[Vm-dev] FFI: Vararg function how to tell callee how many arguments i passed?

[Eliot Miranda]

On Mon, Sep 27, 2010 at 3:12 PM, Igor Stasenko <siguctua at gmail.com> wrote:

>
> On 28 September 2010 00:22, Bert Freudenberg <bert at freudenbergs.de> wrote:
> >
> >
> > On 27.09.2010, at 20:21, Bert Freudenberg wrote:
> >
> > On 27.09.2010, at 19:27, Eliot Miranda wrote:
> >
> >
> > On Mon, Sep 27, 2010 at 2:28 AM, Bert Freudenberg <bert at freudenbergs.de>
> wrote:
> >>
> >>
> >> On 27.09.2010, at 01:23, Igor Stasenko wrote:
> >>
> >> >
> >> > On 27 September 2010 02:12, Levente Uzonyi <leves at elte.hu> wrote:
> >> >>
> >> >> On Mon, 27 Sep 2010, Igor Stasenko wrote:
> >> >>
> >> >>>
> >> >>> Suppose i want to call printf() using FFI.
> >> >>> But it is a variable argument function. What should do to let it
> know
> >> >>> how many arguments i passed?
> >> >>
> >> >> I think you don't tell it, it guesses the number of arguments from
> the first
> >> >> argument. If you pass invalid arguments, something bad will happen.
> :)
> >> >>
> >> >
> >> > Yeah.. i inspected the assembly (gcc -s) for printf call,
> >> > and there is only pushes of arguments , no extra info.
> >>
> >> Vararg functions are specially compiled. They do not use the regular
> platform calling conventions. That's because the same compiled function
> needs to be able to take any type of argument. Whereas e.g. floats are
> normally passed on the float stack, in a vararg function call they might be
> passed on the regular stack. Also they are converted to doubles at the
> calling side, just as chars and shorts are promoted to ints.
> >>
> >> So at the calling site the compiler has to do special magic to construct
> the argument list. It can only do this if you actually provide a vararg
> call. That is, you need to literally write a printf() call to make this
> work. There is no portable way around this - the C FAQ states (*)
> >>
> >> Q: How can I call a function with an argument list built up at run time?
> >> A: There is no guaranteed or portable way to do this.
> >>
> >> Squeak's FFI does not support vararg functions yet. There are other FFIs
> that do (CLISP's vacall library, Rubinius FFI), so taking a look there might
> help if anyone wants to implement this.
> >
> > While the image-level facilities may not exist the VM
> provides primitiveCalloutWithArgs which implements
> ExternalFunction>>invokeWithArguments: and that could be used to do varargs
> calls.  One has to synthesize the ExternalFunction because the primitive
> still needs a type vector for the arguments.   But I think it could do the
> job.
> >
> > No it can't. Maybe re-read what I wrote when you have a little more time
> ;)
> >
> > Retract that.
> > A little off-line conversation with Eliot convinced me that the parameter
> passing isn't actually that much different between regular and varagrs calls
> (if at all). The FFI already does all the non-portable ABI-dependent
> trickery needed to set up the call. So this might in fact just work :)
> > Anyone up for trying?
>
> Well, i made a callout to printf() via NativeBoost in Linux,
> but was unable to determine if it works, because when i run it, it
> prints nothing on console.
> Probably because stdout is closed by default and i need to reopen it first.
>

Did you try using a console VM (if you're using Cog that's
CroquetConsole.exe)?

e.


> > - Bert -
> >
>
> --
> Best regards,
> Igor Stasenko AKA sig.
>
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