[cffi-devel] with-pointer-to-vector-data

[rpgoldman at sift.info]

>>>>> "David" == David Lichteblau <david at lichteblau.com> writes:

    David> Quoting rpgoldman at sift.info (rpgoldman at sift.info):
    >> a pointer for the C code to use, the important thing is to
    >> avoid having the lisp gc move the byte vector around, which is
    >> accomplished by the use of register-lisp-value.

    David> Are you sure?  I believe r-l-v just returns an integer
    David> index into a global table that can later be used to
    David> retrieve the object again.  That keeps it from being
    David> garbage collected, but not from movement in memory.

No, I'm not sure!

The somewhat confusing manual materials are (section 8.1 of the ACL
manual):

"Before accessing a Lisp value from C, it should be registered
first. When a Lisp value is registered, an index is returned as a
`handle' on the Lisp object. A C function is provided that will return
a pointer to the Lisp object given its index. This is preferable to
passing addresses of Lisp objects to C functions, since Lisp objects
will generally move with each garbage collection. If a garbage
collection is triggered from C code - by calling back from C to Lisp -
the addresses of Lisp objects originally passed to the C function from
Lisp may become invalid. And since one will have lost one's only
handle on these Lisp objects, their addresses, there will be no way to
find their new addresses. If instead one were to pass the registration
indices of these Lisp objects, one could readily find their new
addresses using these indices following a call-back to Lisp.

....

Note that when one passes Lisp values to foreign functions that have
been declared using def-foreign-call, most Lisp data types are
converted to corresponding C data types automatically. When one
obtains Lisp values by calling lisp_value(), the conversion must be
performed explicitly by the foreign code."

I read this as saying:

1.  I should register the lisp value so that it doesn't get mangled by
    garbage collection.

2.  The use of def-foreign-call means that the indexing stuff will be
    handled automagically.

Either or both of these might be wrong.

Best,
R