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Defyleiti
Defyleiti

Reputation: 565

Tailing a named pipe in node.js

I'm using node-tail to read a file in linux and send it down to a socket.

node.js sending data read from a text file

var io = require('socket.io');
Tail = require('tail').Tail;


tail = new Tail("/tmp/test.txt");


io.sockets.on('connection', function (socket) {

tail.on("line", function(data) {
  socket.emit('Message', { test: data });

});

});

Receiving side

var socket = io.connect();
  socket.on('Message', function (data) {

    console.log(data.test);

  });

This works but when I try to modify this part 

tail = new Tail("/tmp/test.txt");

to this

tail = new Tail("/tmp/FIFOFILE");

I can't get any data from it.

Is there anyway to read a named pipe in linux? or a package that can read a named pipe?

Upvotes: 2

Views: 2904

Answers (2)

BenJ
BenJ

Reputation: 21

The OS will send an EOF when the last process finishes writing to the FIFO. If only one process is writing to the FIFO then you get an EOF when that process finishes writing its stuff. This EOF triggers Node to close the stream.

The trick to avoiding this is given by @JoshuaWalsh in this answer, namely: you open the pipe yourself FOR READING AND WRITING - even though you have no intention of ever writing to it. This means that the OS sees that there is always at least one process writing to the file and so you never get the EOF.

So... just add in something like:

let fifoHandle = fs.open(fifoPath, fs.constants.O_RDWR,function(){console.log('FIFO open')});

You don't ever have to do anything with fifoHandle - just make sure it sticks around and doesn't get garbage collected.

In fact... in my case I was using createReadStream, and I found that simply adding the fs.constants.O_RDWR to this was enough (even though I have no intention of ever writing to the fifo.

let fifo = fs.createReadStream(fifoPath,{flags: fs.constants.O_RDWR});

fifo.on('data',function(data){
    console.log('Got data:'+data.toString());
}

Upvotes: 1

robertklep
robertklep

Reputation: 203534

I can get it to work in a silly way:

// app.js
process.stdin.resume();
process.stdin.on('data', function(chunk) {
 console.log('D', chunk);
});

And start like this:

node app.js < /tmp/FIFOFILE

If I create a readable stream for the named pipe, it ends after having read the first piece of data written to the named pipe. Not sure why stdin is special.

Upvotes: 4

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