RE: Calling Glibc's malloc(3) from Guile

There is nothing that forbids doing that, to my knowledge (do so the same way you would call other libc functions). But remember to check for ENOMEM/NULL (and eventually free it).

Something to keep in mind though (not specific to malloc): if you put the pointer in some object, and the record type of that object has a record printer, and that record printer dereferences the pointer, then when the object is printed after deallocation, you (almost tautologically) have use-after-free, and before initialisation you have another problem.

While obvious, this can happen in unexpected situations. Consider:

;; ENOMEM / NULL handling omitted.
;; assume that the record printer, when the pointer field isn’t #false,
;; dereferences the pointer and prints something about it.
;; (if it doesn’t check for #false, you also have problems, but different problems)
(define (allocate-thing)
  (let ((r (make-record-thingie))
          (p (malloc-something)))
    ;; even if setting the field is integrated in the constructor,
    ;; then internally Guile will split the allocation and setting the fields
   ;; (although, different than C, it is still initialised after allocation, to #false IIRC)
    (set-some-field! r p)
    (do-some-initialisation! r)
    r))
,trace (allocate-thing)

Because ‘trace’ looks at each procedure call (including do-some-initialisation!) inside and prints the argument, you end up with use before initialisation (a variant with use-after-free can also be written).

(For a non-malloc example, see the bug report (+ patch) about GOOPS methods and ,trace.)

Best regards,
Maxime Devos