Perl Cd Bookshelf [Electronic resources] نسخه متنی

16.8.1. Problem

Wanting simultaneous read and write access to another program is very
common, but surprisingly perilous. That's one reason the built-in
open doesn't permit:

open(my $double_handle, "| program args |") # WRONG

The big problem here is buffering. Because you can't force the other
program to unbuffer its output, you can't guarantee that your reads
won't block. If you block trying to read at the same time the other
process blocks waiting for you to send something, you've achieved the
unholy state of deadlock. There you'll both stay, wedged, until
someone kills your process or the machine reboots.

If you control the other process's buffering because you wrote the
other program and know how it works, then IPC::Open2 may be the
module for you. If you pass undefined scalar values as the first two
arguments, open2 creates new filehandles:

use IPC::Open2;
$pid = open2(my $reader, my $writer, $program);

Alternatively, you can pass in arguments that look like
"<&OTHERFILEHANDLE" or
">&OTHERFILEHANDLE", which specify existing
filehandles for the child process to read from or write to. These
filehandles don't have to be controlled by your program—they
may be connected to other programs, files, or sockets.

You can specify the program either as a list (where the first element
is the program name and the remaining elements are arguments to the
program) or as a single string (which is passed to the shell as a
command to start the program). If you also want control over the
process's standard error, use the IPC::Open3 module and see the next
recipe.

If an error occurs, open2 and
open3 do not return. Instead, they
die with an error message that begins with
"open2" or "open3". To test for
failure, use the eval BLOCK
construct:

eval {
$pid = open2($readme, $writeme, @program_and_arguments);
};
if ($@) {
if ($@ =~ /^open2/) {
warn "open2 failed: $!\n$@\n";
return;
}
die; # reraise unforeseen exception
}

You must call waitpid, as we do in the Solution,
because IPC::Open2 doesn't reap the child process after it exits. See
Recipe 16.19 for details.