* Calling Glibc's malloc(3) from Guile
@ 2025-01-13 20:58 Felix Lechner via Developers list for Guile, the GNU extensibility library
2025-01-19 13:07 ` Maxime Devos
0 siblings, 1 reply; 2+ messages in thread
From: Felix Lechner via Developers list for Guile, the GNU extensibility library @ 2025-01-13 20:58 UTC (permalink / raw)
To: guile-devel
Hi,
May I call Glibc's malloc(3) from Guile? Thanks!
Kind regards,
Felix Lechner
P.S. Following a series of friendly interactions on #guile, I subscribed
to this list. There is no need to copy me separately on a reply.
^ permalink raw reply [flat|nested] 2+ messages in thread
* RE: Calling Glibc's malloc(3) from Guile
2025-01-13 20:58 Calling Glibc's malloc(3) from Guile Felix Lechner via Developers list for Guile, the GNU extensibility library
@ 2025-01-19 13:07 ` Maxime Devos
0 siblings, 0 replies; 2+ messages in thread
From: Maxime Devos @ 2025-01-19 13:07 UTC (permalink / raw)
To: Felix Lechner, guile-devel@gnu.org
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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
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