I have a GenServer that reads/writes to a Linux Pipe or Fifo. I need to do: {:ok, fifo} = :file.open('/path/to/fifo', [:read, :write, :binary]) {:ok, <<protocol_pattern_match>>} = :file.read(fifo, protocol_size) Except :file.read/2 blocks the entire calling process until it is complete similar to r…

So this worked: ~/OSS/elixir[jv-basic-releases %]$ mkfifo test.pipe ~/OSS/elixir[jv-basic-releases %]$ erl Erlang/OTP 21 [erts-10.0] [source] [64-bit] [smp:8:8] [ds:8:8:10] [async-threads:1] [hipe] Eshell V10.0 (abort with ^G) 1> Fifo = open_port("test.pipe", [eof]), 1> receive 1> Msg -> io:forma…

Okay so while investigating this, i discovered something. This code will hang forever no matter what. It doesn’t matter how much data is written to the pipe, the data will never be read by Erlang/Elixir. {:ok, file} = :file.open(path_to_fifo, [:read, :write, :binary]) {:ok, "hello world"} = :file…

[image] ConnorRigby: task = Task.async(:file, :read, [protocol_size]) Doesn’t that need to be used with Task.await to receive the result? I also think tasks use receive blocks that wouldn’t work in a genserver process.

if i manually change my code to do a :file.read(fd, 0) the behaviour works. I recieve the result in handle_info, so i at least am fairly certain Task is not the problem here.

The solution is to use ports, which will also give asynchrony. See previous discussion here: Elixir vs Unix named pipe I think it was recently announced that erlang supports pipes, but I cant recall if in port or file. Or I may be completely misremebering it. Anyway, a port should do nowadays.

Sorry, i should have stated this in the original post, but i have tried this as well and it did not work either. Interactive Elixir (1.7.3) - press Ctrl+C to exit (type h() ENTER for help) iex(1)> System.cmd("mkfifo", ["/tmp/fifo", "-m", "0666"]) {"", 0} iex(2)> port = :erlang.open_port({…

ah ha! i was trying to use spawn which was not needed. Will try this method out and report back.

I can confirm that this does in fact allow reading and writing from a named pipe. However it does add the requirement of buffering now, since you are not telling the port how much data you want to read. Either way thanks for the help!

Good point. Can that be done at the pipe level? I.e. have some UNIX utility that buffers it for you or similar?

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Help with `:file` blocking GenServer process

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ConnorRigby January 25, 2019, 12:41am 18

Right. I chose the port method and it seems to be working well!