NAME
    IO::AIO - Asynchronous Input/Output

SYNOPSIS
     use IO::AIO;

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

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

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

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

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

     my $grp = aio_group sub { print "all stats done\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.

    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 Fcntl;
       use EV;
       use IO::AIO;

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

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

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

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

             close $fh;

             # file contents now in $contents
             print $contents;

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

       aio_open $pathname, $flags, $mode, $callback->($fh)
       aio_close $fh, $callback->($status)
       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_truncate $fh_or_path, $offset, $callback->($status)
       aio_chmod $fh_or_path, $mode, $callback->($status)
       aio_unlink $pathname, $callback->($status)
       aio_mknod $path, $mode, $dev, $callback->($status)
       aio_link $srcpath, $dstpath, $callback->($status)
       aio_symlink $srcpath, $dstpath, $callback->($status)
       aio_readlink $path, $callback->($link)
       aio_rename $srcpath, $dstpath, $callback->($status)
       aio_mkdir $pathname, $mode, $callback->($status)
       aio_rmdir $pathname, $callback->($status)
       aio_readdir $pathname, $callback->($entries)
       aio_readdirx $pathname, $flags, $callback->($entries, $flags)
          IO::AIO::READDIR_DENTS IO::AIO::READDIR_DIRS_FIRST
          IO::AIO::READDIR_STAT_ORDER IO::AIO::READDIR_FOUND_UNKNOWN
       aio_load $path, $data, $callback->($status)
       aio_copy $srcpath, $dstpath, $callback->($status)
       aio_move $srcpath, $dstpath, $callback->($status)
       aio_scandir $path, $maxreq, $callback->($dirs, $nondirs)
       aio_rmtree $path, $callback->($status)
       aio_sync $callback->($status)
       aio_fsync $fh, $callback->($status)
       aio_fdatasync $fh, $callback->($status)
       aio_sync_file_range $fh, $offset, $nbytes, $flags, $callback->($status)
       aio_pathsync $path, $callback->($status)
       aio_msync $scalar, $offset = 0, $length = undef, flags = 0, $callback->($status)
       aio_mtouch $scalar, $offset = 0, $length = undef, flags = 0, $callback->($status)
       aio_mlock $scalar, $offset = 0, $length = undef, $callback->($status)
       aio_mlockall $flags, $callback->($status)
       aio_group $callback->(...)
       aio_nop $callback->()

       $prev_pri = aioreq_pri [$pri]
       aioreq_nice $pri_adjust

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

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

  AIO REQUEST FUNCTIONS
    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 get
    called with the syscall return code (e.g. most syscalls return -1 on
    error, unlike perl, which usually delivers "false") as its sole argument
    after the given syscall has been executed asynchronously.

    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 *must* be absolute and encoded
    as octets. The reason for the former is that at the time the request is
    being executed, the current working directory could have changed.
    Alternatively, you can make sure that you never change the current
    working directory anywhere in the program and then use relative paths.

    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 ASCII or ISO 8859-1, c) use the Encode module
    and encode your pathnames to the locale (or other) encoding in effect in
    the user environment, d) use Glib::filename_from_unicode on unicode
    filenames or e) use something else 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.

    $prev_pri = aioreq_pri [$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.

        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";
              }
           };

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

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

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

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

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

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

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

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

        This call tries to make use of a native "sendfile" 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",
        "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.

        Please note, however, that "aio_sendfile" can read more bytes from
        $in_fh than are written, and there is no way to find out how many
        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.

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

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

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

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

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

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

        Examples:

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

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

        Examples:

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

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

    aio_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 $path, $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 $path, IO::AIO::S_IFIFO | $mode, 0, sub { ...

    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 $path, $callback->($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_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.

    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.

    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 to
        tune behaviour and output format. In case of an error, $entries will
        be "undef".

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

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

            $name is the name of the entry.

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

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

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

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

        IO::AIO::READDIR_DIRS_FIRST
            When this flag is set, then the names will be returned in an
            order where likely directories come first. 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 files beginning with ".", or otherwise files with no dots,
            of which files with short names are tried first.

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

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

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

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

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

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

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

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

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

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

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

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

        Example:

           aio_scandir $dir, 0, sub {
              my ($dirs, $nondirs) = @_;
              print "real directories: @$dirs\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 seperately). 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 $path, $callback->($status)
        Delete a directory tree starting (and including) $path, return the
        status of the final "rmdir" only. This is a composite request that
        uses "aio_scandir" to recurse into and rmdir directories, and unlink
        everything else.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

           aio_mlockall IO::AIO::MCL_FUTURE;

    aio_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::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 "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 outstanding events on the result pipe. You have to call
        this regularly. Returns 0 if all events could be processed, 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" and
        "IO::AIO::max_poll_time".

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

        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
        If there are any outstanding requests and none of them in the result
        phase, wait till the result filehandle becomes ready for reading
        (simply does a "select" on the filehandle. This is useful if you
        want to synchronously wait for some requests to finish).

        See "nreqs" for an example.

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

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

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

    IO::AIO::flush
        Wait till all outstanding AIO requests have been handled.

        Strictly equivalent to:

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

    IO::AIO::max_idle $nthreads
        Limit the number of threads (default: 4) that are allowed to idle
        (i.e., threads that did not get a request to process within 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::max_outstanding $maxreqs
        This is a very bad function to use in interactive programs because
        it blocks, and a bad way to reduce concurrency because it is
        inexact: Better use an "aio_group" together with a feed callback.

        Sets the maximum number of outstanding requests to $nreqs. If you do
        queue up more than this number of requests, the next call to the
        "poll_cb" (and "poll_some" and other functions calling "poll_cb")
        function will block until the limit is no longer exceeded.

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

        You can still queue as many requests as you want. Therefore,
        "max_outstanding" is mainly useful in simple scripts (with low
        values) or as a stop gap to shield against fatal memory overflow
        (with large values).

   STATISTICAL INFORMATION
    IO::AIO::nreqs
        Returns the number of requests currently in the ready, execute or
        pending states (i.e. for which their callback has not been invoked
        yet).

        Example: wait till there are no outstanding requests anymore:

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

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

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

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

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

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

    IO::AIO::fadvise $fh, $offset, $len, $advice
        Simply calls the "posix_fadvise" function (see its manpage for
        details). The following advice constants are avaiable:
        "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 avaiable:
        "IO::AIO::MADV_NORMAL", "IO::AIO::MADV_SEQUENTIAL",
        "IO::AIO::MADV_RANDOM", "IO::AIO::MADV_WILLNEED",
        "IO::AIO::MADV_DONTNEED".

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

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

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

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

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

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

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

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

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

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

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

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

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

        Example:

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

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

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

           my $fast_md5 = md5 $data;

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

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

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

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

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
    This module should do "the right thing" when the process using it forks:

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

    In short: the parent will, after a short pause, continue as if fork had
    not been called, while the child will act as if IO::AIO has not been
    used yet.

  MEMORY USAGE
    Per-request usage:

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

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

    Per-thread usage:

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

KNOWN BUGS
    Known bugs will be fixed in the next release.

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

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

