Dead-MacPtrs

The following information about MacPtrs (and specifically, dead MacPtrs) was posted by Gary Byers to openmcl-devel this morning. Since this info is generally useful, I’m putting it up in a blog entry. I’m also taking the occasion to inaugurate a new blog entry category: GB’s Greatest Hits.


In general, a pointer (to some foreign address) is only meaningful within a session; save-application walks memory, changing the type of any non-NULL pointers it finds to dead-macptr.  Most operations on pointers do typechecking, so attempting to access something leftover from the previous session gets a type error (rather than acessing a foreign address that’s leftover from the previous session.)  The exception for NULL pointers is somewhat arbitrary; it’d be just as reasonable to decide that something pointing to a “small” address (for some value of “small”) isn’t really a pointer “to” anything.

So:

(defvar *some-foreign-pointer* (#_malloc 1000))

(defun read-into-foreign-pointer (fd)
(#_read fd *some-foreign-pointer* 1000))

(save-application “new-image”)

;;; run the new image

(read-into-foreign-pointer some-fd)

will signal a type-error (because save-application has marked the value of *some-foreign-pointer* as being dead.)

In a very simple case like this (where the pointer’s only accessible via a special variable), it’s possible to use ccl:defloadvar:

(ccl:defloadvar *some-foreign-pointer* (#_malloc 1000))

That’ll behave like defstatic (which -is- documented …), but will additionally push (a compiled version of) the initform on a list of pointer-reinitialization functions that’s run when an image starts up. Entries on this list are run in the order in which they’re defined, so:


(ccl:defloadvar *a* (something))

(ccl:defloadvar *b* (something-that-depends-on *a*))

will do the (re)initialization in the same order that the initialization is done.

That’s often adequate.  There are some other mechanisms (it’s possible to destructively change a dead-macptr back into a macptr in cases where EQness matters, and it’s possible to run arbitrary code at image startup time.)

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