Author image Αριστοτέλης Παγκαλτζής
and 1 contributors


Async - Asynchronous evaluation of Perl code (with optional timeouts)


  my $proc = Async->new( sub { any perl code you want executed } );

  if ( $proc->ready ) {
    # the code has finished executing
    if ( $proc->error ) {
      # something went wrong
    } else {
      $result = $proc->result; # The return value of the code

  # or:
  $result = $proc->result( 'force completion' ); # wait for it to finish


This module runs some code in a separate process and retrieves its result. Since the code is running in a separate process, your main program can continue with whatever it was doing while the separate code is executing. This separate code is called an asynchronous computation.


To check if the asynchronous computation is complete you can call the ready() method, which returns true if so, and false if it is still running.

After the asynchronous computation is complete, you should call the error() method to make sure that everything went all right. error() will return undef if the computation completed normally, and an error message otherwise.

Data returned by the computation can be retrieved with the result() method. The data must be a single string; any non-string value returned by the computation will be stringified. (See AsyncData below for how to avoid this.) If the computation has not completed yet, result() will return an undefined value.

result() takes an optional parameter, $force. If $force is true, then the calling process will wait until the asynchronous computation is complete before returning.


  use Async;
  $proc = AsyncTimeout->new( sub { ... }, $timeout, $special );

AsyncTimeout implements a version of Async that has an automatic timeout. If the asynchronous computation does not complete before $timeout seconds have elapsed, it is forcibly terminated and returns a special value $special. The default special value is the string "Timed out\n".

All the other methods for AsyncTimeout are exactly the same as for Async.


  use Async;
  $proc = AsyncData->new( sub { ... } );

AsyncData is just like Async except that instead of returning a string, the asynchronous computation may return any scalar value. If the scalar value is a reference, the result() method will yield a refernce to a copy of this data structure.

The AsyncData module requires that Storable be installed. AsyncData::new will die if Storable is unavailable.

All the other methods for AsyncData are exactly the same as for Async.


The asynchronous computation takes place in a separate process, so nothing it does can affect the main program. For example, if it modifies global variables, changes the current directory, opens and closes filehandles, or calls die, the parent process will be unaware of these things. However, the asynchronous computation does inherit the main program's file handles, so if it reads data from files that the main program had open, that data will not be availble to the main program; similarly the asynchronous computation can write data to the same file as the main program if it inherits an open filehandle for that file.


The errors that are reported by the error() mechanism are: those that are internal to Async itself:

  Couldn't make pipe: (reason)
  Couldn't fork: (reason)
  Read error: (reason)

If your asynchronous computation dies for any reason, that is not considered to be an error; that is the normal termination of the process. Any messages written to STDERR will go to the computation's STDERR, which is normally inherited from the main program, and the result() will be the empty string.


  use Async;
  sub long_running_computation {
    # This function simulates a computation that takes a long time to run
    my ( $x ) = @_;
    sleep 5;
    return $x + 2; # Eureka!
  # Main program:
  my $proc = Async->new( sub { long_running_computation(2) } ) or die;
  # The long-running computation is now executing.
  while (1) {
    print "Main program: The time is now ", scalar( localtime ), "\n";
    my $e;
    if ( $proc->ready ) {
      if ( $e = $proc->error ) {
        print "Something went wrong. The error was: $e\n";
      } else {
        print "The result of the computation is: ", $proc->result, "\n";
      undef $proc;
    # The result is not ready; we can go off and do something else here.
    sleep 1; # One thing we could do is to take nap.


Mark-Jason Dominus


Mark-Jason Dominus has dedicated the work to the Commons by waiving all of his or her rights to the work worldwide under copyright law and all related or neighboring legal rights he or she had in the work, to the extent allowable by law.

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