Hi, Interesting: my program is compiled with -threaded, and when I run it with +RTS -N I get: <program name>: forking not supported with +RTS -N<n> greater than 1 This is probably about forkProcess rather than forkIO/forkOS, but why this limitation? rawSystem works fine from within a thread started by forkIO, but why cannot I use forkProcess? On Tue, Apr 27, 2010 at 12:35 PM, Bryan O'Sullivan wrote:

With 6.12.1 and newer, no "-N" argument implies 1 core, "-Nk" implies k cores, and "-N" without an argument means "use all".

-- Dimitry Golubovsky Anywhere on the Web

On Wed, Apr 28, 2010 at 8:02 AM, Bryan O'Sullivan wrote:

On Tue, Apr 27, 2010 at 7:02 PM, Dimitry Golubovsky wrote:

...

This is probably about forkProcess rather than forkIO/forkOS, but why this limitation?

That's a standard Unix limitation. A child process is forked with one thread, so if the parent has multiple threads running, it's overwhelmingly likely that the child will be unable to execute correctly.

One problem used to be that the I/O manager would get killed but Simon Marlow fixed that. I think we should support forking, with some caveats, so that we can daemonize processes.

Simon, Can the documentation for forkProcess be fixed then? Instead of "forkProcess comes with a giant warning..." it could say: "forkProcess is not supported by GHC in multicore setting". PS I have created ticket #4027 with more detailed proposal. On Wed, Apr 28, 2010 at 4:26 AM, Simon Marlow wrote:

On 28/04/2010 03:02, Dimitry Golubovsky wrote:

...

this limitation? rawSystem works fine from within a thread started by forkIO, but why cannot I use forkProcess?

No good reason, other than it's tremendously difficult to implement.

Thank you. -- Dimitry Golubovsky Anywhere on the Web