Variadic functions and libffi

[Andrew Haley]

On 01/06/2011 11:34 AM, Marcus Shawcroft wrote:
>
>>> that is just like ffi_prep_cif but also takes a 'nnamedargs' to say
>>> the number of fixed args (and hence which arg is the 1st variadic).
>>
>> I don't think this is such a great design, because you'd be doing the
>> calculation about where to put the args at the time of the call.  IMO
>> it makes more sense to do as much precomputation as possible, by
>> calling ffi_prep_cif_variadic with the actual number of fixed args and
>> the actual number of varargs you intend to pass.  In almost all usages
>> of libffi you really will know the number of arguments to be passed
>> when preparing the CIF.  If you can precompute all this stuff you can
>> even use a JIT to generate code for the call.
>
> The prototype David and I discussed has the form:
>
> ffi_status ffi_prep_cif_variadic(ffi_cif *cif, ffi_abi abi, unsigned int
> nargs, unsigned int nnamedargs, ffi_type *rtype, ffi_type **atypes);
>
> Which makes available the total number of arguments, the number of named
> arguments and hence the number of anonymous arguments up front.

Ah, OK.  That seems reasonable.

Paging Anthony Green: I never understood why libffi didn't deal with
varargs -- was it a conscious design decision, or just something you
never got around to?

Andrew.