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


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(
    # Set the program to execute, and optionally some parameters.
    Program     => $program,
    ProgramArgs => \@program_args,
    # Define I/O events to emit.  Most are optional.
    StdinEvent  => 'stdin',     # Flushed all data to the child's STDIN.
    StdoutEvent => 'stdout',    # Received data from the child's STDOUT.
    StderrEvent => 'stderr',    # Received data from the child's STDERR.
    ErrorEvent  => 'oops',          # An I/O error occurred.
    CloseEvent  => 'child_closed',  # Child closed all output handles.
    # Optionally adjust the child process priority, user ID, and/or
    # group ID.  You may need to be root to do this.
    Priority    => +5,
    User        => scalar(getpwnam 'nobody'),
    Group       => getgrnam('nobody'),
    # Optionally 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.
    # Shorthand to set StdinFilter and StdoutFilter together.
    StdioFilter => POE::Filter::Line->new(),    # Or some other filter.
  # Information about the wheel and its process.
  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);  # TERM by default.


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

Wheel::Run does not reap child processes. For that, you need to register a SIGCHLD handler:

  $kernel->sig(CHLD => "your_event");

The session will then receive your_event with details about $? when the wheel's process exits and is reaped. POE will reap child processes as a side effect.

Another way to do it is to register $SIG{CHLD} = ``IGNORE''. Use sparingly and with caution: This may clobber a handler that POE has already registered for SIGCHLD. Why does IGNORE work this way? See the discussion in perldoc perlipc.


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. By default it will try various forms of inter-process communication to build a pipe between the parent and child processes. If a particular method is preferred, it can be set to ``pipe'', ``socketpair'', or ``inet''. It may also be set to ``pty'' if the child process should have its own pseudo tty. Setting it to ``pty-pipe'' gives the child process a stdin and stdout pseudo-tty, but keeps stderr as a pipe, rather than merging stdout and stderr as with ``pty''.

The reasons to define this parameter would be if you want to use ``pty'', if the default pipe type doesn't work properly on your system, or the default pipe type's performance is poor.

Pty conduits require the IO::Pty module.

Winsize is only valid for Conduit = "pty" and used to set the window size of the pty device.

The window size is given as an array reference. The first element is the number of lines, the second the number of columns. The third and the fourth arguments are optional and specify the X and Y dimensions in pixels.

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 the perlvar manpage.

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.

See EVENTS AND PARAMETERS below for a more detailed description of these events and their parameters.

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.

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

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.

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.

When true, NoSetSid disables setsid() in the child process. By default, setsid() is called to execute the child process in a separate Unix session.

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.

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

Note: Do not call exit() explicitly when executing a subroutine. POE::Wheel::Run takes special care to avoid object destructors and END blocks in the child process, and calling exit() will thwart that. You may see ``POE::Kernel's run() method was never called.'' or worse.

ProgramArgs => ARRAY
If specified, ProgramArgs should refer to a list of parameters for the program being run.
  my @parameters = qw(foo bar baz);  # will be passed to Program
  ProgramArgs => \@parameters;

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.

Closes the child process' STDIN and stops the wheel from reporting StdinEvent. It is extremely useful for running utilities that expect to receive EOF on their standard inputs before they respond.

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.

The kill() method will send SIGTERM if SIGNAL is undef or omitted.

Return driver statistics.


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', 'write', 'fork', 'exec' or the name of some other function or task. The actual values aren't yet defined. Note: This is not necessarily a function name.

ARG1 and ARG2 hold numeric and string values for $!, respectively. "$!" will eq "" for read error 0 (child process closed STDOUT or STDERR).

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];
    $errstr = "remote end closed" if $operation eq "read" and !$errnum;
    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 generates new 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";


One common task is scrubbing a child process' environment. This amounts to clearing the contents of %ENV and setting it up with some known, secure values.

Environment scrubbing is easy when the child process is running a subroutine, but it's not so easy---or at least not as intuitive---when executing external programs.

The way we do it is to run a small subroutine in the child process that performs the exec() call for us.

  Program => \&exec_with_scrubbed_env,
  sub exec_with_scrubbed_env {
    delete @ENV{keys @ENV};
    $ENV{PATH} = "/bin";

That deletes everything from the environment, sets a simple, secure PATH, and executes a program with its arguments.



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.


Please see POE for more information about authors and contributors.

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