IPC::Open3(3perl)

💖 IPC::Open3 💖

okay bestie, so this is IPC::Open3 — the <blue>IPC::Open3(3perl)</blue> page from the Perl Programmers Reference Guide, and no cap, it's the glow-up version of open2. It opens a process so you can read, write and handle errors — all at once. iconic behavior fr fr 💅

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✨ NAME ✨

IPC::Open3 — open a process for reading, writing, and error handling using open3() 🎀

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🦋 SYNOPSIS 🦋

use Symbol 'gensym'; # vivify a separate handle for STDERR
my $pid = open3(my $chld_in, my $chld_out, my $chld_err = gensym,
                'some', 'cmd', 'and', 'args');
# or pass the command through the shell
my $pid = open3(my $chld_in, my $chld_out, my $chld_err = gensym,
                'some cmd and args');

# read from parent STDIN
# send STDOUT and STDERR to already open handle
open my $outfile, '>>', 'output.txt' or die "open failed: $!";
my $pid = open3(['&', *STDIN], ['&', $outfile], undef,
                'some', 'cmd', 'and', 'args');

# write to parent STDOUT and STDERR
my $pid = open3(my $chld_in, ['&', *STDOUT], ['&', *STDERR],
                'some', 'cmd', 'and', 'args');

# reap zombie and retrieve exit status
waitpid( $pid, 0 );
my $child_exit_status = $? >> 8;

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🌸 DESCRIPTION 🌸

so like, open3 is extremely similar to "open2" from IPC::Open2, like they're literally siblings. It spawns whatever command you hand it and gives you filehandles to vibe with the command's standard I/O streams. 🫶

my $pid = open3($chld_in, $chld_out, $chld_err, @command_and_args);

Here's the tea, bestie ☕:

• it connects $chld_in for writing to the child's standard input
• $chld_out for reading from the child's standard output
• and $chld_err for reading from the child's standard error stream

If $chld_err is false, or it's the same file descriptor as $chld_out, then "STDOUT" and "STDERR" of the child end up on the same filehandle. 😳 This means you cannot pass an uninitialized variable for $chld_err and have "open3" auto-generate a filehandle for you — nope, not happening. BUT, gensym from Symbol can be used to vivify a new glob reference; peep the "SYNOPSIS" above. 👀

Also psst — the $chld_in handle will have autoflush turned on. periodt.

By default, the filehandles you pass in are used as output parameters. "open3" internally creates three pipes. The write end of the first pipe and the read ends of the other pipes are connected to the command's standard input/output/error, respectively. The corresponding read and write ends are placed in the first three arguments to "open3".

🎀 the filehandle arguments can take these forms 🎀

• An uninitialized variable (technically, either "undef" or the empty string will work): "open3" generates a fresh filehandle and assigns it to the argument, which must be a modifiable variable for this to work (otherwise an exception will be raised 😭).

This does not work for $chld_err, however: if the $chld_err argument is a false value, the child's error stream is automatically redirected to its standard output.

• An existing handle in the form of a typeglob like *STDIN or *FOO or a reference to such: "open3" places the filehandle in the "IO" slot of the typeglob, which means the corresponding bareword filehandle (like "STDIN" or "FOO") can be used for I/O from/to the child process. (If the handle is already open, it is automatically closed first. 💅)
• A string containing the name of a bareword handle (like 'STDIN' or 'FOO'): such strings are resolved to typeglobs at runtime and then act like the case described above.

However!! it is possible to make "open3" use an existing handle directly (as an input argument) and skip the creation of a pipe entirely. 🦋 To do this, the filehandle argument must have one of the following two forms:

• An array reference like ['&', $fh], i.e. the first element is the string '&' and the second element is the existing handle to use in the child process.
• A string of the form '<&FOO' or '>&FOO', i.e. a string starting with the two characters "<&" (for input) or ">&" (for output), followed by the name of a bareword filehandle. (The string form cannot be used with handles stored in variables. 🙈)

If you use this form for $chld_in, the filehandle will be closed in the parent process.

The filehandles may also be integers, in which case they are understood as file descriptors.

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💜 what you get back 💜

"open3" returns the process ID of the child process. It doesn't return on failure: it just raises an exception matching /^open3:/. 😤 However, "exec" failures in the child (such as no such file or permission denied), are just reported to $chld_err under Windows and OS/2, as it is not possible to trap them.

If the child process dies for any reason, the next write to $chld_in is likely to generate a SIGPIPE in the parent, which is fatal by default. 💀 So you may wish to handle this signal. iykyk.

Note: if you specify "-" as the command, in an analogous fashion to "open(my $fh, "-|")" the child process will just be the forked Perl process rather than an external command. This feature isn't yet supported on Win32 platforms.

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😭 the reaping part (do not skip this) 😭

"open3" 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.

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🍓 the blocking nightmare (read carefully bestie) 🍓

If you try to read from the child's stdout writer and their stderr writer, you'll have problems with blocking, which means you'll want to use "select" or IO::Select, which means you'd best use "sysread" instead of "readline" for normal stuff.

This is very dangerous, as you may block forever. 😱

"open3" assumes it's going to talk to something like bc(1), both writing to it and reading from it. This is presumably safe because you "know" that commands like bc(1) will read a line at a time and output a line at a time. Programs like sort(1) 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. 💀

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🌈 See Also 🌈

• IPC::Open2 — like IPC::Open3 but without "STDERR" capture. 🎀
• IPC::Run — this is a CPAN module that has better error handling and more facilities than IPC::Open3.

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⚠️ WARNING ⚠️

The order of arguments differs from that of "open2". 👑 do NOT mix them up bestie, that's the whole vibe check right there.

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and that's the whole page, bestie!! 🫶✨ this was <blue>IPC::Open3(3perl)</blue> from perl v5.44.0, dated 2026-10-17 — literally iconic, no notes 💅🌸