytes written to C<$out_fh>. Only if the result
193	value equals C<$length> one can assume that C<$length> bytes have been
194	read.
195
196	=item aio_readahead $fh,$offset,$length, $callback->($retval)
197
198	C<aio_readahead> populates the page cache with data from a file so that
199	subsequent reads from that file will not block on disk I/O. The C<$offset>
200	argument specifies the starting point from which data is to be read and
201	C<$length> specifies the number of bytes to be read. I/O is performed in
202	whole pages, so that offset is effectively rounded down to a page boundary
203	and bytes are read up to the next page boundary greater than or equal to
204	(off-set+length). C<aio_readahead> does not read beyond the end of the
205	file. The current file offset of the file is left unchanged.
206
207	If that syscall doesn't exist (likely if your OS isn't Linux) it will be
208	emulated by simply reading the data, which would have a similar effect.
209
210	=item aio_stat $fh_or_path, $callback->($status)
211
212	=item aio_lstat $fh, $callback->($status)
213
214	Works like perl's C<stat> or C<lstat> in void context. The callback will
215	be called after the stat and the results will be available using C<stat _>
216	or C<-s _> etc...
217
218	The pathname passed to C<aio_stat> must be absolute. See API NOTES, above,
219	for an explanation.
220
221	Currently, the stats are always 64-bit-stats, i.e. instead of returning an
222	error when stat'ing a large file, the results will be silently truncated
223	unless perl itself is compiled with large file support.
224
225	Example: Print the length of F</etc/passwd>:
226
227	aio_stat "/etc/passwd", sub {
228	$_[0] and die "stat failed: $!";
229	print "size is ", -s _, "\n";
230	};
231
232	=item aio_unlink $pathname, $callback->($status)
233
234	Asynchronously unlink (delete) a file and call the callback with the
235	result code.
236
237	=item aio_rmdir $pathname, $callback->($status)
238
239	Asynchronously rmdir (delete) a directory and call the callback with the
240	result code.
241
242	=item aio_readdir $pathname, $callback->($entries)
243
244	Unlike the POSIX call of the same name, C<aio_readdir> reads an entire
245	directory (i.e. opendir + readdir + closedir). The entries will not be
246	sorted, and will B<NOT> include the C<.> and C<..> entries.
247
248	The callback a single argument which is either C<undef> or an array-ref
249	with the filenames.
250
251	=item aio_scandir $path, $maxreq, $callback->($dirs, $nondirs)
252
253	Scans a directory (similar to C<aio_readdir>) and tries to separate the
254	entries of directory C<$path> into two sets of names, ones you can recurse
255	into (directories), and ones you cannot recurse into (everything else).
256
257	C<aio_scandir> is a composite request that consists of many
258	aio-primitives. C<$maxreq> specifies the maximum number of outstanding
259	aio requests that this function generates. If it is C<< <= 0 >>, then a
260	suitable default will be chosen (currently 8).
261
262	On error, the callback is called without arguments, otherwise it receives
263	two array-refs with path-relative entry names.
264
265	Example:
266
267	aio_scandir $dir, 0, sub {
268	my ($dirs, $nondirs) = @_;
269	print "real directories: @$dirs\n";
270	print "everything else: @$nondirs\n";
271	};
272
273	Implementation notes.
274
275	The C<aio_readdir> cannot be avoided, but C<stat()>'ing every entry can.
276
277	After reading the directory, the modification time, size etc. of the
278	directory before and after the readdir is checked, and if they match, the
279	link count will be used to decide how many entries are directories (if
280	>= 2). Otherwise, no knowledge of the number of subdirectories will be
281	assumed.
282
283	Then entires will be sorted into likely directories (everything without a
284	non-initial dot) and likely non-directories (everything else). Then every
285	entry + C</.> will be C<stat>'ed, likely directories first. This is often
286	faster because filesystems might detect the type of the entry without
287	reading the inode data (e.g. ext2fs filetype feature). If that succeeds,
288	it assumes that the entry is a directory or a symlink to directory (which
289	will be checked seperately).
290
291	If the known number of directories has been reached, the rest of the
292	entries is assumed to be non-directories.
293
294	=cut
295
296	sub aio_scandir($$$) {
297	my ($path, $maxreq, $cb) = @_;
298
299	$maxreq = 8 if $maxreq <= 0;
300
301	# stat once
302	aio_stat $path, sub {
303	return $cb->() if $_[0];
304	my $hash1 = join ":", (stat _)[0,1,3,7,9];
305
306	# read the directory entries
307	aio_readdir $path, sub {
308	my $entries = shift
309	or return $cb->();
310
311	# stat the dir another time
312	aio_stat $path, sub {
313	my $hash2 = join ":", (stat _)[0,1,3,7,9];
314
315	my $ndirs;
316
317	# take the slow route if anything looks fishy
318	if ($hash1 ne $hash2) {
319	$ndirs = -1;
320	} else {
321	# if nlink == 2, we are finished
322	# on non-posix-fs's, we rely on nlink < 2
323	$ndirs = (stat _)[3] - 2
324	or return $cb->([], $entries);
325	}
326
327	# sort into likely dirs and likely nondirs
328	# dirs == files without ".", short entries first
329	$entries = [map $_->[0],
330	sort { $b->[1] cmp $a->[1] }
331	map [$_, sprintf "%s%04d", (/.\./ ? "1" : "0"), length],
332	@$entries];
333
334	my (@dirs, @nondirs);
335
336	my ($statcb, $schedcb);
337	my $nreq = 0;
338
339	$schedcb = sub {
340	if (@$entries) {
341	if ($nreq < $maxreq) {
342	my $ent = pop @$entries;
343	$nreq++;
344	aio_stat "$path/$ent/.", sub { $statcb->($_[0], $ent) };
345	}
346	} elsif (!$nreq) {
347	# finished
348	undef $statcb;
349	undef $schedcb;
350	$cb->(\@dirs, \@nondirs) if $cb;
351	undef $cb;
352	}
353	};
354	$statcb = sub {
355	my ($status, $entry) = @_;
356
357	if ($status < 0) {
358	$nreq--;
359	push @nondirs, $entry;
360	&$schedcb;
361	} else {
362	# need to check for real directory
363	aio_lstat "$path/$entry", sub {
364	$nreq--;
365
366	if (-d _) {
367	push @dirs, $entry;
368
369	if (!--$ndirs) {
370	push @nondirs, @$entries;
371	$entries = [];
372	}
373	} else {
374	push @nondirs, $entry;
375	}
376
377	&$schedcb;
378	}
379	}
380	};
381
382	&$schedcb while @$entries && $nreq < $maxreq;
383	};
384	};
385	};
386	}
387
388	=item aio_fsync $fh, $callback->($status)
389
390	Asynchronously call fsync on the given filehandle and call the callback
391	with the fsync result code.
392
393	=item aio_fdatasync $fh, $callback->($status)
394
395	Asynchronously call fdatasync on the given filehandle and call the
396	callback with the fdatasync result code.
397
398	If this call isn't available because your OS lacks it or it couldn't be
399	detected, it will be emulated by calling C<fsync> instead.
400
401	=back
402
403	=head2 SUPPORT FUNCTIONS
404
405	=over 4
406
407	=item $fileno = IO::AIO::poll_fileno
408
409	Return the I<request result pipe file descriptor>. This filehandle must be
410	polled for reading by some mechanism outside this module (e.g. Event or
411	select, see below or the SYNOPSIS). If the pipe becomes readable you have
412	to call C<poll_cb> to check the results.
413
414	See C<poll_cb> for an example.
415
416	=item IO::AIO::poll_cb
417
418	Process all outstanding events on the result pipe. You have to call this
419	regularly. Returns the number of events processed. Returns immediately
420	when no events are outstanding.
421
422	Example: Install an Event watcher that automatically calls
423	IO::AIO::poll_cb with high priority:
424
425	Event->io (fd => IO::AIO::poll_fileno,
426	poll => 'r', async => 1,
427	cb => \&IO::AIO::poll_cb);
428
429	=item IO::AIO::poll_wait
430
431	Wait till the result filehandle becomes ready for reading (simply does a
432	C<select> on the filehandle. This is useful if you want to synchronously wait
433	for some requests to finish).
434
435	See C<nreqs> for an example.
436
437	=item IO::AIO::nreqs
438
439	Returns the number of requests currently outstanding (i.e. for which their
440	callback has not been invoked yet).
441
442	Example: wait till there are no outstanding requests anymore:
443
444	IO::AIO::poll_wait, IO::AIO::poll_cb
445	while IO::AIO::nreqs;
446
447	=item IO::AIO::flush
448
449	Wait till all outstanding AIO requests have been handled.
450
451	Strictly equivalent to:
452
453	IO::AIO::poll_wait, IO::AIO::poll_cb
454	while IO::AIO::nreqs;
455
456	=item IO::AIO::poll
457
458	Waits until some requests have been handled.
459
460	Strictly equivalent to:
461
462	IO::AIO::poll_wait, IO::AIO::poll_cb
463	if IO::AIO::nreqs;
464
465	=item IO::AIO::min_parallel $nthreads
466
467	Set the minimum number of AIO threads to C<$nthreads>. The current default
468	is C<4>, which means four asynchronous operations can be done at one time
469	(the number of outstanding operations, however, is unlimited).
470
471	IO::AIO starts threads only on demand, when an AIO request is queued and
472	no free thread exists.
473
474	It is recommended to keep the number of threads low, as some Linux
475	kernel versions will scale negatively with the number of threads (higher
476	parallelity => MUCH higher latency). With current Linux 2.6 versions, 4-32
477	threads should be fine.
478
479	Under most circumstances you don't need to call this function, as the
480	module selects a default that is suitable for low to moderate load.
481
482	=item IO::AIO::max_parallel $nthreads
483
484	Sets the maximum number of AIO threads to C<$nthreads>. If more than the
485	specified number of threads are currently running, this function kills
486	them. This function blocks until the limit is reached.
487
488	While C<$nthreads> are zero, aio requests get queued but not executed
489	until the number of threads has been increased again.
490
491	This module automatically runs C<max_parallel 0> at program end, to ensure
492	that all threads are killed and that there are no outstanding requests.
493
494	Under normal circumstances you don't need to call this function.
495
496	=item $oldnreqs = IO::AIO::max_outstanding $nreqs
497
498	Sets the maximum number of outstanding requests to C<$nreqs>. If you
499	try to queue up more than this number of requests, the caller will block until
500	some requests have been handled.
501
502	The default is very large, so normally there is no practical limit. If you
503	queue up many requests in a loop it often improves speed if you set
504	this to a relatively low number, such as C<100>.
505
506	Under normal circumstances you don't need to call this function.
507
508	=back
509
510	=cut
511
512	# support function to convert a fd into a perl filehandle
513	sub _fd2fh {
514	return undef if $_[0] < 0;
515
516	# try to generate nice filehandles
517	my $sym = "IO::AIO::fd#$_[0]";
518	local *$sym;
519
520	open *$sym, "+<&=$_[0]" # usually works under any unix
521	or open *$sym, "<&=$_[0]" # cygwin needs this
522	or open *$sym, ">&=$_[0]" # or this
523	or return undef;
524
525	*$sym
526	}
527
528	min_parallel 4;
529
530	END {
531	max_parallel 0;
532	}
533
534	1;
535
536	=head2 FORK BEHAVIOUR
537
538	Before the fork, IO::AIO enters a quiescent state where no requests
539	can be added in other threads and no results will be processed. After
540	the fork the parent simply leaves the quiescent state and continues
541	request/result processing, while the child clears the request/result
542	queue (so the requests started before the fork will only be handled in
543	the parent). Threats will be started on demand until the limit ste in the
544	parent process has been reached again.
545
546	=head1 SEE ALSO
547
548	L<Coro>, L<Linux::AIO>.
549
550	=head1 AUTHOR
551
552	Marc Lehmann <schmorp@schmorp.de>
553	http://home.schmorp.de/
554
555	=cut
556