[Vm-dev] FFI: Vararg function how to tell callee how many arguments i passed?

Bert Freudenberg bert at freudenbergs.de
Mon Sep 27 09:28:20 UTC 2010

On 27.09.2010, at 01:23, Igor Stasenko wrote:

> On 27 September 2010 02:12, Levente Uzonyi <leves at elte.hu> wrote:
>> On Mon, 27 Sep 2010, Igor Stasenko wrote:
>>> Suppose i want to call printf() using FFI.
>>> But it is a variable argument function. What should do to let it know
>>> how many arguments i passed?
>> I think you don't tell it, it guesses the number of arguments from the first
>> argument. If you pass invalid arguments, something bad will happen. :)
> Yeah.. i inspected the assembly (gcc -s) for printf call,
> and there is only pushes of arguments , no extra info.

Vararg functions are specially compiled. They do not use the regular platform calling conventions. That's because the same compiled function needs to be able to take any type of argument. Whereas e.g. floats are normally passed on the float stack, in a vararg function call they might be passed on the regular stack. Also they are converted to doubles at the calling side, just as chars and shorts are promoted to ints.

So at the calling site the compiler has to do special magic to construct the argument list. It can only do this if you actually provide a vararg call. That is, you need to literally write a printf() call to make this work. There is no portable way around this - the C FAQ states (*)

Q: How can I call a function with an argument list built up at run time?
A: There is no guaranteed or portable way to do this.

Squeak's FFI does not support vararg functions yet. There are other FFIs that do (CLISP's vacall library, Rubinius FFI), so taking a look there might help if anyone wants to implement this. 

- Bert -

(*) http://c-faq.com/varargs/invvarargs.html

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