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Man Pages

Manual Reference Pages  -  IO::AIO (3)

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IO::AIO - Asynchronous Input/Output



 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 ...;


This module implements asynchronous I/O using whatever means your operating system supports. It is implemented as an interface to libeio (<>).

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

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

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

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


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: $!";

      # stating 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

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

   # process events as long as there are some:


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

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

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

During their existance, aio requests travel through the following states, in order:
ready Immediately after a request is created it is put into the ready state, waiting for a thread to execute it.
execute A thread has accepted the request for processing and is currently executing it (e.g. blocking in read).
pending The request has been executed and is waiting for result processing.

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

result The request results are processed synchronously by poll_cb.

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

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



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_mkdir $pathname, $mode, $callback->($status)
   aio_rmdir $pathname, $callback->($status)
   aio_readdir $pathname, $callback->($entries)
   aio_readdirx $pathname, $flags, $callback->($entries, $flags)
   aio_scandir $pathname, $maxreq, $callback->($dirs, $nondirs)
   aio_load $pathname, $data, $callback->($status)
   aio_copy $srcpath, $dstpath, $callback->($status)
   aio_move $srcpath, $dstpath, $callback->($status)
   aio_rmtree $pathname, $callback->($status)
   aio_sync $callback->($status)
   aio_syncfs $fh, $callback->($status)
   aio_fsync $fh, $callback->($status)
   aio_fdatasync $fh, $callback->($status)
   aio_sync_file_range $fh, $offset, $nbytes, $flags, $callback->($status)
   aio_pathsync $pathname, $callback->($status)
   aio_msync $scalar, $offset = 0, $length = undef, flags = 0, $callback->($status)
   aio_mtouch $scalar, $offset = 0, $length = undef, flags = 0, $callback->($status)
   aio_mlock $scalar, $offset = 0, $length = undef, $callback->($status)
   aio_mlockall $flags, $callback->($status)
   aio_group $callback->(...)
   aio_nop $callback->()

   $prev_pri = aioreq_pri [$pri]
   aioreq_nice $pri_adjust

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


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

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

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

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

The pathnames you pass to these routines should be absolute. The reason for this is that at the time the request is being executed, the current working directory could have changed. Alternatively, you can make sure that you never change the current working directory anywhere in the program and then use relative paths. You can also take advantage of IO::AIOs working directory abstraction, that lets you specify paths relative to some previously-opened working directory object - see the description of the IO::AIO::WD class later in this document.

To encode pathnames as octets, either make sure you either: a) always pass in filenames you got from outside (command line, readdir etc.) without tinkering, b) are in your native filesystem encoding, c) use the Encode module and encode your pathnames to the locale (or other) encoding in effect in the user environment, d) use Glib::filename_from_unicode on unicode filenames or e) use something else to ensure your scalar has the correct contents.

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


$prev_pri = aioreq_pri [$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.


   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):


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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Example: Print the length of /etc/passwd:

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

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

   0x0000adf5 adfs
   0x0000adff affs
   0x5346414f afs
   0x09041934 anon-inode filesystem
   0x00000187 autofs
   0x42465331 befs
   0x1badface bfs
   0x42494e4d binfmt_misc
   0x9123683e btrfs
   0x0027e0eb cgroupfs
   0xff534d42 cifs
   0x73757245 coda
   0x012ff7b7 coh
   0x28cd3d45 cramfs
   0x453dcd28 cramfs-wend (wrong endianness)
   0x64626720 debugfs
   0x00001373 devfs
   0x00001cd1 devpts
   0x0000f15f ecryptfs
   0x00414a53 efs
   0x0000137d ext
   0x0000ef53 ext2/ext3/ext4
   0x0000ef51 ext2
   0xf2f52010 f2fs
   0x00004006 fat
   0x65735546 fuseblk
   0x65735543 fusectl
   0x0bad1dea futexfs
   0x01161970 gfs2
   0x47504653 gpfs
   0x00004244 hfs
   0xf995e849 hpfs
   0x00c0ffee hostfs
   0x958458f6 hugetlbfs
   0x2bad1dea inotifyfs
   0x00009660 isofs
   0x000072b6 jffs2
   0x3153464a jfs
   0x6b414653 k-afs
   0x0bd00bd0 lustre
   0x0000137f minix
   0x0000138f minix 30 char names
   0x00002468 minix v2
   0x00002478 minix v2 30 char names
   0x00004d5a minix v3
   0x19800202 mqueue
   0x00004d44 msdos
   0x0000564c novell
   0x00006969 nfs
   0x6e667364 nfsd
   0x00003434 nilfs
   0x5346544e ntfs
   0x00009fa1 openprom
   0x7461636F ocfs2
   0x00009fa0 proc
   0x6165676c pstorefs
   0x0000002f qnx4
   0x68191122 qnx6
   0x858458f6 ramfs
   0x52654973 reiserfs
   0x00007275 romfs
   0x67596969 rpc_pipefs
   0x73636673 securityfs
   0xf97cff8c selinux
   0x0000517b smb
   0x534f434b sockfs
   0x73717368 squashfs
   0x62656572 sysfs
   0x012ff7b6 sysv2
   0x012ff7b5 sysv4
   0x01021994 tmpfs
   0x15013346 udf
   0x00011954 ufs
   0x54190100 ufs byteswapped
   0x00009fa2 usbdevfs
   0x01021997 v9fs
   0xa501fcf5 vxfs
   0xabba1974 xenfs
   0x012ff7b4 xenix
   0x58465342 xfs
   0x012fd16d xia

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

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


   # 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).


   # 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) and FALLOC_FL_ZERO_RANGE, to zero a range (see your fallocate(2) manpage).

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

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

aio_chmod $fh_or_path, $mode, $callback->($status) Works like perl’s chmod function.
aio_unlink $pathname, $callback->($status) Asynchronously unlink (delete) a file and call the callback with the result code.
aio_mknod $pathname, $mode, $dev, $callback->($status) [EXPERIMENTAL]

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

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

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

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

aio_link $srcpath, $dstpath, $callback->($status) Asynchronously create a new link to the existing object at $srcpath at the path $dstpath and call the callback with the result code.
aio_symlink $srcpath, $dstpath, $callback->($status) Asynchronously create a new symbolic link to the existing object at $srcpath at the path $dstpath and call the callback with the result code.
aio_readlink $pathname, $callback->($link) Asynchronously read the symlink specified by $path and pass it to the callback. If an error occurs, nothing or undef gets passed to the callback.
aio_realpath $pathname, $callback->($path) Asynchronously make the path absolute and resolve any symlinks in $path. The resulting path only consists of directories (same as Cwd::realpath).

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

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

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

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

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

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

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

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

The flags are a combination of the following constants, ORed together (the flags will also be passed to the callback, possibly modified):
IO::AIO::READDIR_DENTS When this flag is off, then the callback gets an arrayref consisting of names only (as with aio_readdir), otherwise it gets an arrayref with [$name, $type, $inode] arrayrefs, each describing a single directory entry in more detail.

$name is the name of the entry.

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


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

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

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

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

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

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

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

aio_load $pathname, $data, $callback->($status) This is a composite request that tries to fully load the given file into memory. Status is the same as with aio_read.
aio_copy $srcpath, $dstpath, $callback->($status) Try to copy the file (directories not supported as either source or destination) from $srcpath to $dstpath and call the callback with a status of 0 (ok) or -1 (error, see $!).

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

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

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

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

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

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

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


   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_sync $callback->($status) Asynchronously call sync and call the callback when finished.
aio_fsync $fh, $callback->($status) Asynchronously call fsync on the given filehandle and call the callback with the fsync result code.
aio_fdatasync $fh, $callback->($status) Asynchronously call fdatasync on the given filehandle and call the callback with the fdatasync result code.

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

aio_syncfs $fh, $callback->($status) Asynchronously call the syncfs syscall to sync the filesystem associated to the given filehandle and call the callback with the syncfs result code. If syncfs is not available, calls sync(), but returns -1 and sets errno to ENOSYS nevertheless.
aio_sync_file_range $fh, $offset, $nbytes, $flags, $callback->($status) Sync the data portion of the file specified by $offset and $length to disk (but NOT the metadata), by calling the Linux-specific sync_file_range call. If sync_file_range is not available or it returns ENOSYS, then fdatasync or fsync is being substituted.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

   aio_mlockall IO::AIO::MCL_FUTURE;

aio_fiemap $fh, $start, $length, $flags, $count, $cb->(\@extents) Queries the extents of the given file (by calling the Linux FIEMAP ioctl, see <> 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)):


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

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

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


   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 realpath) will always rely on the string form of the pathname.

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

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

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

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

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

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

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

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

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


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 <B>executeB> state and skipping calling the callback when entering the the <B>resultB> 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.


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.
o The IO::AIO::GRP objects will be cleaned up during calls to IO::AIO::poll_cb, just like any other request.
o They can be canceled like any other request. Canceling will cancel not only the request itself, but also all requests it contains.
o They can also can also be added to other IO::AIO::GRP objects.
o 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.


   # 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.


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

See poll_cb for an example.

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

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

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

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

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

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

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

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

See nreqs for an example.

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

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

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

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

Strictly equivalent to:

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

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

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

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

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

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

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

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

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 somehting like this:

   IO::AIO::max_outstanding 32;

   for my $path (...) {
      aio_stat $path , ...;


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

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

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).

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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


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

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

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

   my $fast_md5 = md5 $data;

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

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

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

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

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

See the splice(2) manpage for details.

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


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 =>


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.


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 will be fixed in the next release.


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


 Marc Lehmann <>

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