[Haskell-cafe] Fwd: Default stdout buffering of child process of createProcess

[Alexander Kjeldaas]

Line buffered would be an inefficient default. Stdout is fully buffered in
c99 if it can be determined that it is not attached to a terminal.

Alexander
On Aug 2, 2014 5:07 AM, "Chris Myzie" <cmyzie28 at gmail.com> wrote:

> I'm reposting this message because I think it only went to the google
> group and not the official haskell-cafe list:
>
> On Friday, August 1, 2014 10:06:32 AM UTC-4, Chris Myzie wrote:
>>
>> As a workaround, I am able to trick the child haskell process into
>> thinking it's running in an interactive terminal by wrapping it with
>> /usr/bin/script:
>>
>> createProcess (proc "/usr/bin/script" ["-qc","./A","/dev/null"]) {
>> std_out = CreatePipe }
>>
>> I still think haskell is using screwy defaults for stdout buffering..
>>
>> On Thursday, July 31, 2014 3:24:47 PM UTC-4, Chris Myzie wrote:
>>>
>>> Hello,
>>>
>>> I'm trying to write a wrapper process that will be able to read any
>>> child process' stdout.  The problem I'm running into is that unless I force
>>> the child's stdout to LineBuffering, it defaults to BlockBuffering.  Is
>>> BlockBuffering really the default in this case?  I don't want to have to
>>> modify all of the child processes that I want to use with this wrapper.
>>>
>>> Below is a simple test case.  A.hs is the child process, and B.hs is the
>>> wrapper.  If I run B.hs, I will get no output unless I uncomment the line
>>> in A.hs.
>>>
>>> Thanks,
>>> Chris
>>>
>>>
>>> ------------------------------ A.hs ---------------------------
>>> import Control.Concurrent
>>> import System.IO
>>>
>>> main :: IO ()
>>> main = do
>>>   -- hSetBuffering stdout LineBuffering
>>>   putStrLn "test" >> threadDelay 1000000 >> main
>>>
>>>
>>> ------------------------------ B.hs ---------------------------
>>> import Control.Monad
>>> import System.IO
>>> import System.Process
>>>
>>> main :: IO ()
>>> main = do
>>>   (_,Just h,_,_) <- createProcess (proc "./A" []) { std_out = CreatePipe
>>> }
>>>   hSetBuffering h LineBuffering
>>>   forever $ hGetLine h >>= putStrLn
>>>
>>
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