IPC::Open2(3) Perl Programmers Reference Guide IPC::Open2(3)NAME
IPC::Open2, open2 - open a process for both reading and
writing
SYNOPSIS
use IPC::Open2;
$pid = open2(\*RDRFH, \*WTRFH, 'some cmd and args');
# or without using the shell
$pid = open2(\*RDRFH, \*WTRFH, 'some', 'cmd', 'and', 'args');
# or with handle autovivification
my($rdrfh, $wtrfh);
$pid = open2($rdrfh, $wtrfh, 'some cmd and args');
# or without using the shell
$pid = open2($rdrfh, $wtrfh, 'some', 'cmd', 'and', 'args');
DESCRIPTION
The open2() function runs the given $cmd and connects
$rdrfh for reading and $wtrfh 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 $rdrfh 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 handle. If $wtrfh is a string that begins with "<&",
then $wtrfh 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 your
self.
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
"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 the waitpid entry in
the perlfunc manpage for more information.
This whole affair is quite dangerous, as you may block
forever. It assumes it's going to talk to something like
bc, both writing to it and reading from it. This is pre
sumably safe because you "know" that commands like bc will
read a line at a time and output a line at a time. Pro
grams 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 pro
cess, you can't control what it does with pipe buffering.
Thus you can't just open a pipe to "cat -v" and continu
ally 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.
WARNING
The order of arguments differs from that of open3().
SEE ALSO
See the IPC::Open3 manpage for an alternative that handles
STDERR as well. This function is really just a wrapper
around open3().
2001-02-22 perl v5.6.1 IPC::Open2(3)
