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IPC::Open2(3pm) Perl Programmers Reference Guide IPC::Open2(3pm)

NAME

IPC::Open2, open2 - open a process for both reading and writing

SYNOPSIS

use IPC::Open2;

$pid = open2(\*CHLDOUT, \*CHLDIN, 'some cmd and args');

# or without using the shell

$pid = open2(\*CHLDOUT, \*CHLDIN, 'some', 'cmd', 'and', 'args');

# or with handle autovivification

my($chldout, $chldin);

$pid = open2($chldout, $chldin, 'some cmd and args');

# or without using the shell

$pid = open2($chldout, $chldin, 'some', 'cmd', 'and', 'args');

DESCRIPTION

The open2() function runs the given $cmd and connects $chldout for

reading and $chldin 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 $chldout 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 $chldin is a string that

begins with "<&", then $chldin 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 excep-

tion 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 nor-

mally 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 bbcc, both writing to it and

reading from it. This is presumably safe because you "know" that com-

mands like bbcc will read a line at a time and output a line at a time. Programs like ssoorrtt 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. WWAARRNNIINNGG The order of arguments differs from that of open3().