IPC::Open3(3pm) Perl Programmers Reference Guide IPC::Open3(3pm)
NAME
IPC::Open3 - open a process for reading, writing, and error handling
using open3()
SYNOPSIS
use Symbol 'gensym'; # vivify a separate handle for STDERR
my $pid = open3(my $chld_in, my $chld_out, my $chld_err = gensym,
'some', 'cmd', 'and', 'args');
# or pass the command through the shell
my $pid = open3(my $chld_in, my $chld_out, my $chld_err = gensym,
'some cmd and args');
# read from parent STDIN
# send STDOUT and STDERR to already open handle
open my $outfile, '>>', 'output.txt' or die "open failed: $!";
my $pid = open3('<&STDIN', $outfile, undef,
'some', 'cmd', 'and', 'args');
# write to parent STDOUT and STDERR
my $pid = open3(my $chld_in, '>&STDOUT', '>&STDERR',
'some', 'cmd', 'and', 'args');
# reap zombie and retrieve exit status
waitpid( $pid, 0 );
my $child_exit_status = $? >> 8;
DESCRIPTION
Extremely similar to open2(), open3() spawns the given command and
connects $chld_out for reading from the child, $chld_in for writing to
the child, and $chld_err for errors. If $chld_err is false, or the
same file descriptor as $chld_out, then STDOUT and STDERR of the child
are on the same filehandle. This means that an autovivified lexical
cannot be used for the STDERR filehandle, but gensym from Symbol can be
used to vivify a new glob reference, see "SYNOPSIS". The $chld_in will
have autoflush turned on.
If $chld_in begins with "<&", then $chld_in will be closed in the
parent, and the child will read from it directly. If $chld_out or
$chld_err begins with ">&", then the child will send output directly to
that filehandle. In both cases, there will be a dup(2) instead of a
pipe(2) made.
If either reader or writer is the empty string or undefined, this will
be replaced by an autogenerated filehandle. If so, you must pass a
valid lvalue in the parameter slot so it can be overwritten in the
caller, or an exception will be raised.
The filehandles may also be integers, in which case they are understood
as file descriptors.
open3() returns the process ID of the child process. It doesn't return
on failure: it just raises an exception matching "/^open3:/". However,
"exec" failures in the child (such as no such file or permission
denied), are just reported to $chld_err under Windows and OS/2, as it
is not possible to trap them.
If the child process dies for any reason, the next write to $chld_in is
likely to generate a SIGPIPE in the parent, which is fatal by default.
So you may wish to handle this signal.
Note if you specify "-" as the command, in an analogous fashion to
"open(my $fh, "-|")" the child process will just be the forked Perl
process rather than an external command. This feature isn't yet
supported on Win32 platforms.
open3() does not wait for and reap the child process after it exits.
Except for short programs where it's acceptable to let the operating
system take care of this, you need to do this yourself. This is
normally as simple as calling "waitpid $pid, 0" when you're done with
the process. Failing to do this can result in an accumulation of
defunct or "zombie" processes. See "waitpid" in perlfunc for more
information.
If you try to read from the child's stdout writer and their stderr
writer, you'll have problems with blocking, which means you'll want to
use select() or IO::Select, which means you'd best use sysread()
instead of readline() for normal stuff.
This is very dangerous, as you may block forever. It assumes it's
going to talk to something like bc(1), both writing to it and reading
from it. This is presumably safe because you "know" that commands like
bc(1) will read a line at a time and output a line at a time. Programs
like sort(1) that read their entire input stream first, however, are
quite apt to cause deadlock.
The big problem with this approach is that if you don't have control
over source code being run in the child process, you can't control what
it does with pipe buffering. Thus you can't just open a pipe to "cat
-v" and continually read and write a line from it.
See Also
IPC::Open2(3)
Like Open3 but without STDERR capture.
IPC::Run(3)
This is a CPAN module that has better error handling and more
facilities than Open3.
WARNING
The order of arguments differs from that of open2().
perl v5.38.2 2023-11-28 IPC::Open3(3pm)
perl 5.38.2 - Generated Sat Dec 7 11:01:09 CST 2024