RE: RFC: variadic closures in x86/x86_64

["Hogan, D. (GE Power & Water)" <D.Hogan@ge.com>]

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.

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

> The patch suffers from the same flaw as my first attempt at adding
> variadic argument support for PowerPC64 ELFv2, namely that you cannot
> add fields to ffi_cif without breaking ABI compatibility.
> Applications linked against an older version of libffi.so will supply
> the old ffi_cif to a new shared library..

That's a good point.  This should be changed to avoid breaking the ABI.