NAME
    IO::AIO - Asynchronous/Advanced 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\n" };
     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.html>).

    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.

    In addition to asynchronous I/O, this module also exports some rather
    arcane interfaces, such as "madvise" or linux's "splice" system call,
    which is why the "A" in "AIO" can also mean *advanced*.

    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::break;
          };
       };

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

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

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, $callback->($path)
       aio_rename $srcpath, $dstpath, $callback->($status)
       aio_rename2 $srcpath, $dstpath, $flags, $callback->($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_fcntl $fh, $cmd, $arg, $callback->($status)
       aio_ioctl $fh, $request, $buf, $callback->($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 = MS_SYNC, $callback->($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::reinit

       $nfd = IO::AIO::get_fdlimit
       IO::AIO::min_fdlimit $nfd

       IO::AIO::sendfile $ofh, $ifh, $offset, $count
       IO::AIO::fadvise $fh, $offset, $len, $advice
       IO::AIO::fexecve $fh, $argv, $envp

       IO::AIO::mmap $scalar, $length, $prot, $flags[, $fh[, $offset]]
       IO::AIO::munmap $scalar
       IO::AIO::mremap $scalar, $new_length, $flags[, $new_address]
       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

       # stat extensions
       $counter = IO::AIO::st_gen
       $seconds = IO::AIO::st_atime, IO::AIO::st_mtime, IO::AIO::st_ctime, IO::AIO::st_btime
       ($atime, $mtime, $ctime, $btime, ...) = IO::AIO::st_xtime
       $nanoseconds = IO::AIO::st_atimensec, IO::AIO::st_mtimensec, IO::AIO::st_ctimensec, IO::AIO::st_btimensec
       $seconds = IO::AIO::st_btimesec
       ($atime, $mtime, $ctime, $btime, ...) = IO::AIO::st_xtimensec

       # very much unportable syscalls
       IO::AIO::accept4 $r_fh, $sockaddr, $sockaddr_len, $flags
       IO::AIO::splice $r_fh, $r_off, $w_fh, $w_off, $length, $flags
       IO::AIO::tee $r_fh, $w_fh, $length, $flags

       $actual_size = IO::AIO::pipesize $r_fh[, $new_size]
       ($rfh, $wfh) = IO::AIO::pipe2 [$flags]

       $fh = IO::AIO::eventfd [$initval, [$flags]]
       $fh = IO::AIO::memfd_create $pathname[, $flags]

       $fh = IO::AIO::timerfd_create $clockid[, $flags]
       ($cur_interval, $cur_value) = IO::AIO::timerfd_settime $fh, $flags, $new_interval, $nbw_value
       ($cur_interval, $cur_value) = IO::AIO::timerfd_gettime $fh

       $fh = IO::AIO::pidfd_open $pid[, $flags]
       $status = IO::AIO::pidfd_send_signal $pidfh, $signal[, $siginfo[, $flags]]
       $fh = IO::AIO::pidfd_getfd $pidfh, $targetfd[, $flags]

  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]\n";
                 ...
              } else {
                 die "open failed: $!\n";
              }
           };

        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", "O_PATH", "O_TMPFILE", "O_TTY_INIT"
        and "O_ACCMODE".

    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 with the actual number of bytes transferred (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>\n";
           };

    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" - fewer 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 kernel 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 almost exactly 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 _" and other tests (with the
        exception of "-B" and "-T").

        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".

        To access higher resolution stat timestamps, see "SUBSECOND STAT
        TIME ACCESS".

        Example: Print the length of /etc/passwd:

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

    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
           }

    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 utimensat(2) or utimes(2) if
        available, otherwise utime(2). If called on a file descriptor, uses
        futimens(2) or 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 frees disk space according to the $mode argument. See
        the linux "fallocate" documentation for details.

        $mode is usually 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.

        IO::AIO also supports "FALLOC_FL_COLLAPSE_RANGE", to remove a range
        (without leaving a hole), "FALLOC_FL_ZERO_RANGE", to zero a range,
        "FALLOC_FL_INSERT_RANGE" to insert a range and
        "FALLOC_FL_UNSHARE_RANGE" to unshare shared blocks (see your
        fallocate(2) manpage).

        The file system block size used by "fallocate" is presumably the
        "f_bsize" returned by "statvfs", but different filesystems and
        filetypes can dictate other limitations.

        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_rename2 $srcpath, $dstpath, $flags, $callback->($status)
        Basically a version of "aio_rename" with an additional $flags
        argument. Calling this with "$flags=0" is the same as calling
        "aio_rename".

        Non-zero flags are currently only supported on GNU/Linux systems
        that support renameat2. Other systems fail with "ENOSYS" in this
        case.

        The following constants are available (missing ones are, as usual
        0), see renameat2(2) for details:

        "IO::AIO::RENAME_NOREPLACE", "IO::AIO::RENAME_EXCHANGE" and
        "IO::AIO::RENAME_WHITEOUT".

    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
            Normally the callback gets an arrayref consisting of names only
            (as with "aio_readdir"). If this flag is set, then the callback
            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/memory reasons, the $type scalars are read-only: you must
            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() most or all files in the given directory, then the
            returned order will likely be faster.

            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 for stat'ing all entries, but likely a more
            optimal order for finding subdirectories.

        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_slurp $pathname, $offset, $length, $data, $callback->($status)
        Opens, reads and closes the given file. The data is put into $data,
        which is resized as required.

        If $offset is negative, then it is counted from the end of the file.

        If $length is zero, then the remaining length of the file is used.
        Also, in this case, the same limitations to modifying $data apply as
        when IO::AIO::mmap is used, i.e. it must only be modified in-place
        with "substr". If the size of the file is known, specifying a
        non-zero $length results in a performance advantage.

        This request is similar to the older "aio_load" request, but since
        it is a single request, it might be more efficient to use.

        Example: load /etc/passwd into $passwd.

           my $passwd;
           aio_slurp "/etc/passwd", 0, 0, $passwd, sub {
              $_[0] >= 0
                 or die "/etc/passwd: $!\n";

              printf "/etc/passwd is %d bytes long, and contains:\n", length $passwd;
              print $passwd;
           };
           IO::AIO::flush;

    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.

        Using "aio_slurp" might be more efficient, as it is a single
        request.

    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 $!).

        Existing destination files will be truncated.

        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 generates 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\n";
              print "everything else: @$nondirs\n";
           };

        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_fcntl $fh, $cmd, $arg, $callback->($status)
    aio_ioctl $fh, $request, $buf, $callback->($status)
        These work just like the "fcntl" and "ioctl" built-in functions,
        except they execute asynchronously and pass the return value to the
        callback.

        Both calls can be used for a lot of things, some of which make more
        sense to run asynchronously in their own thread, while some others
        make less sense. For example, calls that block waiting for external
        events, such as locking, will also lock down an I/O thread while it
        is waiting, which can deadlock the whole I/O system. At the same
        time, there might be no alternative to using a thread to wait.

        So in general, you should only use these calls for things that do
        (filesystem) I/O, not for things that wait for other events
        (network, other processes), although if you are careful and know
        what you are doing, you still can.

        The following constants are available and can be used for normal
        "ioctl" and "fcntl" as well (missing ones are, as usual 0):

        "F_DUPFD_CLOEXEC",

        "F_OFD_GETLK", "F_OFD_SETLK", "F_OFD_GETLKW",

        "FIFREEZE", "FITHAW", "FITRIM", "FICLONE", "FICLONERANGE",
        "FIDEDUPERANGE".

        "F_ADD_SEALS", "F_GET_SEALS", "F_SEAL_SEAL", "F_SEAL_SHRINK",
        "F_SEAL_GROW" and "F_SEAL_WRITE".

        "FS_IOC_GETFLAGS", "FS_IOC_SETFLAGS", "FS_IOC_GETVERSION",
        "FS_IOC_SETVERSION", "FS_IOC_FIEMAP".

        "FS_IOC_FSGETXATTR", "FS_IOC_FSSETXATTR",
        "FS_IOC_SET_ENCRYPTION_POLICY", "FS_IOC_GET_ENCRYPTION_PWSALT",
        "FS_IOC_GET_ENCRYPTION_POLICY", "FS_KEY_DESCRIPTOR_SIZE".

        "FS_SECRM_FL", "FS_UNRM_FL", "FS_COMPR_FL", "FS_SYNC_FL",
        "FS_IMMUTABLE_FL", "FS_APPEND_FL", "FS_NODUMP_FL", "FS_NOATIME_FL",
        "FS_DIRTY_FL", "FS_COMPRBLK_FL", "FS_NOCOMP_FL", "FS_ENCRYPT_FL",
        "FS_BTREE_FL", "FS_INDEX_FL", "FS_JOURNAL_DATA_FL", "FS_NOTAIL_FL",
        "FS_DIRSYNC_FL", "FS_TOPDIR_FL", "FS_FL_USER_MODIFIABLE".

        "FS_XFLAG_REALTIME", "FS_XFLAG_PREALLOC", "FS_XFLAG_IMMUTABLE",
        "FS_XFLAG_APPEND", "FS_XFLAG_SYNC", "FS_XFLAG_NOATIME",
        "FS_XFLAG_NODUMP", "FS_XFLAG_RTINHERIT", "FS_XFLAG_PROJINHERIT",
        "FS_XFLAG_NOSYMLINKS", "FS_XFLAG_EXTSIZE", "FS_XFLAG_EXTSZINHERIT",
        "FS_XFLAG_NODEFRAG", "FS_XFLAG_FILESTREAM", "FS_XFLAG_DAX",
        "FS_XFLAG_HASATTR",

    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 = MS_SYNC,
    $callback->($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 either "IO::AIO::MS_ASYNC" or "IO::AIO::MS_SYNC", plus
        an optional "IO::AIO::MS_INVALIDATE".

    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", "IO::AIO::MCL_FUTURE" and
        "IO::AIO::MCL_ONFAULT").

        On systems that do not implement "mlockall", this function returns
        -1 and sets errno to "ENOSYS". Similarly, flag combinations not
        supported by the system result in a return value of -1 with errno
        being set to "EINVAL".

        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 request 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 a
        large number of extents. The code (only) 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\n";
           };

           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
          };
       };

    The fact 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 "aio_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 compile time 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]\n";
       };

    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\n";
       };

    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\n" };
           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;

        This function can be useful at program aborts, to make sure
        outstanding I/O has been done ("IO::AIO" uses an "END" block which
        already calls this function on normal exits), or when you are merely
        using "IO::AIO" for its more advanced functions, rather than for
        async I/O, e.g.:

           my ($dirs, $nondirs);
           IO::AIO::aio_scandir "/tmp", 0, sub { ($dirs, $nondirs) = @_ };
           IO::AIO::flush;
           # $dirs, $nondirs are now set

    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 bad function to use in interactive programs because it
        blocks, and a bad way to reduce concurrency because it is inexact.
        If you need to issue many requests without being able to call a poll
        function on demand, it is better to use an "aio_group" together with
        a feed callback.

        Its main use is in scripts without an event loop - when you want to
        stat a lot of files, you can write something 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, allowing the loop to progress, but as soon as more than
        32 requests 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 (which, with many paths to stat,
        can use up a lot of memory).

        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).

   SUBSECOND STAT TIME ACCESS
    Both "aio_stat"/"aio_lstat" and perl's "stat"/"lstat" functions can
    generally find access/modification and change times with subsecond time
    accuracy of the system supports it, but perl's built-in functions only
    return the integer part.

    The following functions return the timestamps of the most recent stat
    with subsecond precision on most systems and work both after
    "aio_stat"/"aio_lstat" and perl's "stat"/"lstat" calls. Their return
    value is only meaningful after a successful "stat"/"lstat" call, or
    during/after a successful "aio_stat"/"aio_lstat" callback.

    This is similar to the Time::HiRes "stat" functions, but can return full
    resolution without rounding and work with standard perl "stat",
    alleviating the need to call the special "Time::HiRes" functions, which
    do not act like their perl counterparts.

    On operating systems or file systems where subsecond time resolution is
    not supported or could not be detected, a fractional part of 0 is
    returned, so it is always safe to call these functions.

    $seconds = IO::AIO::st_atime, IO::AIO::st_mtime, IO::AIO::st_ctime,
    IO::AIO::st_btime
        Return the access, modication, change or birth time, respectively,
        including fractional part. Due to the limited precision of floating
        point, the accuracy on most platforms is only a bit better than
        milliseconds for times around now - see the *nsec* function family,
        below, for full accuracy.

        File birth time is only available when the OS and perl support it
        (on FreeBSD and NetBSD at the time of this writing, although support
        is adaptive, so if your OS/perl gains support, IO::AIO can take
        advantage of it). On systems where it isn't available, 0 is
        currently returned, but this might change to "undef" in a future
        version.

    ($atime, $mtime, $ctime, $btime, ...) = IO::AIO::st_xtime
        Returns access, modification, change and birth time all in one go,
        and maybe more times in the future version.

    $nanoseconds = IO::AIO::st_atimensec, IO::AIO::st_mtimensec,
    IO::AIO::st_ctimensec, IO::AIO::st_btimensec
        Return the fractional access, modifcation, change or birth time, in
        nanoseconds, as an integer in the range 0 to 999999999.

        Note that no accessors are provided for access, modification and
        change times - you need to get those from "stat _" if required ("int
        IO::AIO::st_atime" and so on will *not* generally give you the
        correct value).

    $seconds = IO::AIO::st_btimesec
        The (integral) seconds part of the file birth time, if available.

    ($atime, $mtime, $ctime, $btime, ...) = IO::AIO::st_xtimensec
        Like the functions above, but returns all four times in one go (and
        maybe more in future versions).

    $counter = IO::AIO::st_gen
        Returns the generation counter (in practice this is just a random
        number) of the file. This is only available on platforms which have
        this member in their "struct stat" (most BSDs at the time of this
        writing) and generally only to the root usert. If unsupported, 0 is
        returned, but this might change to "undef" in a future version.

    Example: print the high resolution modification time of /etc, using
    "stat", and "IO::AIO::aio_stat".

       if (stat "/etc") {
          printf "stat(/etc) mtime: %f\n", IO::AIO::st_mtime;
       }

       IO::AIO::aio_stat "/etc", sub {
          $_[0]
             and return;

          printf "aio_stat(/etc) mtime: %d.%09d\n", (stat _)[9], IO::AIO::st_mtimensec;
       };

       IO::AIO::flush;

    Output of the awbove on my system, showing reduced and full accuracy:

       stat(/etc) mtime: 1534043702.020808
       aio_stat(/etc) mtime: 1534043702.020807792

   MISCELLANEOUS FUNCTIONS
    IO::AIO implements some functions that are useful when you want to use
    some "Advanced I/O" function not available to in Perl, without going the
    "Asynchronous I/O" route. Many of these have an asynchronous "aio_*"
    counterpart.

    $retval = IO::AIO::fexecve $fh, $argv, $envp
        A more-or-less direct equivalent to the POSIX "fexecve" functions,
        which allows you to specify the program to be executed via a file
        descriptor (or handle). Returns -1 and sets errno to "ENOSYS" if not
        available.

    $numfd = IO::AIO::get_fdlimit
        Tries to find the current file descriptor limit and returns it, or
        "undef" and sets $! in case of an error. The limit is one larger
        than the highest valid file descriptor number.

    IO::AIO::min_fdlimit [$numfd]
        Try to increase the current file descriptor limit(s) to at least
        $numfd by changing the soft or hard file descriptor resource limit.
        If $numfd is missing, it will try to set a very high limit, although
        this is not recommended when you know the actual minimum that you
        require.

        If the limit cannot be raised enough, the function makes a
        best-effort attempt to increase the limit as much as possible, using
        various tricks, while still failing. You can query the resulting
        limit using "IO::AIO::get_fdlimit".

        If an error occurs, returns "undef" and sets $!, otherwise returns
        true.

    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".

        If $offset is negative, counts from the end. If $length is negative,
        the remaining length of the $scalar is used. If possible, $length
        will be reduced to fit into the $scalar.

        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".

        If $offset is negative, counts from the end. If $length is negative,
        the remaining length of the $scalar is used. If possible, $length
        will be reduced to fit into the $scalar.

        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 scalar must exist, but its contents do not matter - this means
        you cannot use a nonexistant array or hash element. When in doubt,
        "undef" the scalar first.

        The only operations allowed on the mmapped scalar are
        "substr"/"vec", which 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 undef'd or destroyed, or when the
        "IO::AIO::mmap" or "IO::AIO::munmap" functions are called on it.

        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 0): "IO::AIO::MAP_ANONYMOUS" (which is set to
        "MAP_ANON" if your system only provides this constant),
        "IO::AIO::MAP_LOCKED", "IO::AIO::MAP_NORESERVE",
        "IO::AIO::MAP_POPULATE", "IO::AIO::MAP_NONBLOCK",
        "IO::AIO::MAP_FIXED", "IO::AIO::MAP_GROWSDOWN",
        "IO::AIO::MAP_32BIT", "IO::AIO::MAP_HUGETLB", "IO::AIO::MAP_STACK",
        "IO::AIO::MAP_FIXED_NOREPLACE", "IO::AIO::MAP_SHARED_VALIDATE",
        "IO::AIO::MAP_SYNC" or "IO::AIO::MAP_UNINITIALIZED".

        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::mremap $scalar, $new_length, $flags = MREMAP_MAYMOVE[,
    $new_address = 0]
        Calls the Linux-specific mremap(2) system call. The $scalar must
        have been mapped by "IO::AIO::mmap", and $flags must currently
        either be 0 or "IO::AIO::MREMAP_MAYMOVE".

        Returns true if successful, and false otherwise. If the underlying
        mmapped region has changed address, then the true value has the
        numerical value 1, otherwise it has the numerical value 0:

           my $success = IO::AIO::mremap $mmapped, 8192, IO::AIO::MREMAP_MAYMOVE
              or die "mremap: $!";

           if ($success*1) {
              warn "scalar has chanegd address in memory\n";
           }

        "IO::AIO::MREMAP_FIXED" and the $new_address argument are currently
        implemented, but not supported and might go away in a future
        version.

        On systems where this call is not supported or is not emulated, this
        call returns falls and sets $! to "ENOSYS".

    IO::AIO::mlockall $flags
        Calls the "eio_mlockall_sync" function, which is like
        "aio_mlockall", but is blocking.

    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".

    $fh = IO::AIO::accept4 $r_fh, $sockaddr, $sockaddr_maxlen, $flags
        Uses the GNU/Linux accept4(2) syscall, if available, to accept a
        socket and return the new file handle on success, or sets $! and
        returns "undef" on error.

        The remote name of the new socket will be stored in $sockaddr, which
        will be extended to allow for at least $sockaddr_maxlen octets. If
        the socket name does not fit into $sockaddr_maxlen octets, this is
        signaled by returning a longer string in $sockaddr, which might or
        might not be truncated.

        To accept name-less sockets, use "undef" for $sockaddr and 0 for
        $sockaddr_maxlen.

        The main reasons to use this syscall rather than portable accept(2)
        are that you can specify "SOCK_NONBLOCK" and/or "SOCK_CLOEXEC" flags
        and you can accept name-less sockets by specifying 0 for
        $sockaddr_maxlen, which is sadly not possible with perl's interface
        to "accept".

    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 its 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.

    ($rfh, $wfh) = IO::AIO::pipe2 [$flags]
        This is a direct interface to the Linux pipe2(2) system call. If
        $flags is missing or 0, then this should be the same as a call to
        perl's built-in "pipe" function and create a new pipe, and works on
        systems that lack the pipe2 syscall. On win32, this case invokes
        "_pipe (..., 4096, O_BINARY)".

        If $flags is non-zero, it tries to invoke the pipe2 system call with
        the given flags (Linux 2.6.27, glibc 2.9).

        On success, the read and write file handles are returned.

        On error, nothing will be returned. If the pipe2 syscall is missing
        and $flags is non-zero, fails with "ENOSYS".

        Please refer to pipe2(2) for more info on the $flags, but at the
        time of this writing, "IO::AIO::O_CLOEXEC", "IO::AIO::O_NONBLOCK"
        and "IO::AIO::O_DIRECT" (Linux 3.4, for packet-based pipes) were
        supported.

        Example: create a pipe race-free w.r.t. threads and fork:

           my ($rfh, $wfh) = IO::AIO::pipe2 IO::AIO::O_CLOEXEC
              or die "pipe2: $!\n";

    $fh = IO::AIO::memfd_create $pathname[, $flags]
        This is a direct interface to the Linux memfd_create(2) system call.
        The (unhelpful) default for $flags is 0, but your default should be
        "IO::AIO::MFD_CLOEXEC".

        On success, the new memfd filehandle is returned, otherwise returns
        "undef". If the memfd_create syscall is missing, fails with
        "ENOSYS".

        Please refer to memfd_create(2) for more info on this call.

        The following $flags values are available: "IO::AIO::MFD_CLOEXEC",
        "IO::AIO::MFD_ALLOW_SEALING", "IO::AIO::MFD_HUGETLB",
        "IO::AIO::MFD_HUGETLB_2MB" and "IO::AIO::MFD_HUGETLB_1GB".

        Example: create a new memfd.

           my $fh = IO::AIO::memfd_create "somenameforprocfd", IO::AIO::MFD_CLOEXEC
              or die "memfd_create: $!\n";

    $fh = IO::AIO::pidfd_open $pid[, $flags]
        This is an interface to the Linux pidfd_open(2) system call. The
        default for $flags is 0.

        On success, a new pidfd filehandle is returned (that is already set
        to close-on-exec), otherwise returns "undef". If the syscall is
        missing, fails with "ENOSYS".

        Example: open pid 6341 as pidfd.

           my $fh = IO::AIO::pidfd_open 6341
              or die "pidfd_open: $!\n";

    $status = IO::AIO::pidfd_send_signal $pidfh, $signal[, $siginfo[,
    $flags]]
        This is an interface to the Linux pidfd_send_signal system call. The
        default for $siginfo is "undef" and the default for $flags is 0.

        Returns the system call status. If the syscall is missing, fails
        with "ENOSYS".

        When specified, $siginfo must be a reference to a hash with one or
        more of the following members:

        code - the "si_code" member
        pid - the "si_pid" member
        uid - the "si_uid" member
        value_int - the "si_value.sival_int" member
        value_ptr - the "si_value.sival_ptr" member, specified as an integer

        Example: send a SIGKILL to the specified process.

           my $status = IO::AIO::pidfd_send_signal $pidfh, 9, undef
              and die "pidfd_send_signal: $!\n";

        Example: send a SIGKILL to the specified process with extra data.

           my $status = IO::AIO::pidfd_send_signal $pidfh, 9,  { code => -1, value_int => 7 }
              and die "pidfd_send_signal: $!\n";

    $fh = IO::AIO::pidfd_getfd $pidfh, $targetfd[, $flags]
        This is an interface to the Linux pidfd_getfd system call. The
        default for $flags is 0.

        On success, returns a dup'ed copy of the target file descriptor
        (specified as an integer) returned (that is already set to
        close-on-exec), otherwise returns "undef". If the syscall is
        missing, fails with "ENOSYS".

        Example: get a copy of standard error of another process and print
        soemthing to it.

           my $errfh = IO::AIO::pidfd_getfd $pidfh, 2
              or die "pidfd_getfd: $!\n";
           print $errfh "stderr\n";

    $fh = IO::AIO::eventfd [$initval, [$flags]]
        This is a direct interface to the Linux eventfd(2) system call. The
        (unhelpful) defaults for $initval and $flags are 0 for both.

        On success, the new eventfd filehandle is returned, otherwise
        returns "undef". If the eventfd syscall is missing, fails with
        "ENOSYS".

        Please refer to eventfd(2) for more info on this call.

        The following symbol flag values are available:
        "IO::AIO::EFD_CLOEXEC", "IO::AIO::EFD_NONBLOCK" and
        "IO::AIO::EFD_SEMAPHORE" (Linux 2.6.30).

        Example: create a new eventfd filehandle:

           $fh = IO::AIO::eventfd 0, IO::AIO::EFD_CLOEXEC
              or die "eventfd: $!\n";

    $fh = IO::AIO::timerfd_create $clockid[, $flags]
        This is a direct interface to the Linux timerfd_create(2) system
        call. The (unhelpful) default for $flags is 0, but your default
        should be "IO::AIO::TFD_CLOEXEC".

        On success, the new timerfd filehandle is returned, otherwise
        returns "undef". If the timerfd_create syscall is missing, fails
        with "ENOSYS".

        Please refer to timerfd_create(2) for more info on this call.

        The following $clockid values are available:
        "IO::AIO::CLOCK_REALTIME", "IO::AIO::CLOCK_MONOTONIC"
        "IO::AIO::CLOCK_CLOCK_BOOTTIME" (Linux 3.15)
        "IO::AIO::CLOCK_CLOCK_REALTIME_ALARM" (Linux 3.11) and
        "IO::AIO::CLOCK_CLOCK_BOOTTIME_ALARM" (Linux 3.11).

        The following $flags values are available (Linux 2.6.27):
        "IO::AIO::TFD_NONBLOCK" and "IO::AIO::TFD_CLOEXEC".

        Example: create a new timerfd and set it to one-second repeated
        alarms, then wait for two alarms:

           my $fh = IO::AIO::timerfd_create IO::AIO::CLOCK_BOOTTIME, IO::AIO::TFD_CLOEXEC
              or die "timerfd_create: $!\n";

           defined IO::AIO::timerfd_settime $fh, 0, 1, 1
              or die "timerfd_settime: $!\n";

           for (1..2) {
              8 == sysread $fh, my $buf, 8
                 or die "timerfd read failure\n";

              printf "number of expirations (likely 1): %d\n",
                 unpack "Q", $buf;
           }

    ($cur_interval, $cur_value) = IO::AIO::timerfd_settime $fh, $flags,
    $new_interval, $nbw_value
        This is a direct interface to the Linux timerfd_settime(2) system
        call. Please refer to its manpage for more info on this call.

        The new itimerspec is specified using two (possibly fractional)
        second values, $new_interval and $new_value).

        On success, the current interval and value are returned (as per
        "timerfd_gettime"). On failure, the empty list is returned.

        The following $flags values are available:
        "IO::AIO::TFD_TIMER_ABSTIME" and "IO::AIO::TFD_TIMER_CANCEL_ON_SET".

        See "IO::AIO::timerfd_create" for a full example.

    ($cur_interval, $cur_value) = IO::AIO::timerfd_gettime $fh
        This is a direct interface to the Linux timerfd_gettime(2) system
        call. Please refer to its manpage for more info on this call.

        On success, returns the current values of interval and value for the
        given timerfd (as potentially fractional second values). On failure,
        the empty list is returned.

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.

  LINUX-SPECIFIC CALLS
    When a call is documented as "linux-specific" then this means it
    originated on GNU/Linux. "IO::AIO" will usually try to autodetect the
    availability and compatibility of such calls regardless of the platform
    it is compiled on, so platforms such as FreeBSD which often implement
    these calls will work. When in doubt, call them and see if they fail wth
    "ENOSYS".

  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 :)

KNOWN ISSUES
    Calls that try to "import" foreign memory areas (such as "IO::AIO::mmap"
    or "IO::AIO::aio_slurp") do not work with generic lvalues, such as
    non-created hash slots or other scalars I didn't think of. It's best to
    avoid such and either use scalar variables or making sure that the
    scalar exists (e.g. by storing "undef") and isn't "funny" (e.g. tied).

    I am not sure anything can be done about this, so this is considered a
    known issue, rather than a bug.

SEE ALSO
    AnyEvent::AIO for easy integration into event loops, Coro::AIO for a
    more natural syntax and IO::FDPass for file descriptor passing.

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

