focal (3) Proc::Background.3pm.gz
NAME
Proc::Background - Generic interface to background process management
VERSION
version 1.21
SYNOPSIS
use Proc::Background;
timeout_system($seconds, $command, $arg1);
timeout_system($seconds, "$command $arg1");
my $proc1 = Proc::Background->new($command, $arg1, $arg2);
my $proc2 = Proc::Background->new("$command $arg1 1>&2");
$proc1->alive;
$proc1->die;
$proc1->wait;
my $time1 = $proc1->start_time;
my $time2 = $proc1->end_time;
# Add an option to kill the process with die when the variable is
# DESTROYed.
my $opts = {'die_upon_destroy' => 1};
my $proc3 = Proc::Background->new($opts, $command, $arg1, $arg2);
$proc3 = undef;
DESCRIPTION
This is a generic interface for placing processes in the background on both Unix and Win32 platforms.
This module lets you start, kill, wait on, retrieve exit values, and see if background processes still
exist.
NAME
Proc::Background - Generic interface to Unix and Win32 background process management
METHODS
new [options] command, [arg, [arg, ...]]
new [options] 'command [arg [arg ...]]'
This creates a new background process. As exec() or system() may be passed an array with a single
single string element containing a command to be passed to the shell or an array with more than one
element to be run without calling the shell, new has the same behavior.
In certain cases new will attempt to find command on the system and fail if it cannot be found.
For Win32 operating systems:
The Win32::Process module is always used to spawn background
processes on the Win32 platform. This module always takes a
single string argument containing the executable's name and
any option arguments. In addition, it requires that the
absolute path to the executable is also passed to it. If
only a single argument is passed to new, then it is split on
whitespace into an array and the first element of the split
array is used at the executable's name. If multiple
arguments are passed to new, then the first element is used
as the executable's name.
If the executable's name is an absolute path, then new
checks to see if the executable exists in the given location
or fails otherwise. If the executable's name is not
absolute, then the executable is searched for using the PATH
environmental variable. The input executable name is always
replaced with the absolute path determined by this process.
In addition, when searching for the executable, the
executable is searched for using the unchanged executable
name and if that is not found, then it is checked by
appending `.exe' to the name in case the name was passed
without the `.exe' suffix.
Finally, the argument array is placed back into a single
string and passed to Win32::Process::Create.
For non-Win32 operating systems, such as Unix:
If more than one argument is passed to new, then new
assumes that the command will not be passed through the
shell and the first argument is the executable's relative
or absolute path. If the first argument is an absolute
path, then it is checked to see if it exists and can be
run, otherwise new fails. If the path is not absolute,
then the PATH environmental variable is checked to see if
the executable can be found. If the executable cannot be
found, then new fails. These steps are taking to prevent
exec() from failing after an fork() without the caller of
new knowing that something failed.
The first argument to new options may be a reference to a hash which contains key/value pairs to
modify Proc::Background's behavior. Currently the only key understood by new is die_upon_destroy.
When this value is set to true, then when the Proc::Background object is being DESTROY'ed for any
reason (i.e. the variable goes out of scope) the process is killed via the die() method.
If anything fails, then new returns an empty list in a list context, an undefined value in a scalar
context, or nothing in a void context.
pid Returns the process ID of the created process. This value is saved even if the process has already
finished.
alive
Return 1 if the process is still active, 0 otherwise.
die, die(@kill_sequence)
Reliably try to kill the process. Returns 1 if the process no longer exists once die has completed,
0 otherwise. This will also return 1 if the process has already died.
@kill_sequence is a list of actions and seconds-to-wait for that action to end the process. The
default is " TERM 2 TERM 8 KILL 3 KILL 7 ". On Unix this sends SIGTERM and SIGKILL; on Windows it
just calls TerminateProcess (graceful termination is still a TODO).
Note that "die()" on Proc::Background 1.10 and earlier on Unix called a sequence of:
->die( ( HUP => 1 )x5, ( QUIT => 1 )x5, ( INT => 1 )x5, ( KILL => 1 )x5 );
which didn't particularly make a lot of sense, since SIGHUP is open to interpretation, and QUIT is
almost always immediately fatal and generates an unneeded coredump. The new default should
accomodate programs that acknowledge a second SIGTERM, and give enough time for it to exit on a laggy
system while still not holding up the main script too much.
wait
$exit= $proc->wait; # blocks forever
$exit= $proc->wait($timeout_seconds); # since version 1.20
Wait for the process to exit. Return the exit status of the command as returned by wait() on the
system. To get the actual exit value, divide by 256 or right bit shift by 8, regardless of the
operating system being used. If the process never existed, this returns undef. This function may be
called multiple times even after the process has exited and it will return the same exit status.
Since version 1.20, you may pass an optional argument of the number of seconds to wait for the
process to exit. This may be fractional, and if it is zero then the wait will be non-blocking. Note
that on Unix this is implemented with "alarm" in Time::HiRes before a call to wait(), so it may not
be compatible with scripts that use alarm() for other purposes, or systems/perls that resume system
calls after a signal. In the event of a timeout, the return will be undef.
exit_code
Returns the exit code of the process, assuming it exited cleanly. Returns "undef" if the process has
not exited yet, and 0 if the process exited with a signal (or TerminateProcess). Since 0 is
ambiguous, check for "exit_signal" first.
exit_signal
Returns the value of the signal the process exited with, assuming it died on a signal. Returns
"undef" if it has not exited yet, and 0 if it did not die to a signal.
start_time
Return the value that the Perl function time() returned when the process was started.
end_time
Return the value that the Perl function time() returned when the exit status was obtained from the
process.
FUNCTIONS
timeout_system timeout, command, [arg, [arg...]]
timeout_system 'timeout command [arg [arg...]]'
Run a command for timeout seconds and if the process did not exit, then kill it. While the timeout
is implemented using sleep(), this function makes sure that the full timeout is reached before
killing the process. timeout_system does not wait for the complete timeout number of seconds before
checking if the process has exited. Rather, it sleeps repeatidly for 1 second and checks to see if
the process still exists.
In a scalar context, timeout_system returns the exit status from the process. In an array context,
timeout_system returns a two element array, where the first element is the exist status from the
process and the second is set to 1 if the process was killed by timeout_system or 0 if the process
exited by itself.
The exit status is the value returned from the wait() call. If the process was killed, then the
return value will include the killing of it. To get the actual exit value, divide by 256.
If something failed in the creation of the process, the subroutine returns an empty list in a list
context, an undefined value in a scalar context, or nothing in a void context.
IMPLEMENTATION
Proc::Background comes with two modules, Proc::Background::Unix and Proc::Background::Win32. Currently,
on Unix platforms Proc::Background uses the Proc::Background::Unix class and on Win32 platforms it uses
Proc::Background::Win32, which makes use of Win32::Process.
The Proc::Background assigns to @ISA either Proc::Background::Unix or Proc::Background::Win32, which does
the OS dependent work. The OS independent work is done in Proc::Background.
Proc::Background uses two variables to keep track of the process. $self->{_os_obj} contains the
operating system object to reference the process. On a Unix systems this is the process id (pid). On
Win32, it is an object returned from the Win32::Process class. When $self->{_os_obj} exists, then the
process is running. When the process dies, this is recorded by deleting $self->{_os_obj} and saving the
exit value $self->{_exit_value}.
Anytime alive is called, a waitpid() is called on the process and the return status, if any, is gathered
and saved for a call to wait. This module does not install a signal handler for SIGCHLD. If for some
reason, the user has installed a signal handler for SIGCHLD, then, then when this module calls waitpid(),
the failure will be noticed and taken as the exited child, but it won't be able to gather the exit
status. In this case, the exit status will be set to 0.
SEE ALSO
IPC::Run
IPC::Run is a much more complete solution for running child processes. It handles dozens of forms of
redirection and pipe pumping, and should probably be your first stop for any complex needs.
However, also note the very large and slightly alarming list of limitations it lists for Win32.
Proc::Background is a much simpler design and should be more reliable for simple needs.
Win32::ShellQuote
If you are running on Win32, this article by helps describe the problem you are up against for
passing argument lists:
Everyone quotes command line arguments the wrong way
<https://blogs.msdn.microsoft.com/twistylittlepassagesallalike/2011/04/23/everyone-quotes-command-
line-arguments-the-wrong-way/> by Daniel Colascione.
This module gives you parsing / quoting per the standard CommandLineToArgvW behavior. But, if you
need to pass arguments to be processed by "cmd.exe" then you need to do additional work.
AUTHORS
• Blair Zajac <blair@orcaware.com> • Michael Conrad <mike@nrdvana.net>
CONTRIBUTORS
• Florian Schlichting <fsfs@debian.org> • Kevin Ryde <user42@zip.com.au> • Salvador Fandiño <sfandino@yahoo.com>
COPYRIGHT AND LICENSE
This software is copyright (c) 2019 by Michael Conrad, (C) 1998-2009 by Blair Zajac.
This is free software; you can redistribute it and/or modify it under the same terms as the Perl 5
programming language system itself.