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So I'm trying to connect to a (Lua) debugger embedded in a program by redirecting the I/O. Currently I create a pair of FIFOs for the read and write streams and connect to them using cat /tmp/dbg_write & cat > /tmp/dbg_read. This is workable and pretty straightforward, but if you don't exit everything just right you have to go back and kill the background cat command. I figured it was time for a learning experience since there seems like there must be a better way. I'm totally stumped though.

Internally I'm just overriding the debugger's I/O functions to use FIFOs, since stdio is a no-go in this case. As far as I know, there is no way to do bidirectional I/O with the standard Lua APIs like sockets, and pulling in native libs is a no-go since it's embedded.

local dbg = require("debugger")

local READ_PIPE, WRITE_PIPE = "/tmp/dbg_read", "/tmp/dbg_write"
os.execute(string.format("mkfifo %s %s", READ_PIPE, WRITE_PIPE))
local input, output = io.open(READ_PIPE, "r"), io.open(WRITE_PIPE, "w")

function dbg.read(prompt)
    dbg.write(prompt)
    return input:read()
end

function dbg.write(str)
    output:write(str)
    output:flush()
end

I also tried:

  • using popen("netcat -lU") to create a Unix domain socket, but pipes are unidirectional. Derp.
  • Using an existing pty. Seems promising except I don't know how to create one separate from a shell that wants to read from it.
  • Using screen to... somehow create a tty session I can interact with. (no idea...)
  • Finding an existing utility that can copy from a file/pipe to stdout while also copying from stdin to another.

Anything obvious that I'm missing?

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  • You seem to have missed out at least 1/2 of what you did. – ctrl-alt-delor Apr 1 '20 at 21:08
  • Could you tell us what you do to have the debugger read and write to those two files? – Eduardo Trápani Apr 1 '20 at 21:29
  • Done. I guess I figured it was sort of irrelevant, but I guess more information is always better. – slembcke Apr 1 '20 at 21:49
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What you can do is create two virtual terminals, back to back, with socat:

$ socat pty,link=/tmp/lua_end pty,link=/tmp/screen_end

Now, from lua, you just read to and write from a single file: /tmp/lua_end

In another shell you do:

$ screen /tmp/screen_end

And that's it, they're going to be connected, through socat.

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