IPC::Open3 (3)

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NAME

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

SYNOPSIS

    $pid = open3(\*CHLD_IN, \*CHLD_OUT, \*CHLD_ERR,
                    'some cmd and args', 'optarg', ...);

    my($wtr, $rdr, $err);
    use Symbol 'gensym'; $err = gensym;
    $pid = open3($wtr, $rdr, $err,
                    'some cmd and args', 'optarg', ...);

    waitpid( $pid, 0 );
    my $child_exit_status = $? >> 8;

DESCRIPTION

Extremely similar to open2(), open3() spawns the given $cmd and connects for reading from the child, for writing to the child, and for errors. If is false, or the same file descriptor as then and of the child are on the same filehandle (this means that an autovivified lexical cannot be used for the filehandle, see ). The will have autoflush turned on.

If

begins with "<&", then will be closed in the parent, and the child will read from it directly. If or 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 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.

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

open3() returns the process

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 under Windows and as it is not possible to trap them.

If the child process dies for any reason, the next write to

is likely to generate a 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(FOO, "-|")" 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 the 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, 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.

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().