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Rocco Caputo


POE::Wheel::Run - event driven fork/exec with added value


  # Program may be scalar or \@array.
  $program = '/usr/bin/cat -';
  $program = [ '/usr/bin/cat', '-' ];

  $wheel = POE::Wheel::Run->new(
    Program     => $program,
    Priority    => +5,                 # Adjust priority.  May need to be root.
    User        => getpwnam('nobody'), # Adjust UID. May need to be root.
    Group       => getgrnam('nobody'), # Adjust GID. May need to be root.
    ErrorEvent  => 'oops',             # Event to emit on errors.
    CloseEvent  => 'child_closed',     # Child closed all output.

    StdinEvent  => 'stdin',  # Event to emit when stdin is flushed to child.
    StdoutEvent => 'stdout', # Event to emit with child stdout information.
    StderrEvent => 'stderr', # Event to emit with child stderr information.

    # Specify different I/O formats.
    StdinFilter  => POE::Filter::Line->new(),   # Child accepts input as lines.
    StdoutFilter => POE::Filter::Stream->new(), # Child output is a stream.
    StderrFilter => POE::Filter::Line->new(),   # Child errors are lines.

    # Set StdinFilter and StdoutFilter together.
    StdioFilter => POE::Filter::Line->new(),    # Or some other filter.

    # Specify different I/O methods.
    StdinDriver  => POE::Driver::SysRW->new(),  # Defaults to SysRW.
    StdoutDriver => POE::Driver::SysRW->new(),  # Same.
    StderrDriver => POE::Driver::SysRW->new(),  # Same.

    # Set StdinDriver and StdoutDriver together.
    StdioDriver  => POE::Driver::SysRW->new(),

  print "Unique wheel ID is  : ", $wheel->ID;
  print "Wheel's child PID is: ", $wheel->PID;

  # Send something to the child's STDIN.
  $wheel->put( 'input for the child' );

  # Kill the child.
  $wheel->kill( -9 );


Wheel::Run spawns child processes and establishes non-blocking, event based communication with them.



new() creates a new Run wheel. If successful, the new wheel represents a child process and the input, output and error pipes that speak with it.

new() accepts lots of stuff. Each parameter is name/value pair.


Conduit describes how Wheel::Run should talk with the child process. It may either be 'pipe' (the default), or 'pty'.

Pty conduits require the IO::Pty module.


CloseOnCall emulates the close-on-exec feature for child processes which are not started by exec(). When it is set to 1, all open file handles whose descriptors are greater than $^F are closed in the child process. This is only effective when POE::Wheel::Run is called with a code reference for its Program parameter.

  CloseOnCall => 1,
  Program => \&some_function,

CloseOnCall defaults to 0 (off) to remain compatible with existing programs.

For more details, please the discussion of $^F in perlvar.


These parameters change the drivers for Wheel::Run. The default drivers are created internally with <POE::Driver::SysRW-new()>>.

StdioDriver changes both StdinDriver and StdoutDriver at the same time.


CloseEvent contains the name of an event to emit when the child process closes all its output handles. This is a consistent notification that the child will not be sending any more output. It does not, however, signal that the client process has stopped accepting input.

ErrorEvent contains the name of an event to emit if something fails. It is optional and if omitted, the wheel will not notify its session if any errors occur. The event receives 5 parameters as follows: ARG0 = the return value of syscall(), ARG1 = errno() - the numeric value of the error generated, ARG2 = error() - a descriptive for the given error, ARG3 = the wheel id, and ARG4 = the handle on which the error ocurred (stdout, stderr, etc.)

Wheel::Run requires at least one of the following three events:

StdinEvent contains the name of an event that Wheel::Run emits whenever all its output has been flushed to the child process' STDIN handle.

StdoutEvent and StderrEvent contain names of events that Wheel::Run emits whenever the child process writes something to its STDOUT or STDERR handles, respectively.


StdioFilter contains an instance of a POE::Filter subclass. The filter describes how the child process performs input and output. Filter will be used to describe the child's stdin and stdout methods. If stderr is also to be used, StderrFilter will need to be specified separately.

Filter is optional. If left blank, it will default to an instance of POE::Filter::Line-new(Literal => "\n");>

StdinFilter and StdoutFilter can be used instead of or in addition to StdioFilter. They will override the default filter's selection in situations where a process' input and output are in different formats.


Group contains a numerical group ID that the child process should run at. This may not be meaningful on systems that have no concept of group IDs. The current process may need to run as root in order to change group IDs. Mileage varies considerably.


Priority contains an offset from the current process's priority. The child will be executed at the current priority plus the offset. The priority offset may be negative, but the current process may need to be running as root for that to work.


Program is the program to exec() once pipes and fork have been set up. Program's type determines how the program will be run.

If Program holds a scalar, it will be executed as exec($scalar). Shell metacharacters will be expanded in this form.

If Program holds an array reference, it will executed as exec(@$array). This form of exec() doesn't expand shell metacharacters.

If Program holds a code reference, it will be called in the forked child process, and then the child will exit. This allows Wheel::Run to fork off bits of long-running code which can accept STDIN input and pass responses to STDOUT and/or STDERR. Note, however, that POE's services are effectively disabled in the child process.

perlfunc has more information about exec() and the different ways to call it.


event() changes the event that Wheel::Run emits when a certain type of event occurs. EVENT_TYPE may be one of the event parameters in Wheel::Run's constructor.

  $wheel->event( StdinEvent  => 'new-stdin-event',
                 StdoutEvent => 'new-stdout-event',
put LIST

put() queues a LIST of different inputs for the child process. They will be flushed asynchronously once the current state returns. Each item in the LIST is processed according to the StdinFilter.


Get StdinFilter, StdoutFilter, or StderrFilter respectively.

set_stdio_filter FILTER_REFERENCE

Set StdinFilter and StdoutFilter at once.

set_stdin_filter FILTER_REFERENCE
set_stdout_filter FILTER_REFERENCE
set_stderr_filter FILTER_REFERENCE

Set StdinFilter, StdoutFilter, or StderrFilter respectively.


Pause or resume StdoutEvent or StderrEvent events. By using these methods a session can control the flow of Stdout and Stderr events coming in from this child process.


Returns the wheel's unique ID, which is not the same as the child process' ID. Every event generated by Wheel::Run includes a wheel ID so that it can be matched up with its generator. This lets a single session manage several wheels without becoming confused about which one generated what event.


Returns the child process' ID. It's useful for matching up to SIGCHLD events, which include child process IDs as well, so that wheels can be destroyed properly when children exit.


Sends a signal to the child process. It's useful for processes which tend to be reluctant to exit when their terminals are closed.



CloseEvent contains the name of the event Wheel::Run emits whenever a child process has closed all its output handles. It signifies that the child will not be sending more information. In addition to the usual POE parameters, each CloseEvent comes with one of its own:

ARG0 contains the wheel's unique ID. This can be used to keep several child processes separate when they're managed by the same session.

A sample close event handler:

  sub close_state {
    my ($heap, $wheel_id) = @_[HEAP, ARG0];

    my $child = delete $heap->{child}->{$wheel_id};
    print "Child ", $child->PID, " has finished.\n";

ErrorEvent contains the name of an event that Wheel::Run emits whenever an error occurs. Every error event comes with four parameters:

ARG0 contains the name of the operation that failed. It may be 'read' or 'write' or 'fork' or 'exec' or something. The actual values aren't yet defined. Note: This is not necessarily a function name.

ARG1 and ARG2 hold numeric and string values for $!, respectively.

ARG3 contains the wheel's unique ID.

ARG4 contains the name of the child filehandle that has the error. It may be "STDIN", "STDOUT", or "STDERR". The sense of ARG0 will be the opposite of what you might normally expect for these handles. For example, Wheel::Run will report a "read" error on "STDOUT" because it tried to read data from that handle.

A sample error event handler:

  sub error_state {
    my ($operation, $errnum, $errstr, $wheel_id) = @_[ARG0..ARG3];
    warn "Wheel $wheel_id generated $operation error $errnum: $errstr\n";

StdinEvent contains the name of an event that Wheel::Run emits whenever everything queued by its put() method has been flushed to the child's STDIN handle.

StdinEvent's ARG0 parameter contains its wheel's unique ID.


StdoutEvent and StderrEvent contain names for events that Wheel::Run emits whenever the child process makes output. StdoutEvent contains information the child wrote to its STDOUT handle, and StderrEvent includes whatever arrived from the child's STDERR handle.

Both of these events come with two parameters. ARG0 contains the information that the child wrote. ARG1 holds the wheel's unique ID.

  sub stdout_state {
    my ($heap, $input, $wheel_id) = @_[HEAP, ARG0, ARG1];
    print "Child process in wheel $wheel_id wrote to STDOUT: $input\n";

  sub stderr_state {
    my ($heap, $input, $wheel_id) = @_[HEAP, ARG0, ARG1];
    print "Child process in wheel $wheel_id wrote to STDERR: $input\n";



The SEE ALSO section in POE contains a table of contents covering the entire POE distribution.


Wheel::Run's constructor doesn't emit proper events when it fails. Instead, it just dies, carps or croaks.

Filter changing hasn't been implemented yet. Let the author know if it's needed. Better yet, patch the file based on the code in Wheel::ReadWrite.

Priority is a delta; there's no way to set it directly to some value.

User must be specified by UID. It would be nice to support login names.

Group must be specified by GID. It would be nice to support group names.

ActiveState Perl doesn't like this module one bit.


Please see POE for more information about authors and contributors.