CODEWITHSUNDEEP
multithreadinghaskellconcurrencyfunctional-programmingpipe
Enlico
Enlico

Reputation: 28384

What is the lifetime of the process spawned via System.Process.createProcess?

I am experimenting with System.Process and Control.Concurrent.Async for the purpose of spawning an external process, as well as threads to control its standard input and output.

However, while playing around with those libraries, I've managed to create a situation where 2 instances of my Haskell program communicate with each other (via the external socat process), the threads of each program communicate via an MVar to decide what to do, the two programs exit with 0, and yet... one of the two external socat processes spawned via System.Process.createProcess is left running, so I have to manually kill it.

Sure, maybe the issue is with the program I wrote beside the usage of createProcess, but to start with, I want to make sure I understand how createProgram syhould be used.

So the question is: once I execute something like this

import System.Process
main :: IO ()
main = do
  args <- getArgs
  (Just i, Just o, Nothing, h) <- createProcess (proc "socat" args)
                                                {std_in = CreatePipe, std_out = CreatePipe}
  -- rest

who or what is responsible for putting down the process spawned by createProcess?

After all, -- rest executes immediately after the call to createProcess, so while -- rest executes, the shell program, in this case socat is running on its own. Is it up to me to guarantee correct lifetime management? Should I make use of h for this purpose?

One experiment is this:

import System.Process
main :: IO ()
main = do
  _ <- createProcess (proc "cat" ["/dev/zero"])
              {std_in = CreatePipe, std_out = CreatePipe}
  return ()

Since cat /dev/zero never returns (I've tried in the Bash shell), shouldn't this program terminate leaving cat running? I.e., after the Haskell program terminates successfully, shouldn't pidof cat return some PID?

I've tried, and it doesn't, making me think that something is cleaning up.

Upvotes: 0

Views: 27

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