IO::Async::ChildManager - facilitates the execution of child processes


This object is used indirectly via an IO::Async::Loop:

 use IO::Async::Loop;

 my $loop = IO::Async::Loop->new;


    command => "/bin/ps",

    on_finish => sub {
       my ( $pid, $exitcode, $stdout, $stderr ) = @_;
       my $status = ( $exitcode >> 8 );
       print "ps [PID $pid] exited with status $status\n";

    command => [ "/bin/ping", "-c4", "" ],

    stdout => {
       on_read => sub {
          my ( $stream, $buffref, $eof ) = @_;
          while( $$buffref =~ s/^(.*)\n// ) {
             print "PING wrote: $1\n";
          return 0;

    on_finish => sub {
       my ( $pid, $exitcode ) = @_;
       my $status = ( $exitcode >> 8 );

 my ( $pipeRd, $pipeWr ) = IO::Async::OS->pipepair;
    command => "/usr/bin/my-command",

    setup => [
       stdin  => [ "open", "<", "/dev/null" ],
       stdout => $pipeWr,
       stderr => [ "open", ">>", "/var/log/mycmd.log" ],
       chdir  => "/",

    on_exit => sub {
       my ( $pid, $exitcode ) = @_;
       my $status = ( $exitcode >> 8 );
       print "Command exited with status $status\n";

    code => sub {
       do_something; # executes in a child process
       return 1;

    on_exit => sub {
       my ( $pid, $exitcode, $dollarbang, $dollarat ) = @_;
       my $status = ( $exitcode >> 8 );
       print "Child process exited with status $status\n";
       print " OS error was $dollarbang, exception was $dollarat\n";


This module extends the functionality of the containing IO::Async::Loop to manage the execution of child processes. It acts as a central point to store PID values of currently-running children, and to call the appropriate continuation handler code when the process terminates. It provides useful wrapper methods that set up filehandles and other child process details, and to capture the child process's STDOUT and STDERR streams.


When active, the following methods are available on the containing Loop object.

$pid = $loop->spawn_child( %params )

This method creates a new child process to run a given code block or command. The %params hash takes the following keys:

command => ARRAY or STRING

Either a reference to an array containing the command and its arguments, or a plain string containing the command. This value is passed into perl's exec function.

code => CODE

A block of code to execute in the child process. It will be called in scalar context inside an eval block.

setup => ARRAY

A reference to an array which gives file descriptors to set up in the child process before running the code or command. See below.

on_exit => CODE

A continuation to be called when the child processes exits. It will be invoked in the following way:

 $on_exit->( $pid, $exitcode, $dollarbang, $dollarat )

The second argument is passed the plain perl $? value.

Exactly one of the command or code keys must be specified.

If the command key is used, the given array or string is executed using the exec function.

If the code key is used, the return value will be used as the exit(2) code from the child if it returns (or 255 if it returned undef or thows an exception).

 Case          | ($exitcode >> 8)       | $dollarbang | $dollarat
 exec succeeds | exit code from program |     0       |    ""
 exec fails    |         255            |     $!      |    ""
 $code returns |     return value       |     $!      |    ""
 $code dies    |         255            |     $!      |    $@

It is usually more convenient to use the open_child method in simple cases where an external program is being started in order to interact with it via file IO, or even run_child when only the final result is required, rather than interaction while it is running.

setup array

This array gives a list of file descriptor operations to perform in the child process after it has been fork(2)ed from the parent, before running the code or command. It consists of name/value pairs which are ordered; the operations are performed in the order given.

fdn => ARRAY

Gives an operation on file descriptor n. The first element of the array defines the operation to be performed:

[ 'close' ]

The file descriptor will be closed.

[ 'dup', $io ]

The file descriptor will be dup2(2)ed from the given IO handle.

[ 'open', $mode, $file ]

The file descriptor will be opened from the named file in the given mode. The $mode string should be in the form usually given to the open function; such as '<' or '>>'.

[ 'keep' ]

The file descriptor will not be closed; it will be left as-is.

A non-reference value may be passed as a shortcut, where it would contain the name of the operation with no arguments (i.e. for the close and keep operations).


Shortcut for passing fdn, where n is the fileno of the IO reference. In this case, the key must be a reference that implements the fileno method. This is mostly useful for

 $handle => 'keep'
fdn => IO

A shortcut for the dup case given above.

stdin => ...
stdout => ...
stderr => ...

Shortcuts for fd0, fd1 and fd2 respectively.

env => HASH

A reference to a hash to set as the child process's environment.

Note that this will entirely set a new environment, completely replacing the existing one. If you want to simply add new keys or change the values of some keys without removing the other existing ones, you can simply copy %ENV into the hash before setting new keys:

 env => {
    ANOTHER => "key here",
nice => INT

Change the child process's scheduling priority using POSIX::nice.

chdir => STRING

Change the child process's working directory using chdir.

setuid => INT
setgid => INT

Change the child process's effective UID or GID.

setgroups => ARRAY

Change the child process's groups list, to those groups whose numbers are given in the ARRAY reference.

On most systems, only the privileged superuser change user or group IDs. IO::Async will NOT check before detaching the child process whether this is the case.

If setting both the primary GID and the supplementary groups list, it is suggested to set the primary GID first. Moreover, some operating systems may require that the supplementary groups list contains the primary GID.

If no directions for what to do with stdin, stdout and stderr are given, a default of keep is implied. All other file descriptors will be closed, unless a keep operation is given for them.

If setuid is used, be sure to place it after any other operations that might require superuser privileges, such as setgid or opening special files.


Paul Evans <>