Arcueid’s foreign function interface

One of the big problems I had with the last release of Arcueid last year was with continuations that needed to be created within C code itself. I made use of several troublesome techniques for dealing with this issue, which was the cause of some of the problems the current 0.0.x series code has. I already made use of a technique based on Simon Tatham’s C coroutines to set this up, but it looks like I’m going to have to take the technique further and make more intensive use of it within the core for the new release that I’m working on.

I have written a set of rather ugly macros that hide most of the gory details of these Arcueid Foreign Functions, so it becomes possible to write a foreign function that looks like this:

AFFDEF(doubler, func, a, b)
{
  AVAR(rval);
  AFBEGIN;

  AFCALL(AV(func), AV(a), AV(b));
  AV(rval) = INT2FIX(2 * FIX2INT(AFCRV));
  ARETURN(AV(func));
  AFEND;
}
AFFEND

The definitions for these macros are in arcueid.h in the experimental branch of the code on Github, and probably look extremely strange. The AFBEGIN/AFEND macro pair essentially turns the body of the function into a large switch statement (yes, this is an instance of Duff’s device). Another oddity is that the AFCALL macro, perversely, uses a return statement to do its job! This is because these functions are executed by means of a sort of trampoline. The AFCALL macro basically does what one would expect a Lisp function application to do: create and save a continuation, push arguments on the stack, then jump to the function, by returning to the trampoline with the callee set up for the trampoline to execute. When the called function returns, the trampoline restores the continuation that the foreign function created.

Continuations are created for C functions within the AFCALL macro by using the __LINE__ macro provided by the C preprocessor. This is used as the “offset into the function” as it were, and goes into the continuation as such. It is then used as a case label within the giant switch statement that the AFBEGIN/AFEND macros created. Thus, for the trampoline to restore such a C continuation, it has to call the function again with the offset available for the switch statement to see.

There are a lot of other weird techniques in other macros there as well, such as for AFFDEF(…). That one makes some rather creative use of C’s variadic macros in order to make an Arcueid Foreign Function definition that looks more reasonable. The techniques to do this I are based on answers I found here.

This method of defining foreign functions looks reasonably clean, I should think, and the macros do a good job of hiding the black magic that the foreign function interface has to do. There are also mechanisms for causing the the trampoline to pause execution on a non-ready file descriptor for instance, which caused me a lot of problems in my first attempt.

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~ by stormwyrm on 2013-03-01.

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