Provided by: libio-aio-perl_4.60-1build1_amd64 bug

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 existence, 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
          $nfd = IO::AIO::get_fdlimit [EXPERIMENTAL]
          IO::AIO::min_fdlimit $nfd [EXPERIMENTAL]

          IO::AIO::sendfile $ofh, $ifh, $offset, $count
          IO::AIO::fadvise $fh, $offset, $len, $advice
          IO::AIO::mmap $scalar, $length, $prot, $flags[, $fh[, $offset]]
          IO::AIO::munmap $scalar
          IO::AIO::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

   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 occurred, 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" lets 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 (missing ones are, as usual 0):

           "F_DUPFD_CLOEXEC",

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

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

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

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

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

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

              aio_mlockall IO::AIO::MCL_FUTURE;

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

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

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

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

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

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

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

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

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

           At the time of this writing (Linux 3.2), this 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)
           Asynchronously canonicalise the given pathname and convert it to an IO::AIO::WD object
           representing it. If possible and supported on the system, also open a directory fd to
           speed up pathname resolution relative to this working directory.

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

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

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

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

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

          aio_realpath $wd, sub {
             warn "path is $_[0]\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 minimum amount of time "poll_cb" is allowed to use).

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

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

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

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

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

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

       CONTROLLING THE NUMBER OF THREADS

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

           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, but as soon as
           more thna 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.

           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 avdantage 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 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.

       $numfd = IO::AIO::get_fdlimit
           This function is EXPERIMENTAL and subject to change.

           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]
           This function is EXPERIMENTAL and subject to change.

           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
           nonexistent 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" or "IO::AIO::MAP_STACK".

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

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

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

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

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

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

           See the splice(2) manpage for details.

       IO::AIO::tee $r_fh, $w_fh, $length, $flags
           Calls the GNU/Linux tee(2) syscall, see 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";

       $fh = IO::AIO::eventfd [$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::O_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.

           On success, the new timerfd filehandle is returned, otherwise returns "undef". If the
           eventfd 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.

AUTHOR

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