IPC::Open2(3p) Perl Programmers Reference Guide IPC::Open2(3p)
IPC::Open2, open2 - open a process for both reading and
writing
use IPC::Open2;
$pid = open2(\*CHLD_OUT, \*CHLD_IN, 'some cmd and args');
# or without using the shell
$pid = open2(\*CHLD_OUT, \*CHLD_IN, 'some', 'cmd', 'and', 'args');
# or with handle autovivification
my($chld_out, $chld_in);
$pid = open2($chld_out, $chld_in, 'some cmd and args');
# or without using the shell
$pid = open2($chld_out, $chld_in, 'some', 'cmd', 'and', 'args');
The open2() function runs the given $cmd and connects
$chld_out for reading and $chld_in for writing. It's what
you think should work when you try
$pid = open(HANDLE, "|cmd args|");
The write filehandle will have autoflush turned on.
If $chld_out is a string (that is, a bareword filehandle
rather than a glob or a reference) and it begins with ">&",
then the child will send output directly to that file han-
dle. If $chld_in is a string that begins with "<&", then
$chld_in will be closed in the parent, and the child will
read from it directly. In both cases, there will be a
dup(2) instead of a pipe(2) made.
If either reader or writer is the null string, 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.
open2() returns the process ID of the child process. It
doesn't return on failure: it just raises an exception
matching "/^open2:/". However, "exec" failures in the child
are not detected. You'll have to trap SIGPIPE yourself.
open2() 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 "wait-
pid $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.
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IPC::Open2(3p) Perl Programmers Reference Guide IPC::Open2(3p)
This whole affair is quite dangerous, as you may block for-
ever. It assumes it's going to talk to something like bc,
both writing to it and reading from it. This is presumably
safe because you "know" that commands like bc will read a
line at a time and output a line at a time. Programs like
sort 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.
The IO::Pty and Expect modules from CPAN can help with this,
as they provide a real tty (well, a pseudo-tty, actually),
which gets you back to line buffering in the invoked command
again.
The order of arguments differs from that of open3().
See IPC::Open3 for an alternative that handles STDERR as
well. This function is really just a wrapper around
open3().
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