[Pharo-project] [Vm-dev] Re: Can OSProcess functionality be implemented using FFI instead of plugin?

Eliot Miranda eliot.miranda at gmail.com
Sun Jan 17 01:59:12 UTC 2016

Hi Mariano,

On Sat, Jan 16, 2016 at 6:00 AM, Mariano Martinez Peck <
marianopeck at gmail.com> wrote:

> Hi all,
> Sorry for reviving an old thread but I thought it was better to continue
> the discussion here because of the context.
> As you may have read, the other day I released a first approeach to a
> subset of OSProcess based on FFI (posix_spwan() family of functions):
> https://github.com/marianopeck/OSSubprocess
>  And with that in mind, I wanted to share a few things with you. The main
> 2 problems I found with implementing this with FFI was:
> 1) We have all already agree and discussed that fork+exec cannot be done
> in separate FFI calls. So at the very min you need either a plugin method
> that does the fork()+exec() OR wrapping a lib like posix_spwan()
> 2) The other main problem, is, as you all said (and mostly  Nicolas), is
> the problems with the preprocessor (constants, macros, etc).
> With all that said, I was able to get my stuff working. However, I am
> still using some primitives of OSProcess plugin because of 2).
> I read Eliot idea and what I don't like is the need of a C compiler in the
> user machine. I think that's a high constrain. Then Igor suggested that WE
> (developers and maintainers of a certain tool) are the ones that compiles
> the little C program to extract constant values etc and then WE provide as
> part of our source code, some packages with some SharedPool depending on
> the platform/OS. And Igor approach looked a bit better to me.

You misunderstand the proposal.  The C compiler is needed /only when
changing the set of constants/, i.e. when /developing/ the interface.  The
C compiler is /not/ needed when deploying.

The idea is to
a) at development time, e.g. when a new variable is added to a SharedPool
containing platform constants, a C program is autogenerated that outputs in
some format a description of the names and values of all the constants
defined in the pool.  One convenient notation is e.g. STON.  For the
purposes of this discussion let's assume we're using ston, but any format
the image an parse (or indeed a shared object the image can load on teh
current pkatform) will do.  The output of the autogenerated C program would
be called something like <SharedPoolName>.<PlatformName>.ston, e.g.
UnixConstants.MacOSX64.ston or UnixConstants.Linux32.ston.  The ston files
can easily be parsed by facilities in the Smalltalk image.

b) when deploying the system to a set of platforms one includes all the
relevant platform-specific ston files.

c) at startup the image checks its current platform.  If the platform is
the same that it was saved on, no action is taken.  But if the platform as
changed then the relevant ston file is selected, parsed, and the values for
the variables in the shared pool updated to reflect the values of the
current platform.

So the C compiler is only needed when developing the interface, not when
deploying it.

Then Nicolas made a point that if we plan to manage all that complexity at
> the image level it may become a hell too.
> So.... what if we take a simpler (probably not better) approach and we
> consider the "c program that exports constants and sizes" a VM Plugin?
> Let's say we have a UnixPreprocessorPlugin (that would work for OSX, Linux
> and other's Unix I imagine for the time being) which provides a function
> (that is exported) which answers an array of arrays. For each constant, we
> include the name of the constant, the value, and the sizeof().  Then from
> image side, we simply do one FFI call, we get the large array and we adapt
> it to a SharedPool or whatever kind of object representing that info.

This is what I suggestred in teh first place.  That what is autogenerated
is a shared object (be it a plgin or a dll doesn't matter, it is machine
code generated by a C compiler form an autogenerated C program compiled
with the platform's C compiler) that can be loaded at run-time and
interrogated to fetch the values of a set of variables.  ut I think that
the textual notation suggested above is simpler.  The test files are easier
to distribute and change.

> I know that different users will need different constants. But let's say
> the infrastructure (plugin etc) is already done. And let's say I am a user
> that I want to build something with FFI and I need some constants that I
> see are not defined. Then I can simply add the ones I need in the plugin,
> and next VM release will have those. If Cog gets moved to Github, then this
> is even easier. Everybody can do a PR with the constants he needs. And in
> fact, if we have the infrastructure in place, I think that we each of us
> spend half an hour, we may have almost everything we need.
> For example, I can add myself all those for signals (to use kill() from
> FFI), all those from fcntl (to make none blocking pipes), all those from
> wait()/waitpid() family (so that I can do a waitpid() with WNOHANG), etc
> etc etc.
> I know it's not the best approach but it's something that could be done
> very easily and would allow A LOT of stuff to be moved to FFI just because
> we have no access to preprocess constants or sizeof()  (to know how to
> allocate). I also know this won't cover macros and other stuff. But still.
> If you think this is a good idea, I can spend the time to do it.
> Cheers,
> On Thu, May 10, 2012 at 10:09 AM, Nick Ager <nick.ager at gmail.com> wrote:
>> <snip>
>> Well, like opendbx, maybe because opengl has quite standard interface...
>> </snip>
>> and
>> <snip>
>> It's not that it's not doable, it's that we gonna reinvent gaz plant
>> and it gonna be so boring...
>> I'd like to see a proof of concept, even if we restrict to libc, libm,
>> kernel.dll, msvcrt.dll ...
>> </snip>
>> <snip>
>> Is the unix style select()
>> ubiquitous or should I use WaitForMultipleObject() on Windows? Are
>> specification of read/write streams implementation machine independant
>> (bsd/sysv/others...)
>> </snip>
>> Perhaps *a* way forward is to try to find existing projects which have
>> already created cross-platform abstractions for platform specific
>> functionality. Then we can use FFI to access that interface in a similar
>> way to OpenGL and OpenDBX. For example NodeJs works across unixes - perhaps
>> they have a useful cross-platform abstraction, boost  has abstractions of
>> IPC etc
>> Nick
> --
> Mariano
> http://marianopeck.wordpress.com

best, Eliot
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