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RE: RFC: variadic closures in x86/x86_64
- From: "Hogan, D. (GE Power & Water)" <D dot Hogan at ge dot com>
- To: Alan Modra <amodra at gmail dot com>, Andrew Haley <aph at redhat dot com>
- Cc: Jakub Jelinek <jakub at redhat dot com>, "libffi-discuss at sourceware dot org" <libffi-discuss at sourceware dot org>
- Date: Wed, 4 Dec 2013 08:02:46 +0000
- Subject: RE: RFC: variadic closures in x86/x86_64
- Authentication-results: sourceware.org; auth=none
- References: <F023C084BCC16446BDA5B664305741E8090811 at ALPMBAPA05 dot e2k dot ad dot ge dot com> <52931854 dot 6080007 at redhat dot com> <20131125093715 dot GU892 at tucnak dot redhat dot com> <5293221D dot 4010505 at redhat dot com> <20131126142723 dot GD9211 at bubble dot grove dot modra dot org>
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.