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GC safety and return values




Some of the functions defined in the interface, for example:

     scheme_value SCHEME_REAL_PART(scheme_value) (may GC)

return a scheme_value as a C return value.

This seems inherently error-prone.

For example:

    x = SCHEME_CONS (SCHEME_REAL_PART (a), SCHEME_REAL_PART (b));

although a very natural seeming C statement, is incorrect code.  SCM
makes that mistake (though its not a mistake within the goals of the
SCM project itself), Guile inherited it (where it really is a
mistake), and it proved to be a serious obstacle to, for example,
implementing either precise or copying GC.

For starters, I would like to see all return values passed via
output parameters so that one would have to write:

	SCHEME_REAL_PART (&tmp1, a);
	SCHEME_REAL_PART (&tmp2, b);
        SCHEME_CONS (&x, tmp1, tmp2);

In fact I'd go beyond that in a few ways -- but let me just start with
this and see where we get.

-t