Sep 16, 2014

Nontrivial pipes

I’m going to take a simple concept, UNIX pipes, and basically frame (or misframe?) a nontrivial scenario.

$ cat /proc/cpuinfo | grep CPU | wc -l

We all know pipes, of course. You pipe stuff in and pipe stuff out. Easy enough. I think the piping in the example above is context-free. Each pipe has a stream of continuous bytes for its input and output. This is the simplest case.

Now let’s say we’re writing a program to simulate a user running a CLI application, like R. In our simulator, we’ll execute R as a child process. Then, since R is a CLI program, we’ll have to work with its stdin and stdout file descriptors (which are pipes).

It’s easy enough to simulate a user typing in commands – simply write data to R’s stdin. To get the output, read from its stdout. We know this, and the entire scenario is simple enough. Let’s also call it “context-free.”

But what if you wanted to capture the output for each command separately? Now we’re moving away from the context-free, infinite streams of bytes. It’s more like chunked data. If you send R


you’ll get back

 [1]  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20

Obviously the response, the printed vector, is part of the same “chunk” but the “context-free pipe” has no awareness of that.

This is where things get nontrivial. Our simulator needs to send c(1:20), or some other form of input, and read the response from R. How long is the response? How many bytes do I need to read from the pipe? How will I know when it has finished outputting data?


Please, don’t actually build a simulator like this for R. There are better ways to do this, like telling R to output to a file.

However, if you “zoom out” and think about UNIX pipes in general, these concepts are important. This is why most protocols over TCP include a payload length. You can think of TCP sockets as pipes, and think about how these questions are formed in that scenario.

Anyway, I hope that made sense. Monday night thoughts :P.

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