file descriptors

[Brandon S. Allbery KF8NH]

On 2008 Oct 30, at 10:08, Johannes Waldmann wrote:
> are there any known issues
> with file handles/descriptors in ghc-compiled executables?
>
> My program has a lot of calls to System.Process.runInteractiveProcess
> and I'm running into unpredictable behaviour (sometimes the program
> just silently dies, sometimes it gets stuck)

runInteractiveProcess returns 3 filehandles and a process handle.   
Either can cause problems:  if you use too many filehandles, the  
select()-based mechanism in the Haskell runtime will fail (select()  
has a fairly low limit on filehandles as compared to poll(), epoll(),  
and other mechanisms); if you spawn too many subprocesses without  
reaping them (waitForProcess / getProcessExitCode) you will hit the  
child process limit and not be able to create new child processes.

A third problem is that you can deadlock if you write too much data  
without reading any back.  Your best bet is to forkIO so input and  
output are independent.

> Or perhaps you see anything that's wrong with my code here
> http://dfa.imn.htwk-leipzig.de/cgi-bin/cvsweb/box/src/SMT/Time.hs?rev=1.9

And there's a fourth problem:  you hGetContents after blocking  
waitForProcess.  If the output is larger than the size of a pipe  
buffer (often 512 bytes), the subprocess will block waiting for the  
pipe to be read, while you're waiting for it to exit before reading  
from the pipe.

-- 
brandon s. allbery [solaris,freebsd,perl,pugs,haskell] allbery at kf8nh.com
system administrator [openafs,heimdal,too many hats] allbery at ece.cmu.edu
electrical and computer engineering, carnegie mellon university    KF8NH