IO::AIO - Asynchronous Input/Output


 use IO::AIO;

 aio_open "/etc/passwd", 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 ...;

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

 # 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 =>


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 Fcntl;
   use EV;
   use IO::AIO;

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

   # queue the request to open /etc/passwd
   aio_open "/etc/passwd", 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

   # 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:


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


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


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).


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.


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).



All the aio_* calls are more or less thin wrappers around the syscall with the same name (sans aio_). The arguments are similar or identical, and they all accept an additional (and optional) $callback argument which must be a code reference. This code reference will get called with the syscall return code (e.g. most syscalls return -1 on error, unlike perl, which usually delivers "false") as its sole argument after the given syscall has been executed asynchronously.

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

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

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

To encode pathnames as octets, either make sure you either: a) always pass in filenames you got from outside (command line, readdir etc.) without tinkering, b) are ASCII or ISO 8859-1, c) use the Encode module and encode your pathnames to the locale (or other) encoding in effect in the user environment, d) use Glib::filename_from_unicode on unicode filenames or e) use something else to ensure your scalar has the correct contents.

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

$prev_pri = aioreq_pri [$pri]

Returns the priority value that would be used for the next request and, if $pri is given, sets the priority for the next aio request.

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

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

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

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

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

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

aio_open $pathname, $flags, $mode, $callback->($fh)

Asynchronously open or create a file and call the callback with a newly created filehandle for the file.

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

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

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


   aio_open "/etc/passwd", O_RDONLY, 0, sub {
      if ($_[0]) {
         print "open successful, fh is $_[0]\n";
      } else {
         die "open failed: $!\n";
aio_close $fh, $callback->($status)

Asynchronously close a file and call the callback with the result code.

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

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

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

aio_read $fh,$offset,$length, $data,$dataoffset, $callback->($retval)
aio_write $fh,$offset,$length, $data,$dataoffset, $callback->($retval)

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

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

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

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

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

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

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

   aio_read $fh, 7, 15, $buffer, 0, sub {
      $_[0] > 0 or die "read error: $!";
      print "read $_[0] bytes: <$buffer>\n";
aio_sendfile $out_fh, $in_fh, $in_offset, $length, $callback->($retval)

Tries to copy $length bytes from $in_fh to $out_fh. It starts reading at byte offset $in_offset, and starts writing at the current file offset of $out_fh. Because of that, it is not safe to issue more than one aio_sendfile per $out_fh, as they will interfere with each other.

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

If the native sendfile call fails or is not implemented, it will be emulated, so you can call aio_sendfile on any type of filehandle regardless of the limitations of the operating system.

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

aio_readahead $fh,$offset,$length, $callback->($retval)

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

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

aio_stat $fh_or_path, $callback->($status)
aio_lstat $fh, $callback->($status)

Works like perl's stat or lstat in void context. The callback will be called after the stat and the results will be available using stat _ or -s _ etc...

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

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

Example: Print the length of /etc/passwd:

   aio_stat "/etc/passwd", sub {
      $_[0] and die "stat failed: $!";
      print "size is ", -s _, "\n";
aio_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_chmod $fh_or_path, $mode, $callback->($status)

Works like perl's chmod function.

Asynchronously unlink (delete) a file and call the callback with the result code.

aio_mknod $path, $mode, $dev, $callback->($status)


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

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

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

Asynchronously create a new link to the existing object at $srcpath at the path $dstpath and call the callback with the result code.

Asynchronously create a new symbolic link to the existing object at $srcpath at the path $dstpath and call the callback with the result code.

Asynchronously read the symlink specified by $path and pass it to the callback. If an error occurs, nothing or undef gets passed to the callback.

aio_rename $srcpath, $dstpath, $callback->($status)

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

aio_mkdir $pathname, $mode, $callback->($status)

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

aio_rmdir $pathname, $callback->($status)

Asynchronously rmdir (delete) a directory and call the callback with the result code.

aio_readdir $pathname, $callback->($entries)

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

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

aio_readdirx $pathname, $flags, $callback->($entries, $flags)

Quite similar to aio_readdir, but the $flags argument allows to tune behaviour and output format. In case of an error, $entries will be undef.

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


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

$name is the name of the entry.

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


IO::AIO::DT_UNKNOWN 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.


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

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


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.


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

aio_load $path, $data, $callback->($status)

This is a composite request that tries to fully load the given file into memory. Status is the same as with aio_read.

aio_copy $srcpath, $dstpath, $callback->($status)

Try to copy the file (directories not supported as either source or destination) from $srcpath to $dstpath and call the callback with the 0 (error) or -1 ok.

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 the 0 (error) or -1 ok.

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

aio_scandir $path, $maxreq, $callback->($dirs, $nondirs)

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

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

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


   aio_scandir $dir, 0, sub {
      my ($dirs, $nondirs) = @_;
      print "real directories: @$dirs\n";
      print "everything else: @$nondirs\n";

Implementation notes.

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

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

Otherwise, after reading the directory, the modification time, size etc. of the directory before and after the readdir is checked, and if they match (and isn't the current time), the link count will be used to decide how many entries are directories (if >= 2). Otherwise, no knowledge of the number of subdirectories will be assumed.

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

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

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

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

aio_rmtree $path, $callback->($status)

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

aio_sync $callback->($status)

Asynchronously call sync and call the callback when finished.

aio_fsync $fh, $callback->($status)

Asynchronously call fsync on the given filehandle and call the callback with the fsync result code.

aio_fdatasync $fh, $callback->($status)

Asynchronously call fdatasync on the given filehandle and call the callback with the fdatasync result code.

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

aio_sync_file_range $fh, $offset, $nbytes, $flags, $callback->($status)

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

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

aio_pathsync $path, $callback->($status)

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

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

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

aio_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.


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.


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.


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.

$grp->result (...)

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

$grp->errno ([$errno])

Sets the group errno value to $errno, or the current value of errno when the argument is missing.

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

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

feed $grp $callback->($grp)

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

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

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

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

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


   # 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.


Process some outstanding events on the result pipe. You have to call this regularly. Returns 0 if all events could be processed, or -1 if it returned earlier for whatever reason. Returns immediately when no events are outstanding. The amount of events processed depends on the settings of IO::AIO::max_poll_req and IO::AIO::max_poll_time.

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

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

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

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

See nreqs for an example.


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

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::min_parallel $nthreads

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

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

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

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

IO::AIO::max_parallel $nthreads

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

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

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

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

IO::AIO::max_idle $nthreads

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

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

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

IO::AIO::max_outstanding $maxreqs

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

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

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

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



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;

Returns the number of requests currently in the ready state (not yet executed).


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


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

IO::AIO::sendfile $ofh, $ifh, $offset, $count

Calls the eio_sendfile_sync function, which is like aio_sendfile, but is blocking (this makes most sense if you know the input data is likely cached already and the output filehandle is set to non-blocking operations).

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

IO::AIO::fadvise $fh, $offset, $len, $advice

Simply calls the posix_fadvise function (see it's manpage for details). The following advice constants are avaiable: IO::AIO::FADV_NORMAL, IO::AIO::FADV_SEQUENTIAL, IO::AIO::FADV_RANDOM, IO::AIO::FADV_NOREUSE, IO::AIO::FADV_WILLNEED, IO::AIO::FADV_DONTNEED.

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


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

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

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


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 <>