From: Scott Rubin <slr2777@...>
Date: 2004-06-17T14:48:30+09:00
Subject: Re: Bidirectional named pipes on Linux

Phil Tomson wrote:
> I've got two programs which need to communicate (which eliminates 
> un-named pipes).  Information needs to be sent in both directions such 
> that one program (say it's called the master) sends a couple of bytes 
> to and then waits for a response from the other program (say it's called the 
> slave) and so on.  It looks like if you open the pipe (or FIFO file) from 
> both sides in read/write mode that the pipe is opened non-blocking so what 
> ends up happening is that the side which sends first just keeps sending and 
> doesn't wait for a response.  
> 
> ...yeah, I should probably be using sockets, but this is a sort of 
> 'legacy' system.
> 
> phil

Phil,

I'm doing this exact same thing on a linux system so that a ruby process 
and a C process can communicate.  It works perfectly, here is generally 
how I did it.  First, pipes are one way. You can have one end reading 
and one end writing at the same time, but both sides may not write 
simultaneously and both ends may not read simultaneously. Pipes are 
strictly fifos, that's why the command to make named ones is mkfifo. 
One way in, one way out. So you can either make two pipes, one for each 
direction or switch who is reading and who is writing, this is difficult 
if you ask me.

Next, just write to it as if it is a file. The data will stay in the 
pipe until the other end opens it and then it spills out all over if you 
don't catch it. It can only be read out once. So here is how I think you 
should do it..

Make two fifos, one for each direction. The process that sends first 
sends data down the out fifo and calls select on the in fifo.  Select 
will cause it to block until it is ready to read. The other process 
starts out selecting on the incoming fifo and when the data finally 
comes in it can send the response on the outgoing fifo.

Try to think of it like two guys with those vacuum tubes sending 
messages in canisters. There is just one canister and the tubes are one 
way. The guy puts the canister in the pipe that goes out and then sits 
looking at the other pipe waiting for a response.

I hope this helps. I know its not bi-directional, but it really can't be.

-Scott