Re: RFC: variadic closures in x86/x86_64

[Andrew Haley <aph at redhat dot com>]

On 12/04/2013 08:02 AM, Hogan, D. (GE Power & Water) wrote:
> On Wed, Nov 27, 2013 at 00:57:23PM +1030, Alan Modra wrote:
>> The claim to fame looks to be the ability to call variadic functions
>> without describing the arguments via ffi_prep_cif_var at the point of
>> call.  Instead you do so in the function consuming the args.  I'm not
>> sure what that gains you..
> 
> This is specifically for variadic callbacks.  If it wasn't a callback,
> I could use ffi_prep_cif_var.
> 
> To give a little background, FMI is a standard for (among other things)
> model exchange so you can use a dynamic system model in various
> modeling or simulation environments.  The model is exposed as a C shared
> library.  The shared library executes callbacks (some variadic) provided
> by a FMU driver.

Please, I'm finding this extremely difficult to understand.  Where is
the variadic function?  You say that a C shared library "executes" a
callback, but what does that mean?

At the point of the call, the caller knows for certain what arguments
are passed, and the type of these arguments.

> JFMI allows you to drive a FMU from Java.  The Java FMU driver provides
> a Java implementation for the C callbacks.

Right, so a C program thinks it's calling C, but in fact it's calling
Java, and a libffi closure provides the glue.

> The nonvariadic callbacks

Ah!  OK, so the function that is called as a C function is a libffi
closure, and it is called in a variadic form.

> were already supported by JFMI through libffi and JNA.  In order to
> handle variadic callbacks, libffi and JNA need to be modified so you
> can access the variadic arguments inside of a callback.  This patch
> adds the libffi support.
> 
> I cannot construct a ffi_prep_cif_var because the model's C shared
> library is the one running a variadic callback in the driver.

Again, I don't know what it means to "run a variadic callback." Does
this mean that the shared library contains this callback function, or
that it calls it?

> I don't
> know how many arguments or what types it will provide in the calls.  I
> have to rely on printf style formatting flags in order to know what to
> access from inside of the callback.

OK, so I'm guessing that the C model's shared library calls the variadic
function.

Why do you not define a variadic C function which does this:

void f1(int n, ...) {
  va_list ap;

  jintArray argsArray = (*env_p)->NewIntArray(n);
  jint *args = (*env_p)->GetIntArrayElements(argsArray, NULL);

  int argno = 0;
  va_start(ap, n);
  while (argno < n) {
	args[argno++] = va_arg(ap, int);
  }
  va_end(ap);
  (*env_p)->ReleaseIntArrayElements(argsArray, args, 0);
  (*env_p)->CallStaticVoidMethod(clsH, printargs, argsArray);
}

to call this variadic Java code:

    public static void printargs(int... args) {
        for (int i : args)
            System.out.print(i + " ");
        System.out.println();
    }

There is no reason that JNA cannot use this mechanism, is there?

Andrew.