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 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, 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 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 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, 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(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 catpure.
- 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().