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Date: Fri, 25 Jun 93 13:31:50 -0400
From: Jonathan Rees <jar@martigny.ai.mit.edu>
To: sheldon@lcs.mit.edu
Cc: jar@martigny.ai.mit.edu
Subject: call-back
Message-Id: <9306251331.aa11673@mintaka.lcs.mit.edu>
Hmm... I think the convention is:
1. Scheme calls C (e.g. the rpc server) as usual using EXTERNAL-CALL.
2. C calls the VM's entry point restart(value).
3. That causes the original Scheme code to think that your C function
has returned value, when in fact it hasn't.
4. If Scheme later executes a VM-RETURN instruction, then
the C call to restart() will return with the specified value.
So you need to write some Scheme code that looks vaguely like this:
(let ((communication-block (make-vector 100)))
(let loop ((status (external-call rpc-server communication-block)))
(if status ;Boolean
(begin (store-somewhere-in-communication-block!
(apply (choose-a-procedure ...)
(extract-some-arguments ...)))
;; communication-block might be moved by GC; give new
;; address back to C
(loop (vm-return 0 communication-block))))))
The procedure, number of arguments, and the arguments themselves must
be encoded in the argument block somehow. You must not put Scheme
values (including the argument block) in C variables that are live
across calls to the VM. Therefore the procedures have to numbered
(stored in a vector, say) or something.
Apparently the first argument to VM-RETURN is ignored.
(define-primitive op/vm-return (fixnum-> any->) ;from vm/prim.scm
(lambda (key value)
(set! *val* value)
;; TTreturn_value = 0;
;; return(0L);}
return-option/exit)) ; the VM returns this value
The relevant code from scheme48vm.c (actually, the source from
vm/resume.scm):
long restart(long value)
{
(set! *val* value)
(let loop ()
(let ((option (interpret)))
;; option_880X = TTrun_machine((long)Tinterpret);
(cond ((= option return-option/exit)
*val*)
((= option return-option/external-call)
(set! *val* (call-external-value ; type inference hack
(fetch (address-after-header
(external-value *val*)))
*nargs*
(pointer-to-top-of-stack)))
(stack-add (- 0 (+ *nargs* 1))) ; remove proc and args
(loop))
(else
(error "unkown VM return option" option)
-1))))
}
The relevant code from unix.c:
call_external_value( long proc, long nargs, long *args )
{
return ((long(*)())proc)(nargs, args);
}
/* Driver loop for tail-recursive calls */
long TTreturn_value;
long TTrun_machine(proc)
long (*proc) (void);
{
while (proc != 0)
proc = (long (*) (void)) (*proc)();
return TTreturn_value;
}
These are the contents of the former NiCE NeXT User Group NeXTSTEP/OpenStep software archive, currently hosted by Netfuture.ch.