How to multiplex named pipe / fifo

[ORESoftware]

https://stackoverflow.com/questions/49937798/how-to-multiplex-named-pipe-fifo

this doesn't exactly have to do with Node.js, but hoping I can get some answers here..

Say we have a named pipe:

mkfifo my_named_pipe

say there are multiple writers writing to this named pipe:

node x.js > ${my_named_pipe} &
node y.js > ${my_named_pipe} &
node z.js > ${my_named_pipe} &

something like that - is there a reliable way to multiplex it, so that one whole message gets through each time, or can a named pipe reliable read from only one writer?

It leads me to wonder how we multiplex ports/sockets etc, I don't know how it's done.

[bnoordhuis]

Writes <= PIPE_BUF bytes (at least 512 but bigger on some platforms) are atomic and won't be intermixed.

[ORESoftware]

interesting, I was guessing that writes smaller than a certain size would be atomic, but you are saying that writes bigger than a certain size will be atomic.

[ORESoftware]

I am wondering - is that the same way that sockets are multiplexed? By having small enough writes?

I realized I misread what you wrote - looking at this answer:

https://unix.stackexchange.com/questions/68146/what-are-guarantees-for-concurrent-writes-into-a-named-pipe/68157?noredirect=1#comment793578_68157

It looks like writes less than PIPE_BUF in size won't be interleaved.

However it seems to me that not-interleaving is not sufficient enough to multiplex. I assume the way to multiplex would be have to some information in each write that tells it "which bin" to go into.

[bnoordhuis]

Yes, that's right. And yes, the "at least 512" means you can assume writes <= 512 will be atomic everywhere. Larger writes may be atomic too but that depends on the platform.

[ORESoftware]

@bnoordhuis how do I discover what the value is for PIPE_BUF on Ubuntu or MacOS?

if I tried echo $PIPE_BUF, that yields nothing, must be some C thing.

is there a way to set the value to something bigger for a specific named pipe?
https://unix.stackexchange.com/questions/439196/change-buffer-size-of-named-pipe

[bnoordhuis]

how do I discover what the value is for PIPE_BUF on Ubuntu or MacOS?

grep -wr PIPE_BUF /usr/include should find it.

is there a way to set the value to something bigger for a specific named pipe?

I don't think so. Nothing that's portable anyway.

[gireeshpunathil]

closing as answered.