IPC::Open3(3p) | Perl Programmers Reference Guide | IPC::Open3(3p)

# NAME

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

# 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;

# DESCRIPTION

Extremely similar to open2(), open3() spawns the given command and connects $chld_out for reading from the child, $chld_in for writing to the child, and $chld_err for errors. If $chld_err is false, or the same file descriptor as $chld_out, then STDOUT and STDERR of the child are on the same filehandle. This means that an autovivified lexical cannot be used for the STDERR filehandle, but gensym from Symbol can be used to vivify a new glob reference, see "SYNOPSIS". The $chld_in will have autoflush turned on.

If $chld_in begins with "<&", then $chld_in will be closed in the parent, and the child will read from it directly. If $chld_out or $chld_err begins with ">&", then the child will send output directly to that filehandle. In both cases, there will be a dup(2) instead of a pipe(2) made.

If either reader or writer is the empty string or undefined, 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.

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

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.

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.

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.

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. It 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.

# See Also

IPC::Open2
  Like Open3 but without STDERR capture.
IPC::Run
  This is a CPAN module that has better error handling and more facilities than Open3.

# WARNING

The order of arguments differs from that of open2().

2023-02-15 | perl v5.40.1