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NAME

       IO::AIO - Asynchronous Input/Output

SYNOPSIS

        use IO::AIO;

        aio_open "/etc/passwd", IO::AIO::O_RDONLY, 0, sub {
           my $fh = shift
              or die "/etc/passwd: $!";
           ...
        };

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

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

        # version 2+ has request and group objects
        use IO::AIO 2;

        aioreq_pri 4; # give next request a very high priority
        my $req = aio_unlink "/tmp/file", sub { };
        $req->cancel; # cancel request if still in queue

        my $grp = aio_group sub { print "all stats done
" };
        add $grp aio_stat "..." for ...;

DESCRIPTION

       This module implements asynchronous I/O using whatever means your
       operating system supports. It is implemented as an interface to
       "libeio" (<http://software.schmorp.de/pkg/libeio.php>).

       Asynchronous means that operations that can normally block your program
       (e.g. reading from disk) will be done asynchronously: the operation
       will still block, but you can do something else in the meantime. This
       is extremely useful for programs that need to stay interactive even
       when doing heavy I/O (GUI programs, high performance network servers
       etc.), but can also be used to easily do operations in parallel that
       are normally done sequentially, e.g. stat'ing many files, which is much
       faster on a RAID volume or over NFS when you do a number of stat
       operations concurrently.

       While most of this works on all types of file descriptors (for example
       sockets), using these functions on file descriptors that support
       nonblocking operation (again, sockets, pipes etc.) is very inefficient.
       Use an event loop for that (such as the EV module): IO::AIO will
       naturally fit into such an event loop itself.

       In this version, a number of threads are started that execute your
       requests and signal their completion. You don't need thread support in
       perl, and the threads created by this module will not be visible to
       perl. 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 or restricted (GNU/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 in the presence of other (Perl-) threads,
       it is currently not reentrant in any way, so use appropriate locking
       yourself, always call "poll_cb" from within the same thread, or never
       call "poll_cb" (or other "aio_" functions) recursively.

   EXAMPLE
       This is a simple example that uses the EV module and loads /etc/passwd
       asynchronously:

          use EV;
          use IO::AIO;

          # register the IO::AIO callback with EV
          my $aio_w = EV::io IO::AIO::poll_fileno, EV::READ, \&IO::AIO::poll_cb;

          # queue the request to open /etc/passwd
          aio_open "/etc/passwd", IO::AIO::O_RDONLY, 0, sub {
             my $fh = shift
                or die "error while opening: $!";

             # stat'ing filehandles is generally non-blocking
             my $size = -s $fh;

             # queue a request to read the file
             my $contents;
             aio_read $fh, 0, $size, $contents, 0, sub {
                $_[0] == $size
                   or die "short read: $!";

                close $fh;

                # file contents now in $contents
                print $contents;

                # exit event loop and program
                EV::unloop;
             };
          };

          # possibly queue up other requests, or open GUI windows,
          # check for sockets etc. etc.

          # process events as long as there are some:
          EV::loop;

REQUEST ANATOMY AND LIFETIME

       Every "aio_*" function creates a request. which is a C data structure
       not directly visible to Perl.

       If called in non-void context, every request function returns a Perl
       object representing the request. In void context, nothing is returned,
       which saves a bit of memory.

       The perl object is a fairly standard ref-to-hash object. The hash
       contents are not used by IO::AIO so you are free to store anything you
       like in it.

       During their existance, aio requests travel through the following
       states, in order:

       ready
           Immediately after a request is created it is put into the ready
           state, waiting for a thread to execute it.

       execute
           A thread has accepted the request for processing and is currently
           executing it (e.g. blocking in read).

       pending
           The request has been executed and is waiting for result processing.

           While request submission and execution is fully asynchronous,
           result processing is not and relies on the perl interpreter calling
           "poll_cb" (or another function with the same effect).

       result
           The request results are processed synchronously by "poll_cb".

           The "poll_cb" function will process all outstanding aio requests by
           calling their callbacks, freeing memory associated with them and
           managing any groups they are contained in.

       done
           Request has reached the end of its lifetime and holds no resources
           anymore (except possibly for the Perl object, but its connection to
           the actual aio request is severed and calling its methods will
           either do nothing or result in a runtime error).

FUNCTIONS

   QUICK OVERVIEW
       This section simply lists the prototypes most of the functions for
       quick reference. See the following sections for function-by-function
       documentation.

          aio_wd $pathname, $callback->($wd)
          aio_open $pathname, $flags, $mode, $callback->($fh)
          aio_close $fh, $callback->($status)
          aio_seek  $fh,$offset,$whence, $callback->($offs)
          aio_read  $fh,$offset,$length, $data,$dataoffset, $callback->($retval)
          aio_write $fh,$offset,$length, $data,$dataoffset, $callback->($retval)
          aio_sendfile $out_fh, $in_fh, $in_offset, $length, $callback->($retval)
          aio_readahead $fh,$offset,$length, $callback->($retval)
          aio_stat  $fh_or_path, $callback->($status)
          aio_lstat $fh, $callback->($status)
          aio_statvfs $fh_or_path, $callback->($statvfs)
          aio_utime $fh_or_path, $atime, $mtime, $callback->($status)
          aio_chown $fh_or_path, $uid, $gid, $callback->($status)
          aio_chmod $fh_or_path, $mode, $callback->($status)
          aio_truncate $fh_or_path, $offset, $callback->($status)
          aio_allocate $fh, $mode, $offset, $len, $callback->($status)
          aio_fiemap $fh, $start, $length, $flags, $count, $cb->(\@extents)
          aio_unlink $pathname, $callback->($status)
          aio_mknod $pathname, $mode, $dev, $callback->($status)
          aio_link $srcpath, $dstpath, $callback->($status)
          aio_symlink $srcpath, $dstpath, $callback->($status)
          aio_readlink $pathname, $callback->($link)
          aio_realpath $pathname, $callback->($link)
          aio_rename $srcpath, $dstpath, $callback->($status)
          aio_mkdir $pathname, $mode, $callback->($status)
          aio_rmdir $pathname, $callback->($status)
          aio_readdir $pathname, $callback->($entries)
          aio_readdirx $pathname, $flags, $callback->($entries, $flags)
             IO::AIO::READDIR_DENTS IO::AIO::READDIR_DIRS_FIRST
             IO::AIO::READDIR_STAT_ORDER IO::AIO::READDIR_FOUND_UNKNOWN
          aio_scandir $pathname, $maxreq, $callback->($dirs, $nondirs)
          aio_load $pathname, $data, $callback->($status)
          aio_copy $srcpath, $dstpath, $callback->($status)
          aio_move $srcpath, $dstpath, $callback->($status)
          aio_rmtree $pathname, $callback->($status)
          aio_sync $callback->($status)
          aio_syncfs $fh, $callback->($status)
          aio_fsync $fh, $callback->($status)
          aio_fdatasync $fh, $callback->($status)
          aio_sync_file_range $fh, $offset, $nbytes, $flags, $callback->($status)
          aio_pathsync $pathname, $callback->($status)
          aio_msync $scalar, $offset = 0, $length = undef, flags = 0, $callback->($status)
          aio_mtouch $scalar, $offset = 0, $length = undef, flags = 0, $callback->($status)
          aio_mlock $scalar, $offset = 0, $length = undef, $callback->($status)
          aio_mlockall $flags, $callback->($status)
          aio_group $callback->(...)
          aio_nop $callback->()

          $prev_pri = aioreq_pri [$pri]
          aioreq_nice $pri_adjust

          IO::AIO::poll_wait
          IO::AIO::poll_cb
          IO::AIO::poll
          IO::AIO::flush
          IO::AIO::max_poll_reqs $nreqs
          IO::AIO::max_poll_time $seconds
          IO::AIO::min_parallel $nthreads
          IO::AIO::max_parallel $nthreads
          IO::AIO::max_idle $nthreads
          IO::AIO::idle_timeout $seconds
          IO::AIO::max_outstanding $maxreqs
          IO::AIO::nreqs
          IO::AIO::nready
          IO::AIO::npending

          IO::AIO::sendfile $ofh, $ifh, $offset, $count
          IO::AIO::fadvise $fh, $offset, $len, $advice
          IO::AIO::mmap $scalar, $length, $prot, $flags[, $fh[, $offset]]
          IO::AIO::munmap $scalar
          IO::AIO::madvise $scalar, $offset, $length, $advice
          IO::AIO::mprotect $scalar, $offset, $length, $protect
          IO::AIO::munlock $scalar, $offset = 0, $length = undef
          IO::AIO::munlockall

   API NOTES
       All the "aio_*" calls are more or less thin wrappers around the syscall
       with the same name (sans "aio_"). The arguments are similar or
       identical, and they all accept an additional (and optional) $callback
       argument which must be a code reference. This code reference will be
       called after the syscall has been executed in an asynchronous fashion.
       The results of the request will be passed as arguments to the callback
       (and, if an error occured, in $!) - for most requests the syscall
       return code (e.g.  most syscalls return "-1" on error, unlike perl,
       which usually delivers "false").

       Some requests (such as "aio_readdir") pass the actual results and
       communicate failures by passing "undef".

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

       All functions return request objects of type IO::AIO::REQ that allow
       further manipulation of those requests while they are in-flight.

       The pathnames you pass to these routines should be absolute. The reason
       for this 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 anywhere in the
       program and then use relative paths. You can also take advantage of
       IO::AIOs working directory abstraction, that lets you specify paths
       relative to some previously-opened "working directory object" - see the
       description of the "IO::AIO::WD" class later in this document.

       To encode pathnames as octets, either make sure you either: a) always
       pass in filenames you got from outside (command line, readdir etc.)
       without tinkering, b) are in your native filesystem encoding, 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 to ensure your scalar has the correct contents.

       This works, btw. independent of the internal UTF-8 bit, which IO::AIO
       handles correctly whether it is set or not.

   AIO REQUEST FUNCTIONS
       $prev_pri = aioreq_pri [$pri]
           Returns the priority value that would be used for the next request
           and, if $pri is given, sets the priority for the next aio request.

           The default priority is 0, the minimum and maximum priorities are
           "-4" and 4, respectively. Requests with higher priority will be
           serviced first.

           The priority will be reset to 0 after each call to one of the
           "aio_*" functions.

           Example: open a file with low priority, then read something from it
           with higher priority so the read request is serviced before other
           low priority open requests (potentially spamming the cache):

              aioreq_pri -3;
              aio_open ..., sub {
                 return unless $_[0];

                 aioreq_pri -2;
                 aio_read $_[0], ..., sub {
                    ...
                 };
              };

       aioreq_nice $pri_adjust
           Similar to "aioreq_pri", but subtracts the given value from the
           current priority, so the effect is cumulative.

       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 (or "undef" in case of an
           error).

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

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

           Likewise, $mode specifies the mode of the newly created file, if it
           didn't exist and "O_CREAT" has been given, just like perl's
           "sysopen", except that it is mandatory (i.e. use 0 if you don't
           create new files, and 0666 or 0777 if you do). Note that the $mode
           will be modified by the umask in effect then the request is being
           executed, so better never change the umask.

           Example:

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

           In addition to all the common open modes/flags ("O_RDONLY",
           "O_WRONLY", "O_RDWR", "O_CREAT", "O_TRUNC", "O_EXCL" and
           "O_APPEND"), the following POSIX and non-POSIX constants are
           available (missing ones on your system are, as usual, 0):

           "O_ASYNC", "O_DIRECT", "O_NOATIME", "O_CLOEXEC", "O_NOCTTY",
           "O_NOFOLLOW", "O_NONBLOCK", "O_EXEC", "O_SEARCH", "O_DIRECTORY",
           "O_DSYNC", "O_RSYNC", "O_SYNC" and "O_TTY_INIT".

       aio_close $fh, $callback->($status)
           Asynchronously close a file and call the callback with the result
           code.

           Unfortunately, you can't do this to perl. Perl insists very
           strongly on closing the file descriptor associated with the
           filehandle itself.

           Therefore, "aio_close" will not close the filehandle - instead it
           will use dup2 to overwrite the file descriptor with the write-end
           of a pipe (the pipe fd will be created on demand and will be
           cached).

           Or in other words: the file descriptor will be closed, but it will
           not be free for reuse until the perl filehandle is closed.

       aio_seek $fh, $offset, $whence, $callback->($offs)
           Seeks the filehandle to the new $offset, similarly to perl's
           "sysseek". The $whence can use the traditional values (0 for
           "IO::AIO::SEEK_SET", 1 for "IO::AIO::SEEK_CUR" or 2 for
           "IO::AIO::SEEK_END").

           The resulting absolute offset will be passed to the callback, or
           "-1" in case of an error.

           In theory, the $whence constants could be different than the
           corresponding values from Fcntl, but perl guarantees they are the
           same, so don't panic.

           As a GNU/Linux (and maybe Solaris) extension, also the constants
           "IO::AIO::SEEK_DATA" and "IO::AIO::SEEK_HOLE" are available, if
           they could be found. No guarantees about suitability for use in
           "aio_seek" or Perl's "sysseek" can be made though, although I would
           naively assume they "just work".

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

           "aio_read" will, like "sysread", shrink or grow the $data scalar to
           offset plus the actual number of bytes read.

           If $offset is undefined, then the current file descriptor offset
           will be used (and updated), otherwise the file descriptor offset
           will not be changed by these calls.

           If $length is undefined in "aio_write", use the remaining length of
           $data.

           If $dataoffset is less than zero, it will be counted from the end
           of $data.

           The $data scalar MUST NOT be modified in any way while the request
           is outstanding. Modifying it can result in segfaults or World War
           III (if the necessary/optional hardware is installed).

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

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

       aio_sendfile $out_fh, $in_fh, $in_offset, $length, $callback->($retval)
           Tries to copy $length bytes from $in_fh to $out_fh. It starts
           reading at byte offset $in_offset, and starts writing at the
           current file offset of $out_fh. Because of that, it is not safe to
           issue more than one "aio_sendfile" per $out_fh, as they will
           interfere with each other. The same $in_fh works fine though, as
           this function does not move or use the file offset of $in_fh.

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

           Unlike with other "aio_" functions, it makes a lot of sense to use
           "aio_sendfile" on non-blocking sockets, as long as one end
           (typically the $in_fh) is a file - the file I/O will then be
           asynchronous, while the socket I/O will be non-blocking. Note,
           however, that you can run into a trap where "aio_sendfile" reads
           some data with readahead, then fails to write all data, and when
           the socket is ready the next time, the data in the cache is already
           lost, forcing "aio_sendfile" to again hit the disk. Explicit
           "aio_read" + "aio_write" let's you better control resource usage.

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

           If a native sendfile cannot be found or it fails with "ENOSYS",
           "EINVAL", "ENOTSUP", "EOPNOTSUPP", "EAFNOSUPPORT", "EPROTOTYPE" or
           "ENOTSOCK", it will be emulated, so you can call "aio_sendfile" on
           any type of filehandle regardless of the limitations of the
           operating system.

           As native sendfile syscalls (as practically any non-POSIX interface
           hacked together in a hurry to improve benchmark numbers) tend to be
           rather buggy on many systems, this implementation tries to work
           around some known bugs in Linux and FreeBSD kernels (probably
           others, too), but that might fail, so you really really should
           check the return value of "aio_sendfile" - fewre bytes than
           expected might have been transferred.

       aio_readahead $fh,$offset,$length, $callback->($retval)
           "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 $offset argument specifies the starting point from which data
           is to be read and $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).
           "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.

       aio_stat  $fh_or_path, $callback->($status)
       aio_lstat $fh, $callback->($status)
           Works like perl's "stat" or "lstat" in void context. The callback
           will be called after the stat and the results will be available
           using "stat _" or "-s _" etc...

           The pathname passed to "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.

           To help interpret the mode and dev/rdev stat values, IO::AIO offers
           the following constants and functions (if not implemented, the
           constants will be 0 and the functions will either "croak" or fall
           back on traditional behaviour).

           "S_IFMT", "S_IFIFO", "S_IFCHR", "S_IFBLK", "S_IFLNK", "S_IFREG",
           "S_IFDIR", "S_IFWHT", "S_IFSOCK", "IO::AIO::major $dev_t",
           "IO::AIO::minor $dev_t", "IO::AIO::makedev $major, $minor".

           Example: Print the length of /etc/passwd:

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

       aio_statvfs $fh_or_path, $callback->($statvfs)
           Works like the POSIX "statvfs" or "fstatvfs" syscalls, depending on
           whether a file handle or path was passed.

           On success, the callback is passed a hash reference with the
           following members: "bsize", "frsize", "blocks", "bfree", "bavail",
           "files", "ffree", "favail", "fsid", "flag" and "namemax". On
           failure, "undef" is passed.

           The following POSIX IO::AIO::ST_* constants are defined:
           "ST_RDONLY" and "ST_NOSUID".

           The following non-POSIX IO::AIO::ST_* flag masks are defined to
           their correct value when available, or to 0 on systems that do not
           support them:  "ST_NODEV", "ST_NOEXEC", "ST_SYNCHRONOUS",
           "ST_MANDLOCK", "ST_WRITE", "ST_APPEND", "ST_IMMUTABLE",
           "ST_NOATIME", "ST_NODIRATIME" and "ST_RELATIME".

           Example: stat "/wd" and dump out the data if successful.

              aio_statvfs "/wd", sub {
                 my $f = $_[0]
                    or die "statvfs: $!";

                 use Data::Dumper;
                 say Dumper $f;
              };

              # result:
              {
                 bsize   => 1024,
                 bfree   => 4333064312,
                 blocks  => 10253828096,
                 files   => 2050765568,
                 flag    => 4096,
                 favail  => 2042092649,
                 bavail  => 4333064312,
                 ffree   => 2042092649,
                 namemax => 255,
                 frsize  => 1024,
                 fsid    => 1810
              }

           Here is a (likely partial - send me updates!) list of fsid values
           used by Linux - it is safe to hardcode these when $^O is "linux":

              0x0000adf5 adfs
              0x0000adff affs
              0x5346414f afs
              0x09041934 anon-inode filesystem
              0x00000187 autofs
              0x42465331 befs
              0x1badface bfs
              0x42494e4d binfmt_misc
              0x9123683e btrfs
              0x0027e0eb cgroupfs
              0xff534d42 cifs
              0x73757245 coda
              0x012ff7b7 coh
              0x28cd3d45 cramfs
              0x453dcd28 cramfs-wend (wrong endianness)
              0x64626720 debugfs
              0x00001373 devfs
              0x00001cd1 devpts
              0x0000f15f ecryptfs
              0x00414a53 efs
              0x0000137d ext
              0x0000ef53 ext2/ext3
              0x0000ef51 ext2
              0x00004006 fat
              0x65735546 fuseblk
              0x65735543 fusectl
              0x0bad1dea futexfs
              0x01161970 gfs2
              0x47504653 gpfs
              0x00004244 hfs
              0xf995e849 hpfs
              0x958458f6 hugetlbfs
              0x2bad1dea inotifyfs
              0x00009660 isofs
              0x000072b6 jffs2
              0x3153464a jfs
              0x6b414653 k-afs
              0x0bd00bd0 lustre
              0x0000137f minix
              0x0000138f minix 30 char names
              0x00002468 minix v2
              0x00002478 minix v2 30 char names
              0x00004d5a minix v3
              0x19800202 mqueue
              0x00004d44 msdos
              0x0000564c novell
              0x00006969 nfs
              0x6e667364 nfsd
              0x00003434 nilfs
              0x5346544e ntfs
              0x00009fa1 openprom
              0x7461636F ocfs2
              0x00009fa0 proc
              0x6165676c pstorefs
              0x0000002f qnx4
              0x858458f6 ramfs
              0x52654973 reiserfs
              0x00007275 romfs
              0x67596969 rpc_pipefs
              0x73636673 securityfs
              0xf97cff8c selinux
              0x0000517b smb
              0x534f434b sockfs
              0x73717368 squashfs
              0x62656572 sysfs
              0x012ff7b6 sysv2
              0x012ff7b5 sysv4
              0x01021994 tmpfs
              0x15013346 udf
              0x00011954 ufs
              0x54190100 ufs byteswapped
              0x00009fa2 usbdevfs
              0x01021997 v9fs
              0xa501fcf5 vxfs
              0xabba1974 xenfs
              0x012ff7b4 xenix
              0x58465342 xfs
              0x012fd16d xia

       aio_utime $fh_or_path, $atime, $mtime, $callback->($status)
           Works like perl's "utime" function (including the special case of
           $atime and $mtime being undef). Fractional times are supported if
           the underlying syscalls support them.

           When called with a pathname, uses utimes(2) if available, otherwise
           utime(2). If called on a file descriptor, uses futimes(2) if
           available, otherwise returns ENOSYS, so this is not portable.

           Examples:

              # set atime and mtime to current time (basically touch(1)):
              aio_utime "path", undef, undef;
              # set atime to current time and mtime to beginning of the epoch:
              aio_utime "path", time, undef; # undef==0

       aio_chown $fh_or_path, $uid, $gid, $callback->($status)
           Works like perl's "chown" function, except that "undef" for either
           $uid or $gid is being interpreted as "do not change" (but -1 can
           also be used).

           Examples:

              # same as "chown root path" in the shell:
              aio_chown "path", 0, -1;
              # same as above:
              aio_chown "path", 0, undef;

       aio_truncate $fh_or_path, $offset, $callback->($status)
           Works like truncate(2) or ftruncate(2).

       aio_allocate $fh, $mode, $offset, $len, $callback->($status)
           Allocates or freed disk space according to the $mode argument. See
           the linux "fallocate" docuemntation for details.

           $mode can currently be 0 or "IO::AIO::FALLOC_FL_KEEP_SIZE" to
           allocate space, or "IO::AIO::FALLOC_FL_PUNCH_HOLE |
           IO::AIO::FALLOC_FL_KEEP_SIZE", to deallocate a file range.

           The file system block size used by "fallocate" is presumably the
           "f_bsize" returned by "statvfs".

           If "fallocate" isn't available or cannot be emulated (currently no
           emulation will be attempted), passes "-1" and sets $! to "ENOSYS".

       aio_chmod $fh_or_path, $mode, $callback->($status)
           Works like perl's "chmod" function.

       aio_unlink $pathname, $callback->($status)
           Asynchronously unlink (delete) a file and call the callback with
           the result code.

       aio_mknod $pathname, $mode, $dev, $callback->($status)
           [EXPERIMENTAL]

           Asynchronously create a device node (or fifo). See mknod(2).

           The only (POSIX-) portable way of calling this function is:

              aio_mknod $pathname, IO::AIO::S_IFIFO | $mode, 0, sub { ...

           See "aio_stat" for info about some potentially helpful extra
           constants and functions.

       aio_link $srcpath, $dstpath, $callback->($status)
           Asynchronously create a new link to the existing object at $srcpath
           at the path $dstpath and call the callback with the result code.

       aio_symlink $srcpath, $dstpath, $callback->($status)
           Asynchronously create a new symbolic link to the existing object at
           $srcpath at the path $dstpath and call the callback with the result
           code.

       aio_readlink $pathname, $callback->($link)
           Asynchronously read the symlink specified by $path and pass it to
           the callback. If an error occurs, nothing or undef gets passed to
           the callback.

       aio_realpath $pathname, $callback->($path)
           Asynchronously make the path absolute and resolve any symlinks in
           $path. The resulting path only consists of directories (same as
           Cwd::realpath).

           This request can be used to get the absolute path of the current
           working directory by passing it a path of . (a single dot).

       aio_rename $srcpath, $dstpath, $callback->($status)
           Asynchronously rename the object at $srcpath to $dstpath, just as
           rename(2) and call the callback with the result code.

           On systems that support the AIO::WD working directory abstraction
           natively, the case "[$wd, "."]" as $srcpath is specialcased -
           instead of failing, "rename" is called on the absolute path of $wd.

       aio_mkdir $pathname, $mode, $callback->($status)
           Asynchronously mkdir (create) a directory and call the callback
           with the result code. $mode will be modified by the umask at the
           time the request is executed, so do not change your umask.

       aio_rmdir $pathname, $callback->($status)
           Asynchronously rmdir (delete) a directory and call the callback
           with the result code.

           On systems that support the AIO::WD working directory abstraction
           natively, the case "[$wd, "."]" is specialcased - instead of
           failing, "rmdir" is called on the absolute path of $wd.

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

           The callback is passed a single argument which is either "undef" or
           an array-ref with the filenames.

       aio_readdirx $pathname, $flags, $callback->($entries, $flags)
           Quite similar to "aio_readdir", but the $flags argument allows one
           to tune behaviour and output format. In case of an error, $entries
           will be "undef".

           The flags are a combination of the following constants, ORed
           together (the flags will also be passed to the callback, possibly
           modified):

           IO::AIO::READDIR_DENTS
               When this flag is off, then the callback gets an arrayref
               consisting of names only (as with "aio_readdir"), otherwise it
               gets an arrayref with "[$name, $type, $inode]" arrayrefs, each
               describing a single directory entry in more detail.

               $name is the name of the entry.

               $type is one of the "IO::AIO::DT_xxx" constants:

               "IO::AIO::DT_UNKNOWN", "IO::AIO::DT_FIFO", "IO::AIO::DT_CHR",
               "IO::AIO::DT_DIR", "IO::AIO::DT_BLK", "IO::AIO::DT_REG",
               "IO::AIO::DT_LNK", "IO::AIO::DT_SOCK", "IO::AIO::DT_WHT".

               "IO::AIO::DT_UNKNOWN" means just that: readdir does not know.
               If you need to know, you have to run stat yourself. Also, for
               speed reasons, the $type scalars are read-only: you can not
               modify them.

               $inode is the inode number (which might not be exact on systems
               with 64 bit inode numbers and 32 bit perls). This field has
               unspecified content on systems that do not deliver the inode
               information.

           IO::AIO::READDIR_DIRS_FIRST
               When this flag is set, then the names will be returned in an
               order where likely directories come first, in optimal stat
               order. This is useful when you need to quickly find
               directories, or you want to find all directories while avoiding
               to stat() each entry.

               If the system returns type information in readdir, then this is
               used to find directories directly. Otherwise, likely
               directories are names beginning with ".", or otherwise names
               with no dots, of which names with short names are tried first.

           IO::AIO::READDIR_STAT_ORDER
               When this flag is set, then the names will be returned in an
               order suitable for stat()'ing each one. That is, when you plan
               to stat() all files in the given directory, then the returned
               order will likely be fastest.

               If both this flag and "IO::AIO::READDIR_DIRS_FIRST" are
               specified, then the likely dirs come first, resulting in a less
               optimal stat order.

           IO::AIO::READDIR_FOUND_UNKNOWN
               This flag should not be set when calling "aio_readdirx".
               Instead, it is being set by "aio_readdirx", when any of the
               $type's found were "IO::AIO::DT_UNKNOWN". The absence of this
               flag therefore indicates that all $type's are known, which can
               be used to speed up some algorithms.

       aio_load $pathname, $data, $callback->($status)
           This is a composite request that tries to fully load the given file
           into memory. Status is the same as with aio_read.

       aio_copy $srcpath, $dstpath, $callback->($status)
           Try to copy the file (directories not supported as either source or
           destination) from $srcpath to $dstpath and call the callback with a
           status of 0 (ok) or "-1" (error, see $!).

           This is a composite request that creates the destination file with
           mode 0200 and copies the contents of the source file into it using
           "aio_sendfile", followed by restoring atime, mtime, access mode and
           uid/gid, in that order.

           If an error occurs, the partial destination file will be unlinked,
           if possible, except when setting atime, mtime, access mode and
           uid/gid, where errors are being ignored.

       aio_move $srcpath, $dstpath, $callback->($status)
           Try to move the file (directories not supported as either source or
           destination) from $srcpath to $dstpath and call the callback with a
           status of 0 (ok) or "-1" (error, see $!).

           This is a composite request that tries to rename(2) the file first;
           if rename fails with "EXDEV", it copies the file with "aio_copy"
           and, if that is successful, unlinks the $srcpath.

       aio_scandir $pathname, $maxreq, $callback->($dirs, $nondirs)
           Scans a directory (similar to "aio_readdir") but additionally tries
           to efficiently separate the entries of directory $path into two
           sets of names, directories you can recurse into (directories), and
           ones you cannot recurse into (everything else, including symlinks
           to directories).

           "aio_scandir" is a composite request that creates of many sub
           requests_ $maxreq specifies the maximum number of outstanding aio
           requests that this function generates. If it is "<= 0", then a
           suitable default will be chosen (currently 4).

           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
";
                 print "everything else: @$nondirs
";
              };

           Implementation notes.

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

           If readdir returns file type information, then this is used
           directly to find directories.

           Otherwise, after reading the directory, the modification time, size
           etc.  of the directory before and after the readdir is checked, and
           if they match (and isn't the current time), 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 entries will be sorted into likely directories a non-initial
           dot currently) and likely non-directories (see "aio_readdirx").
           Then every entry plus an appended "/." will be "stat"'ed, likely
           directories first, in order of their inode numbers. If that
           succeeds, it assumes that the entry is a directory or a symlink to
           directory (which will be checked separately). This is often faster
           than stat'ing the entry itself because filesystems might detect the
           type of the entry without reading the inode data (e.g. ext2fs
           filetype feature), even on systems that cannot return the filetype
           information on readdir.

           If the known number of directories (link count - 2) has been
           reached, the rest of the entries is assumed to be non-directories.

           This only works with certainty on POSIX (= UNIX) filesystems, which
           fortunately are the vast majority of filesystems around.

           It will also likely work on non-POSIX filesystems with reduced
           efficiency as those tend to return 0 or 1 as link counts, which
           disables the directory counting heuristic.

       aio_rmtree $pathname, $callback->($status)
           Delete a directory tree starting (and including) $path, return the
           status of the final "rmdir" only. This is a composite request that
           uses "aio_scandir" to recurse into and rmdir directories, and
           unlink everything else.

       aio_sync $callback->($status)
           Asynchronously call sync and call the callback when finished.

       aio_fsync $fh, $callback->($status)
           Asynchronously call fsync on the given filehandle and call the
           callback with the fsync result code.

       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 "fsync"
           instead.

       aio_syncfs $fh, $callback->($status)
           Asynchronously call the syncfs syscall to sync the filesystem
           associated to the given filehandle and call the callback with the
           syncfs result code. If syncfs is not available, calls sync(), but
           returns "-1" and sets errno to "ENOSYS" nevertheless.

       aio_sync_file_range $fh, $offset, $nbytes, $flags, $callback->($status)
           Sync the data portion of the file specified by $offset and $length
           to disk (but NOT the metadata), by calling the Linux-specific
           sync_file_range call. If sync_file_range is not available or it
           returns ENOSYS, then fdatasync or fsync is being substituted.

           $flags can be a combination of
           "IO::AIO::SYNC_FILE_RANGE_WAIT_BEFORE",
           "IO::AIO::SYNC_FILE_RANGE_WRITE" and
           "IO::AIO::SYNC_FILE_RANGE_WAIT_AFTER": refer to the sync_file_range
           manpage for details.

       aio_pathsync $pathname, $callback->($status)
           This request tries to open, fsync and close the given path. This is
           a composite request intended to sync directories after directory
           operations (E.g. rename). This might not work on all operating
           systems or have any specific effect, but usually it makes sure that
           directory changes get written to disc. It works for anything that
           can be opened for read-only, not just directories.

           Future versions of this function might fall back to other methods
           when "fsync" on the directory fails (such as calling "sync").

           Passes 0 when everything went ok, and "-1" on error.

       aio_msync $scalar, $offset = 0, $length = undef, flags = 0,
       $callback->($status)
           This is a rather advanced IO::AIO call, which only works on
           mmap(2)ed scalars (see the "IO::AIO::mmap" function, although it
           also works on data scalars managed by the Sys::Mmap or Mmap
           modules, note that the scalar must only be modified in-place while
           an aio operation is pending on it).

           It calls the "msync" function of your OS, if available, with the
           memory area starting at $offset in the string and ending $length
           bytes later. If $length is negative, counts from the end, and if
           $length is "undef", then it goes till the end of the string. The
           flags can be a combination of "IO::AIO::MS_ASYNC",
           "IO::AIO::MS_INVALIDATE" and "IO::AIO::MS_SYNC".

       aio_mtouch $scalar, $offset = 0, $length = undef, flags = 0,
       $callback->($status)
           This is a rather advanced IO::AIO call, which works best on
           mmap(2)ed scalars.

           It touches (reads or writes) all memory pages in the specified
           range inside the scalar. All caveats and parameters are the same as
           for "aio_msync", above, except for flags, which must be either 0
           (which reads all pages and ensures they are instantiated) or
           "IO::AIO::MT_MODIFY", which modifies the memory pages (by reading
           and writing an octet from it, which dirties the page).

       aio_mlock $scalar, $offset = 0, $length = undef, $callback->($status)
           This is a rather advanced IO::AIO call, which works best on
           mmap(2)ed scalars.

           It reads in all the pages of the underlying storage into memory (if
           any) and locks them, so they are not getting swapped/paged out or
           removed.

           If $length is undefined, then the scalar will be locked till the
           end.

           On systems that do not implement "mlock", this function returns
           "-1" and sets errno to "ENOSYS".

           Note that the corresponding "munlock" is synchronous and is
           documented under "MISCELLANEOUS FUNCTIONS".

           Example: open a file, mmap and mlock it - both will be undone when
           $data gets destroyed.

              open my $fh, "<", $path or die "$path: $!";
              my $data;
              IO::AIO::mmap $data, -s $fh, IO::AIO::PROT_READ, IO::AIO::MAP_SHARED, $fh;
              aio_mlock $data; # mlock in background

       aio_mlockall $flags, $callback->($status)
           Calls the "mlockall" function with the given $flags (a combination
           of "IO::AIO::MCL_CURRENT" and "IO::AIO::MCL_FUTURE").

           On systems that do not implement "mlockall", this function returns
           "-1" and sets errno to "ENOSYS".

           Note that the corresponding "munlockall" is synchronous and is
           documented under "MISCELLANEOUS FUNCTIONS".

           Example: asynchronously lock all current and future pages into
           memory.

              aio_mlockall IO::AIO::MCL_FUTURE;

       aio_fiemap $fh, $start, $length, $flags, $count, $cb->(\@extents)
           Queries the extents of the given file (by calling the Linux
           "FIEMAP" ioctl, see <http://cvs.schmorp.de/IO-AIO/doc/fiemap.txt>
           for details). If the ioctl is not available on your OS, then this
           request will fail with "ENOSYS".

           $start is the starting offset to query extents for, $length is the
           size of the range to query - if it is "undef", then the whole file
           will be queried.

           $flags is a combination of flags ("IO::AIO::FIEMAP_FLAG_SYNC" or
           "IO::AIO::FIEMAP_FLAG_XATTR" - "IO::AIO::FIEMAP_FLAGS_COMPAT" is
           also exported), and is normally 0 or "IO::AIO::FIEMAP_FLAG_SYNC" to
           query the data portion.

           $count is the maximum number of extent records to return. If it is
           "undef", then IO::AIO queries all extents of the range. As a very
           special case, if it is 0, then the callback receives the number of
           extents instead of the extents themselves (which is unreliable, see
           below).

           If an error occurs, the callback receives no arguments. The special
           "errno" value "IO::AIO::EBADR" is available to test for flag
           errors.

           Otherwise, the callback receives an array reference with extent
           structures. Each extent structure is an array reference itself,
           with the following members:

              [$logical, $physical, $length, $flags]

           Flags is any combination of the following flag values (typically
           either 0 or "IO::AIO::FIEMAP_EXTENT_LAST" (1)):

           "IO::AIO::FIEMAP_EXTENT_LAST", "IO::AIO::FIEMAP_EXTENT_UNKNOWN",
           "IO::AIO::FIEMAP_EXTENT_DELALLOC",
           "IO::AIO::FIEMAP_EXTENT_ENCODED",
           "IO::AIO::FIEMAP_EXTENT_DATA_ENCRYPTED",
           "IO::AIO::FIEMAP_EXTENT_NOT_ALIGNED",
           "IO::AIO::FIEMAP_EXTENT_DATA_INLINE",
           "IO::AIO::FIEMAP_EXTENT_DATA_TAIL",
           "IO::AIO::FIEMAP_EXTENT_UNWRITTEN", "IO::AIO::FIEMAP_EXTENT_MERGED"
           or "IO::AIO::FIEMAP_EXTENT_SHARED".

           At the time of this writing (Linux 3.2), this requets is unreliable
           unless $count is "undef", as the kernel has all sorts of bugs
           preventing it to return all extents of a range for files with large
           number of extents. The code works around all these issues if $count
           is undef.

       aio_group $callback->(...)
           This is a very special aio request: Instead of doing something, it
           is a container for other aio requests, which is useful if you want
           to bundle many requests into a single, composite, request with a
           definite callback and the ability to cancel the whole request with
           its subrequests.

           Returns an object of class IO::AIO::GRP. See its documentation
           below for more info.

           Example:

              my $grp = aio_group sub {
                 print "all stats done
";
              };

              add $grp
                 (aio_stat ...),
                 (aio_stat ...),
                 ...;

       aio_nop $callback->()
           This is a special request - it does nothing in itself and is only
           used for side effects, such as when you want to add a dummy request
           to a group so that finishing the requests in the group depends on
           executing the given code.

           While this request does nothing, it still goes through the
           execution phase and still requires a worker thread. Thus, the
           callback will not be executed immediately but only after other
           requests in the queue have entered their execution phase. This can
           be used to measure request latency.

       IO::AIO::aio_busy $fractional_seconds, $callback->()  *NOT EXPORTED*
           Mainly used for debugging and benchmarking, this aio request puts
           one of the request workers to sleep for the given time.

           While it is theoretically handy to have simple I/O scheduling
           requests like sleep and file handle readable/writable, the overhead
           this creates is immense (it blocks a thread for a long time) so do
           not use this function except to put your application under
           artificial I/O pressure.

   IO::AIO::WD - multiple working directories
       Your process only has one current working directory, which is used by
       all threads. This makes it hard to use relative paths (some other
       component could call "chdir" at any time, and it is hard to control
       when the path will be used by IO::AIO).

       One solution for this is to always use absolute paths. This usually
       works, but can be quite slow (the kernel has to walk the whole path on
       every access), and can also be a hassle to implement.

       Newer POSIX systems have a number of functions (openat, fdopendir,
       futimensat and so on) that make it possible to specify working
       directories per operation.

       For portability, and because the clowns who "designed", or shall I
       write, perpetrated this new interface were obviously half-drunk, this
       abstraction cannot be perfect, though.

       IO::AIO allows you to convert directory paths into a so-called
       IO::AIO::WD object. This object stores the canonicalised, absolute
       version of the path, and on systems that allow it, also a directory
       file descriptor.

       Everywhere where a pathname is accepted by IO::AIO (e.g. in "aio_stat"
       or "aio_unlink"), one can specify an array reference with an
       IO::AIO::WD object and a pathname instead (or the IO::AIO::WD object
       alone, which gets interpreted as "[$wd, "."]"). If the pathname is
       absolute, the IO::AIO::WD object is ignored, otherwise the pathname is
       resolved relative to that IO::AIO::WD object.

       For example, to get a wd object for /etc and then stat passwd inside,
       you would write:

          aio_wd "/etc", sub {
             my $etcdir = shift;

             # although $etcdir can be undef on error, there is generally no reason
             # to check for errors here, as aio_stat will fail with ENOENT
             # when $etcdir is undef.

             aio_stat [$etcdir, "passwd"], sub {
                # yay
             };
          };

       That "aio_wd" is a request and not a normal function shows that
       creating an IO::AIO::WD object is itself a potentially blocking
       operation, which is why it is done asynchronously.

       To stat the directory obtained with "aio_wd" above, one could write
       either of the following three request calls:

          aio_lstat "/etc"    , sub { ...  # pathname as normal string
          aio_lstat [$wd, "."], sub { ...  # "." relative to $wd (i.e. $wd itself)
          aio_lstat $wd       , sub { ...  # shorthand for the previous

       As with normal pathnames, IO::AIO keeps a copy of the working directory
       object and the pathname string, so you could write the following
       without causing any issues due to $path getting reused:

          my $path = [$wd, undef];

          for my $name (qw(abc def ghi)) {
             $path->[1] = $name;
             aio_stat $path, sub {
                # ...
             };
          }

       There are some caveats: when directories get renamed (or deleted), the
       pathname string doesn't change, so will point to the new directory (or
       nowhere at all), while the directory fd, if available on the system,
       will still point to the original directory. Most functions accepting a
       pathname will use the directory fd on newer systems, and the string on
       older systems. Some functions (such as realpath) will always rely on
       the string form of the pathname.

       So this functionality is mainly useful to get some protection against
       "chdir", to easily get an absolute path out of a relative path for
       future reference, and to speed up doing many operations in the same
       directory (e.g. when stat'ing all files in a directory).

       The following functions implement this working directory abstraction:

       aio_wd $pathname, $callback->($wd)
           Asynchonously canonicalise the given pathname and convert it to an
           IO::AIO::WD object representing it. If possible and supported on
           the system, also open a directory fd to speed up pathname
           resolution relative to this working directory.

           If something goes wrong, then "undef" is passwd to the callback
           instead of a working directory object and $! is set appropriately.
           Since passing "undef" as working directory component of a pathname
           fails the request with "ENOENT", there is often no need for error
           checking in the "aio_wd" callback, as future requests using the
           value will fail in the expected way.

       IO::AIO::CWD
           This is a compiletime constant (object) that represents the process
           current working directory.

           Specifying this object as working directory object for a pathname
           is as if the pathname would be specified directly, without a
           directory object. For example, these calls are functionally
           identical:

              aio_stat "somefile", sub { ... };
              aio_stat [IO::AIO::CWD, "somefile"], sub { ... };

       To recover the path associated with an IO::AIO::WD object, you can use
       "aio_realpath":

          aio_realpath $wd, sub {
             warn "path is $_[0]
";
          };

       Currently, "aio_statvfs" always, and "aio_rename" and "aio_rmdir"
       sometimes, fall back to using an absolue path.

   IO::AIO::REQ CLASS
       All non-aggregate "aio_*" functions return an object of this class when
       called in non-void context.

       cancel $req
           Cancels the request, if possible. Has the effect of skipping
           execution when entering the execute state and skipping calling the
           callback when entering the the result state, but will leave the
           request otherwise untouched (with the exception of readdir). That
           means that requests that currently execute will not be stopped and
           resources held by the request will not be freed prematurely.

       cb $req $callback->(...)
           Replace (or simply set) the callback registered to the request.

   IO::AIO::GRP CLASS
       This class is a subclass of IO::AIO::REQ, so all its methods apply to
       objects of this class, too.

       A IO::AIO::GRP object is a special request that can contain multiple
       other aio requests.

       You create one by calling the "aio_group" constructing function with a
       callback that will be called when all contained requests have entered
       the "done" state:

          my $grp = aio_group sub {
             print "all requests are done
";
          };

       You add requests by calling the "add" method with one or more
       "IO::AIO::REQ" objects:

          $grp->add (aio_unlink "...");

          add $grp aio_stat "...", sub {
             $_[0] or return $grp->result ("error");

             # add another request dynamically, if first succeeded
             add $grp aio_open "...", sub {
                $grp->result ("ok");
             };
          };

       This makes it very easy to create composite requests (see the source of
       "aio_move" for an application) that work and feel like simple requests.

       ·   The IO::AIO::GRP objects will be cleaned up during calls to
           "IO::AIO::poll_cb", just like any other request.

       ·   They can be canceled like any other request. Canceling will cancel
           not only the request itself, but also all requests it contains.

       ·   They can also can also be added to other IO::AIO::GRP objects.

       ·   You must not add requests to a group from within the group callback
           (or any later time).

       Their lifetime, simplified, looks like this: when they are empty, they
       will finish very quickly. If they contain only requests that are in the
       "done" state, they will also finish. Otherwise they will continue to
       exist.

       That means after creating a group you have some time to add requests
       (precisely before the callback has been invoked, which is only done
       within the "poll_cb"). And in the callbacks of those requests, you can
       add further requests to the group. And only when all those requests
       have finished will the the group itself finish.

       add $grp ...
       $grp->add (...)
           Add one or more requests to the group. Any type of IO::AIO::REQ can
           be added, including other groups, as long as you do not create
           circular dependencies.

           Returns all its arguments.

       $grp->cancel_subs
           Cancel all subrequests and clears any feeder, but not the group
           request itself. Useful when you queued a lot of events but got a
           result early.

           The group request will finish normally (you cannot add requests to
           the group).

       $grp->result (...)
           Set the result value(s) that will be passed to the group callback
           when all subrequests have finished and set the groups errno to the
           current value of errno (just like calling "errno" without an error
           number). By default, no argument will be passed and errno is zero.

       $grp->errno ([$errno])
           Sets the group errno value to $errno, or the current value of errno
           when the argument is missing.

           Every aio request has an associated errno value that is restored
           when the callback is invoked. This method lets you change this
           value from its default (0).

           Calling "result" will also set errno, so make sure you either set
           $!  before the call to "result", or call c<errno> after it.

       feed $grp $callback->($grp)
           Sets a feeder/generator on this group: every group can have an
           attached generator that generates requests if idle. The idea behind
           this is that, although you could just queue as many requests as you
           want in a group, this might starve other requests for a potentially
           long time. For example, "aio_scandir" might generate hundreds of
           thousands of "aio_stat" requests, delaying any later requests for a
           long time.

           To avoid this, and allow incremental generation of requests, you
           can instead a group and set a feeder on it that generates those
           requests. The feed callback will be called whenever there are few
           enough (see "limit", below) requests active in the group itself and
           is expected to queue more requests.

           The feed callback can queue as many requests as it likes (i.e.
           "add" does not impose any limits).

           If the feed does not queue more requests when called, it will be
           automatically removed from the group.

           If the feed limit is 0 when this method is called, it will be set
           to 2 automatically.

           Example:

              # stat all files in @files, but only ever use four aio requests concurrently:

              my $grp = aio_group sub { print "finished
" };
              limit $grp 4;
              feed $grp sub {
                 my $file = pop @files
                    or return;

                 add $grp aio_stat $file, sub { ... };
              };

       limit $grp $num
           Sets the feeder limit for the group: The feeder will be called
           whenever the group contains less than this many requests.

           Setting the limit to 0 will pause the feeding process.

           The default value for the limit is 0, but note that setting a
           feeder automatically bumps it up to 2.

   SUPPORT FUNCTIONS
       EVENT PROCESSING AND EVENT LOOP INTEGRATION

       $fileno = IO::AIO::poll_fileno
           Return the request result pipe file descriptor. This filehandle
           must be polled for reading by some mechanism outside this module
           (e.g. EV, Glib, select and so on, see below or the SYNOPSIS). If
           the pipe becomes readable you have to call "poll_cb" to check the
           results.

           See "poll_cb" for an example.

       IO::AIO::poll_cb
           Process some requests that have reached the result phase (i.e. they
           have been executed but the results are not yet reported). You have
           to call this "regularly" to finish outstanding requests.

           Returns 0 if all events could be processed (or there were no events
           to process), or "-1" if it returned earlier for whatever reason.
           Returns immediately when no events are outstanding. The amount of
           events processed depends on the settings of
           "IO::AIO::max_poll_req", "IO::AIO::max_poll_time" and
           "IO::AIO::max_outstanding".

           If not all requests were processed for whatever reason, the poll
           file descriptor will still be ready when "poll_cb" returns, so
           normally you don't have to do anything special to have it called
           later.

           Apart from calling "IO::AIO::poll_cb" when the event filehandle
           becomes ready, it can be beneficial to call this function from
           loops which submit a lot of requests, to make sure the results get
           processed when they become available and not just when the loop is
           finished and the event loop takes over again. This function returns
           very fast when there are no outstanding requests.

           Example: Install an Event watcher that automatically calls
           IO::AIO::poll_cb with high priority (more examples can be found in
           the SYNOPSIS section, at the top of this document):

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

       IO::AIO::poll_wait
           Wait until either at least one request is in the result phase or no
           requests are outstanding anymore.

           This is useful if you want to synchronously wait for some requests
           to become ready, without actually handling them.

           See "nreqs" for an example.

       IO::AIO::poll
           Waits until some requests have been handled.

           Returns the number of requests processed, but is otherwise strictly
           equivalent to:

              IO::AIO::poll_wait, IO::AIO::poll_cb

       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;

       IO::AIO::max_poll_reqs $nreqs
       IO::AIO::max_poll_time $seconds
           These set the maximum number of requests (default 0, meaning
           infinity) that are being processed by "IO::AIO::poll_cb" in one
           call, respectively the maximum amount of time (default 0, meaning
           infinity) spent in "IO::AIO::poll_cb" to process requests (more
           correctly the mininum amount of time "poll_cb" is allowed to use).

           Setting "max_poll_time" to a non-zero value creates an overhead of
           one syscall per request processed, which is not normally a problem
           unless your callbacks are really really fast or your OS is really
           really slow (I am not mentioning Solaris here). Using
           "max_poll_reqs" incurs no overhead.

           Setting these is useful if you want to ensure some level of
           interactiveness when perl is not fast enough to process all
           requests in time.

           For interactive programs, values such as 0.01 to 0.1 should be
           fine.

           Example: Install an Event watcher that automatically calls
           IO::AIO::poll_cb with low priority, to ensure that other parts of
           the program get the CPU sometimes even under high AIO load.

              # try not to spend much more than 0.1s in poll_cb
              IO::AIO::max_poll_time 0.1;

              # use a low priority so other tasks have priority
              Event->io (fd => IO::AIO::poll_fileno,
                         poll => 'r', nice => 1,
                         cb => &IO::AIO::poll_cb);

       CONTROLLING THE NUMBER OF THREADS

       IO::AIO::min_parallel $nthreads
           Set the minimum number of AIO threads to $nthreads. The current
           default is 8, which means eight asynchronous operations can execute
           concurrently at any one time (the number of outstanding requests,
           however, is unlimited).

           IO::AIO starts threads only on demand, when an AIO request is
           queued and no free thread exists. Please note that queueing up a
           hundred requests can create demand for a hundred threads, even if
           it turns out that everything is in the cache and could have been
           processed faster by a single thread.

           It is recommended to keep the number of threads relatively 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.

       IO::AIO::max_parallel $nthreads
           Sets the maximum number of AIO threads to $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 $nthreads are zero, aio requests get queued but not executed
           until the number of threads has been increased again.

           This module automatically runs "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.

       IO::AIO::max_idle $nthreads
           Limit the number of threads (default: 4) that are allowed to idle
           (i.e., threads that did not get a request to process within the
           idle timeout (default: 10 seconds). That means if a thread becomes
           idle while $nthreads other threads are also idle, it will free its
           resources and exit.

           This is useful when you allow a large number of threads (e.g. 100
           or 1000) to allow for extremely high load situations, but want to
           free resources under normal circumstances (1000 threads can easily
           consume 30MB of RAM).

           The default is probably ok in most situations, especially if thread
           creation is fast. If thread creation is very slow on your system
           you might want to use larger values.

       IO::AIO::idle_timeout $seconds
           Sets the minimum idle timeout (default 10) after which worker
           threads are allowed to exit. SEe "IO::AIO::max_idle".

       IO::AIO::max_outstanding $maxreqs
           Sets the maximum number of outstanding requests to $nreqs. If you
           do queue up more than this number of requests, the next call to
           "IO::AIO::poll_cb" (and other functions calling "poll_cb", such as
           "IO::AIO::flush" or "IO::AIO::poll") will block until the limit is
           no longer exceeded.

           In other words, this setting does not enforce a queue limit, but
           can be used to make poll functions block if the limit is exceeded.

           This is a very bad function to use in interactive programs because
           it blocks, and a bad way to reduce concurrency because it is
           inexact: Better use an "aio_group" together with a feed callback.

           It's main use is in scripts without an event loop - when you want
           to stat a lot of files, you can write somehting like this:

              IO::AIO::max_outstanding 32;

              for my $path (...) {
                 aio_stat $path , ...;
                 IO::AIO::poll_cb;
              }

              IO::AIO::flush;

           The call to "poll_cb" inside the loop will normally return
           instantly, but as soon as more thna 32 reqeusts are in-flight, it
           will block until some requests have been handled. This keeps the
           loop from pushing a large number of "aio_stat" requests onto the
           queue.

           The default value for "max_outstanding" is very large, so there is
           no practical limit on the number of outstanding requests.

       STATISTICAL INFORMATION

       IO::AIO::nreqs
           Returns the number of requests currently in the ready, execute or
           pending states (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;

       IO::AIO::nready
           Returns the number of requests currently in the ready state (not
           yet executed).

       IO::AIO::npending
           Returns the number of requests currently in the pending state
           (executed, but not yet processed by poll_cb).

       MISCELLANEOUS FUNCTIONS

       IO::AIO implements some functions that might be useful, but are not
       asynchronous.

       IO::AIO::sendfile $ofh, $ifh, $offset, $count
           Calls the "eio_sendfile_sync" function, which is like
           "aio_sendfile", but is blocking (this makes most sense if you know
           the input data is likely cached already and the output filehandle
           is set to non-blocking operations).

           Returns the number of bytes copied, or "-1" on error.

       IO::AIO::fadvise $fh, $offset, $len, $advice
           Simply calls the "posix_fadvise" function (see its manpage for
           details). The following advice constants are available:
           "IO::AIO::FADV_NORMAL", "IO::AIO::FADV_SEQUENTIAL",
           "IO::AIO::FADV_RANDOM", "IO::AIO::FADV_NOREUSE",
           "IO::AIO::FADV_WILLNEED", "IO::AIO::FADV_DONTNEED".

           On systems that do not implement "posix_fadvise", this function
           returns ENOSYS, otherwise the return value of "posix_fadvise".

       IO::AIO::madvise $scalar, $offset, $len, $advice
           Simply calls the "posix_madvise" function (see its manpage for
           details). The following advice constants are available:
           "IO::AIO::MADV_NORMAL", "IO::AIO::MADV_SEQUENTIAL",
           "IO::AIO::MADV_RANDOM", "IO::AIO::MADV_WILLNEED",
           "IO::AIO::MADV_DONTNEED".

           On systems that do not implement "posix_madvise", this function
           returns ENOSYS, otherwise the return value of "posix_madvise".

       IO::AIO::mprotect $scalar, $offset, $len, $protect
           Simply calls the "mprotect" function on the preferably AIO::mmap'ed
           $scalar (see its manpage for details). The following protect
           constants are available: "IO::AIO::PROT_NONE",
           "IO::AIO::PROT_READ", "IO::AIO::PROT_WRITE", "IO::AIO::PROT_EXEC".

           On systems that do not implement "mprotect", this function returns
           ENOSYS, otherwise the return value of "mprotect".

       IO::AIO::mmap $scalar, $length, $prot, $flags, $fh[, $offset]
           Memory-maps a file (or anonymous memory range) and attaches it to
           the given $scalar, which will act like a string scalar. Returns
           true on success, and false otherwise.

           The only operations allowed on the scalar are "substr"/"vec" that
           don't change the string length, and most read-only operations such
           as copying it or searching it with regexes and so on.

           Anything else is unsafe and will, at best, result in memory leaks.

           The memory map associated with the $scalar is automatically removed
           when the $scalar is destroyed, or when the "IO::AIO::mmap" or
           "IO::AIO::munmap" functions are called.

           This calls the "mmap"(2) function internally. See your system's
           manual page for details on the $length, $prot and $flags
           parameters.

           The $length must be larger than zero and smaller than the actual
           filesize.

           $prot is a combination of "IO::AIO::PROT_NONE",
           "IO::AIO::PROT_EXEC", "IO::AIO::PROT_READ" and/or
           "IO::AIO::PROT_WRITE",

           $flags can be a combination of "IO::AIO::MAP_SHARED" or
           "IO::AIO::MAP_PRIVATE", or a number of system-specific flags (when
           not available, the are defined as 0): "IO::AIO::MAP_ANONYMOUS"
           (which is set to "MAP_ANON" if your system only provides this
           constant), "IO::AIO::MAP_HUGETLB", "IO::AIO::MAP_LOCKED",
           "IO::AIO::MAP_NORESERVE", "IO::AIO::MAP_POPULATE" or
           "IO::AIO::MAP_NONBLOCK"

           If $fh is "undef", then a file descriptor of "-1" is passed.

           $offset is the offset from the start of the file - it generally
           must be a multiple of "IO::AIO::PAGESIZE" and defaults to 0.

           Example:

              use Digest::MD5;
              use IO::AIO;

              open my $fh, "<verybigfile"
                 or die "$!";

              IO::AIO::mmap my $data, -s $fh, IO::AIO::PROT_READ, IO::AIO::MAP_SHARED, $fh
                 or die "verybigfile: $!";

              my $fast_md5 = md5 $data;

       IO::AIO::munmap $scalar
           Removes a previous mmap and undefines the $scalar.

       IO::AIO::munlock $scalar, $offset = 0, $length = undef
           Calls the "munlock" function, undoing the effects of a previous
           "aio_mlock" call (see its description for details).

       IO::AIO::munlockall
           Calls the "munlockall" function.

           On systems that do not implement "munlockall", this function
           returns ENOSYS, otherwise the return value of "munlockall".

       IO::AIO::splice $r_fh, $r_off, $w_fh, $w_off, $length, $flags
           Calls the GNU/Linux splice(2) syscall, if available. If $r_off or
           $w_off are "undef", then "NULL" is passed for these, otherwise they
           should be the file offset.

           $r_fh and $w_fh should not refer to the same file, as splice might
           silently corrupt the data in this case.

           The following symbol flag values are available:
           "IO::AIO::SPLICE_F_MOVE", "IO::AIO::SPLICE_F_NONBLOCK",
           "IO::AIO::SPLICE_F_MORE" and "IO::AIO::SPLICE_F_GIFT".

           See the splice(2) manpage for details.

       IO::AIO::tee $r_fh, $w_fh, $length, $flags
           Calls the GNU/Linux tee(2) syscall, see it's manpage and the
           description for "IO::AIO::splice" above for details.

       $actual_size = IO::AIO::pipesize $r_fh[, $new_size]
           Attempts to query or change the pipe buffer size. Obviously works
           only on pipes, and currently works only on GNU/Linux systems, and
           fails with "-1"/"ENOSYS" everywhere else. If anybody knows how to
           influence pipe buffer size on other systems, drop me a note.

EVENT LOOP INTEGRATION

       It is recommended to use AnyEvent::AIO to integrate IO::AIO
       automatically into many event loops:

        # AnyEvent integration (EV, Event, Glib, Tk, POE, urxvt, pureperl...)
        use AnyEvent::AIO;

       You can also integrate IO::AIO manually into many event loops, here are
       some examples of how to do this:

        # EV integration
        my $aio_w = EV::io IO::AIO::poll_fileno, EV::READ, \&IO::AIO::poll_cb;

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

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

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

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

   FORK BEHAVIOUR
       Usage of pthreads in a program changes the semantics of fork
       considerably. Specifically, only async-safe functions can be called
       after fork. Perl doesn't know about this, so in general, you cannot
       call fork with defined behaviour in perl if pthreads are involved.
       IO::AIO uses pthreads, so this applies, but many other extensions and
       (for inexplicable reasons) perl itself often is linked against
       pthreads, so this limitation applies to quite a lot of perls.

       This module no longer tries to fight your OS, or POSIX. That means
       IO::AIO only works in the process that loaded it. Forking is fully
       supported, but using IO::AIO in the child is not.

       You might get around by not using IO::AIO before (or after) forking.
       You could also try to call the IO::AIO::reinit function in the child:

       IO::AIO::reinit
           Abandons all current requests and I/O threads and simply
           reinitialises all data structures. This is not an operation
           supported by any standards, but happens to work on GNU/Linux and
           some newer BSD systems.

           The only reasonable use for this function is to call it after
           forking, if "IO::AIO" was used in the parent. Calling it while
           IO::AIO is active in the process will result in undefined
           behaviour. Calling it at any time will also result in any undefined
           (by POSIX) behaviour.

   MEMORY USAGE
       Per-request usage:

       Each aio request uses - depending on your architecture - around 100-200
       bytes of memory. In addition, stat requests need a stat buffer
       (possibly a few hundred bytes), readdir requires a result buffer and so
       on. Perl scalars and other data passed into aio requests will also be
       locked and will consume memory till the request has entered the done
       state.

       This is not awfully much, so queuing lots of requests is not usually a
       problem.

       Per-thread usage:

       In the execution phase, some aio requests require more memory for
       temporary buffers, and each thread requires a stack and other data
       structures (usually around 16k-128k, depending on the OS).

KNOWN BUGS

       Known bugs will be fixed in the next release.

SEE ALSO

       AnyEvent::AIO for easy integration into event loops, Coro::AIO for a
       more natural syntax.

AUTHOR

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



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