NAME¶
IPC::Open2 - open a process for both reading and writing using open2()
SYNOPSIS¶
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');
waitpid( $pid, 0 );
my $child_exit_status = $? >> 8;
DESCRIPTION¶
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 handle. 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 "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.
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 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.
WARNING¶
The order of arguments differs from that of
open3().
SEE ALSO¶
See IPC::Open3 for an alternative that handles STDERR as well. This function is
really just a wrapper around
open3().