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Re: Guile GC in C++
Re: Guile GC in C++
Tue, 06 Jul 2021 10:59:27 +0200
Gnus/5.13 (Gnus v5.13) Emacs/28.0.50 (gnu/linux)
Jean Abou Samra <email@example.com> writes:
> I am struggling to understand how to write
> C++ code that correctly protects and unprotects
> smobs from GC. Take this code from lily/grob-pq-engraver.cc:
> Grob_pq_engraver::process_acknowledged ()
> std::sort (started_now_.begin (), started_now_.end ());
> SCM lst = SCM_EOL;
> SCM *tail = &lst;
> for (vsize i = 0; i < started_now_.size (); i++)
> *tail = scm_acons (started_now_[i].end_.smobbed_copy (),
> started_now_[i].grob_->self_scm (),
> tail = SCM_CDRLOC (*tail);
> SCM busy = get_property (this, "busyGrobs");
> busy = scm_merge_x (lst, busy, ly_grob_pq_less_p_proc);
> set_property (context (), "busyGrobs", busy);
> started_now_.clear ();
> Looks nice, but what does SCM_CDRLOC do? It apparently
> takes an SCM, and returns a pointer to an SCM. What's
> the purpose?
Essentially tail = &scm_cdr (*tail);
but scm_cdr is not an lvalue.
> I can't find it documented anywhere
> in the Guile manual (whether for 1.8 or later releases).
> If I omit the call to SCM_CDRLOC, do I risk introducing
> Heisenbug segmentation faults?
No. Garbage protection here does not work through tail (though I have
no idea how the Boehm GC would treat it) but through lst.
> More generally, how does this all work? I see that when
> one has an SCM at hand, one has to start caring about
> GC when storing it in members (such as in engravers).
> For example, if I were the author of the below code, how
> would I understand that the mmrest_event_ should be
> Part_combine_iterator::kill_mmrest (Context *c)
> if (!mmrest_event_)
> mmrest_event_ = new Stream_event
> (Lily::ly_make_event_class (ly_symbol2scm
> set_property (mmrest_event_, "duration", SCM_EOL);
> mmrest_event_->unprotect ();
> c->event_source ()->broadcast (mmrest_event_);
Any Smob created with new is initially protected and needs exactly one
call to unprotect to be subject to garbage protection. A Simple_smob is
created as an SCM value via smobbed_copy () and is dependent on garbage
protection via this SCM value from the beginning. No call to unprotect
Some information may be in lily/include/smobs.hh .
- Guile GC in C++, Jean Abou Samra, 2021/07/06
- Re: Guile GC in C++,
David Kastrup <=