Node: Can't read from process.stdin.fd in Node.js 6.10

[jamesseanwright]

I'm currently writing a simple CLI with Node.js 6.1.0. Here's an example of how I'm trying to read from stdin:

'use strict';

const fs = require('fs');

const BUFFER_LENGTH = 8;
const buffer = Buffer.alloc(BUFFER_LENGTH);
fs.readSync(process.stdin.fd, buffer, 0, BUFFER_LENGTH);
console.log(buffer.toString());

When running this on OS X Yosemite (10.10.5), node exits with Error: EAGAIN: resource temporarily unavailable, read. Interestingly, it works if I replace process.stdin.fd with fs.openSync('/dev/stdin', 'rs').

Another point of interest is that I can use process.stdin.fd with Node.js 4.4.0 as long as I replace Buffer.alloc with a call to the deprecated Buffer constructor via new.

Finally, process.stdin.fd and fs.openSync('/dev/stdin', 'rs') don't return the same FD Number; on my system, the return values are 0 and 14 respectively.

Am | missing something obvious or is there a potentially bug with process.stdin.fd on OS X/Node.js 6?

Thanks for your time,
James

[bnoordhuis]

There is a long and technical explanation but the summary is that, on OS X, process.stdin.fd is not the real tty file descriptor because of a kernel bug workaround.

Aside: the .fd property is not intended to be used like that. It is undocumented and mainly exists for debugging purposes.

I'll close the issue as working-as-intended but let me know if you have more questions.

[jamesseanwright]

Aah, I didn't realise it was an undocumented API! Thanks for clearing that up!

[CMCDragonkai]

I get the same error on Linux, not just on Mac, is there no way to directly read from STDIN fd, without going through streams?

[bnoordhuis]

@CMCDragonkai You can fs.open('/dev/tty') and read from that file descriptor. You'll have to make sure nothing in your program uses process.stdin though or you'll get mixed results.

[CMCDragonkai]

@bnoordhuis What kind of mixed results are you implying?

I figured to use process.stdin.fd on Windows, but on Linux/Mac to open /dev/fd/0 instead.

[bnoordhuis]

@CMCDragonkai With one resource and two readers it's indeterminate who reads what: reader A may read the first line, B the second line, A the third line, etc.

[CMCDragonkai]

In my case, that's the expected behaviour. Thanks!

[rolfen]

Aside: the .fd property is not intended to be used like that. It is undocumented and mainly exists for debugging purposes.

It is currently documented:

https://nodejs.org/api/process.html#process_process_stdin_fd

[jamesseanwright]

Aside: the .fd property is not intended to be used like that. It is undocumented and mainly exists for debugging purposes.

It is actually documented:

https://nodejs.org/api/process.html#process_process_stdin_fd

I don't think it was documented when I raised the issue, but it's worth noting that a lot has changed since version 6.