I like the idea of Thread.fork coupled with OS.Process.system except I do want
to be able to capture the called processes’ output streams dynamically, and to
be able to kill the called processes dynamically. Neither of these are
possible with system.
I should have perhaps said that the problems actually seem to arise with the
end of the child process: waitpid and similar in the parent seem to give rise
to odd errors (processes reporting as failed when there’s nothing to indicate
that that’s really the case).
Having said that, it would be nice if there were some way to test a more atomic
version of the fork/dup2/exec dance. (Could it be done with an FFI call to C
code that made all the calls in one function?)
Michael
On 5/6/17, 22:01, "polyml-boun...@inf.ed.ac.uk on behalf of David Matthews"
<polyml-boun...@inf.ed.ac.uk on behalf of david.matth...@prolingua.co.uk> wrote:
On 05/06/2017 11:27, Makarius wrote:
> On 05/06/17 10:35, michael.norr...@data61.csiro.au wrote:
>> Both this (with Posix.Process open):
>>
>> case fork() of
>> NONE => (exece(proc,proc::args,env')
>>
>> and this:
>>
>> case fork() of
>> exece(executable,nm_args,env)
>>
>> seem to be causing us grief, particularly on a recent Linux, PolyML 5.7
and under conditions of some memory load.
>>
>> Is there a better idiom we might use?
>
> For Isabelle, I have made many variations and refinements in the past 10
> years -- without fork/exec due to portability problems. The present
> state in this quest for perfection is here (both for ML and Scala):
>
>
http://isabelle.in.tum.de/repos/isabelle/file/Isabelle2016-1/src/Pure/System/bash.ML
>
http://isabelle.in.tum.de/repos/isabelle/file/Isabelle2016-1/src/Pure/System/bash.scala
>
http://isabelle.in.tum.de/repos/isabelle/file/Isabelle2016-1/Admin/bash_process/bash_process.c
>
>
> The main idea is to use plain OS.Process.system (Posix) or
> Windows.simpleExecute with a Thread.fork around it. Thus a synchronous
> process execution is management asynchronously by a separate thread.
I would certainly recommend the use of OS.Process.system rather than
fork and exec. I wonder if there is a problem if there is a need for a
GC or some other action before the child process can actually perform
the exece ? Looking at the code, Poly/ML sets a "singleThreaded" flag
in the child process to indicate that it should not expect any other
threads to be running but there is a lot of state in the run-time system
associated with threads. It may be that this state would have to be
explicitly reset.
OS.Process.system does use fork/exec underneath on Unix but the exec
occurs immediately after the fork within the same C function. There is
also special-case code for OS.Process.system on Cygwin since fork/exec
is very inefficient on Windows.
David
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