From: Andrew Haley <aph@redhat.com>
To: "Hogan, D. (GE Power & Water)" <D.Hogan@ge.com>
Cc: Alan Modra <amodra@gmail.com>, Jakub Jelinek <jakub@redhat.com>,
"libffi-discuss@sourceware.org"
<libffi-discuss@sourceware.org>
Subject: Re: RFC: variadic closures in x86/x86_64
Date: Wed, 04 Dec 2013 12:21:00 -0000 [thread overview]
Message-ID: <529F1E3E.4010401@redhat.com> (raw)
In-Reply-To: <F023C084BCC16446BDA5B664305741E8091C7D@ALPMBAPA05.e2k.ad.ge.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.
next prev parent reply other threads:[~2013-12-04 12:21 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-11-25 2:46 Hogan, D. (GE Power & Water)
2013-11-25 9:29 ` Andrew Haley
2013-11-25 9:37 ` Jakub Jelinek
2013-11-25 10:10 ` Andrew Haley
2013-11-26 14:27 ` Alan Modra
2013-12-04 8:03 ` Hogan, D. (GE Power & Water)
2013-12-04 11:05 ` Philip Ashmore
[not found] ` <F023C084BCC16446BDA5B664305741E8091E8B@ALPMBAPA05.e2k.ad.ge.com>
2013-12-05 13:03 ` Philip Ashmore
2013-12-04 12:21 ` Andrew Haley [this message]
2013-12-04 18:28 ` Hogan, D. (GE Power & Water)
2013-12-04 18:51 ` Andrew Haley
2013-12-05 0:47 ` Hogan, D. (GE Power & Water)
2013-12-05 8:33 ` Andrew Haley
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