[p5sagit/p5-mst-13.2.git] / pod / modpods / Open2.pod

=head1 NAME

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

=head1 SYNOPSIS

    use IPC::Open2;
    $pid = open2('rdr', 'wtr', 'some cmd and args');
      # or
    $pid = open2('rdr', 'wtr', 'some', 'cmd', 'and', 'args');

=head1 DESCRIPTION

The open2() function spawns the given $cmd and connects $rdr for
reading and $wtr for writing.  It's what you think should work 
when you try

    open(HANDLE, "|cmd args");

open2() returns the process ID of the child process.  It doesn't return on
failure: it just raises an exception matching C</^open2:/>.

=head1 WARNING 

It will not create these file handles for you.  You have to do this yourself.
So don't pass it empty variables expecting them to get filled in for you.

Additionally, this is very dangerous as you may block forever.
It assumes it's going to talk to something like B<bc>, both writing to
it and reading from it.  This is presumably safe because you "know"
that commands like B<bc> will read a line at a time and output a line at
a time.  Programs like B<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 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.

=head1 SEE ALSO

See L<open3> for an alternative that handles STDERR as well.
Commit	Line	Data
a0d0e21e	1	=head1 NAME
	2
	3	IPC::Open2, open2 - open a process for both reading and writing
	4
	5	=head1 SYNOPSIS
	6
	7	use IPC::Open2;
	8	$pid = open2('rdr', 'wtr', 'some cmd and args');
	9	# or
	10	$pid = open2('rdr', 'wtr', 'some', 'cmd', 'and', 'args');
	11
	12	=head1 DESCRIPTION
	13
	14	The open2() function spawns the given $cmd and connects $rdr for
	15	reading and $wtr for writing. It's what you think should work
	16	when you try
	17
	18	open(HANDLE, "\|cmd args");
	19
	20	open2() returns the process ID of the child process. It doesn't return on
	21	failure: it just raises an exception matching C</^open2:/>.
	22
	23	=head1 WARNING
	24
	25	It will not create these file handles for you. You have to do this yourself.
	26	So don't pass it empty variables expecting them to get filled in for you.
	27
	28	Additionally, this is very dangerous as you may block forever.
	29	It assumes it's going to talk to something like B<bc>, both writing to
	30	it and reading from it. This is presumably safe because you "know"
	31	that commands like B<bc> will read a line at a time and output a line at
	32	a time. Programs like B<sort> that read their entire input stream first,
748a9306	33	however, are quite apt to cause deadlock.
	34
	35	The big problem with this approach is that if you don't have control
	36	over source code being run in the the child process, you can't control what it does
	37	with pipe buffering. Thus you can't just open a pipe to "cat -v" and continually
	38	read and write a line from it.
	39
	40	=head1 SEE ALSO
	41
	42	See L<open3> for an alternative that handles STDERR as well.
	43