From: ptkwt@... (Phil Tomson)
Date: 2004-06-18T02:23:27+09:00
Subject: Re: Bidirectional named pipes on Linux

In article <40d0f617$1@buckaroo.cs.rit.edu>,
Scott Rubin  <slr2777@cs.rit.edu> wrote:
>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.
>

You wouldn't happen to have the code which shows the select call, would 
you?

Phil