Pipe problems

tran_sonic has asked for the wisdom of the Perl Monks concerning the following question:

I've got two processes A and B. A spawns B by using the
open2 method from the IPC::Open2 module which gives A the
file handles READ and WRITE. I'm using the pipe to send
text strings between A and B.

The problem is that when A tries to send text to B, B
hangs.

Here are examples:

### A ###
...
open2( \*READ, \*WRITE, 'B' );
print WRITE "Hello there.\n";
...

### B ###
$input = <STDIN>;
print "echo back: $input";
...
[download]

I know that <> waits for a '\n', by default, before it
returns and I'd like to keep it that way.

I'd appreciate any help I receive. Thanks.

Comment on Pipe problems Download Code

Replies are listed 'Best First'.
Re: Pipe problems by bluto (Curate) on Aug 31, 2004 at 15:30 UTC
Make sure B has autoflush turned on before the print for STDOUT... `$\| = 1; print ...` [download] Also, terminate the print with a '\n'. Though you haven't shown it, I'm assuming A reads back the line printed from B. (If not, B can hang on the print waiting for A to read it if B prints a large enough string, filling the pipe.) Update: From your code, it looks like A sends the '\n' to B so the comment about terminting the print may not apply.	[reply] [d/l]
Re^2: Pipe problems by sgifford (Prior) on Aug 31, 2004 at 19:36 UTC
Autoflush on the `WRITE` filehandle in A seems more likely to be the problem. In newer versions of Perl you can use: `use IO::Handle; # ... WRITE->autoflush(1);` [download] In older versions you should use: `$oldfh = select(WRITE); $\| = 1; select($oldfh);` [download] Update: bluto's right, IPC::Open2 does this for you. I usually just use `pipe`, which doesn't.	[reply] [d/l] [select]
Re^3: Pipe problems by bluto (Curate) on Aug 31, 2004 at 21:13 UTC
This won't help this problem. According to even older perl docs for IPC::Open2 autoflush is turned on for the parent's write handle.	[reply]
Re: Pipe problems by Random_Walk (Prior) on Aug 31, 2004 at 15:09 UTC
I am trying this and it works fine for me, perl version 5.005_03. Is B up to anything else that makes it hang ? Can you run the test code below (with the right path to B) on your system ? `#!/usr/local/bin/perl -w #### A #### use IPC::Open2; open2( \READ, \WRITE, '/home/random/B' ); print WRITE "Hello there.\n"; my $in=<READ>; print "$0 read: $in\n" #!/usr/local/bin/perl -w #### B #### $input = <STDIN>; print "echo back: $input";` [download] Regards, R. update I think the following reply has the answer, set autoflush with $\|=1 in B or A may hang while the data from B is hanging around in buffer.	[reply] [d/l]

update