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Custom blocks and finalization
-
Markus Mottl
-
Markus Mottl
- Dr. Thomas Fischbacher
- Dr. Thomas Fischbacher
-
Markus Mottl
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Date: | 2009-05-06 (16:43) |
From: | Dr. Thomas Fischbacher <t.fischbacher@s...> |
Subject: | Re: [Caml-list] Custom blocks and finalization |
Markus Mottl wrote: > Yes, if your C-finalizer e.g. needs to remove global roots, this may > not be sound. I am not totally sure this is actually true in the > current implementation, but at least the documentation clearly > indicates that any interaction with the OCaml-runtime from within > C-finalizers is prohibited. You may essentially just extract C-values > / pointers from the OCaml-value to be reclaimed and manipulate those. > I take it you are referring to this part of the documentation (18.9.1 bottom): ===> Note: the finalize, compare, hash, serialize and deserialize functions attached to custom block descriptors must never trigger a garbage collection. Within these functions, do not call any of the Caml allocation functions, and do not perform a callback into Caml code. Do not use CAMLparam to register the parameters to these functions, and do not use CAMLreturn to return the result. <=== Let's have a look at byterun/globroots.c: /* Un-register a global C root */ CAMLexport void caml_remove_global_root(value *r) { struct global_root * update[NUM_LEVELS]; struct global_root * e, * f; int i; /* Init "cursor" to list head */ e = (struct global_root *) &caml_global_roots; /* Find element in list */ for (i = caml_global_roots.level; i >= 0; i--) { while (1) { f = e->forward[i]; if (f == NULL || f->root >= r) break; e = f; } update[i] = e; } e = e->forward[0]; /* If not found, nothing to do */ if (e == NULL || e->root != r) return; /* Rebuild list without node */ for (i = 0; i <= caml_global_roots.level; i++) { if (update[i]->forward[i] == e) update[i]->forward[i] = e->forward[i]; } /* Reclaim list element */ caml_stat_free(e); /* Down-correct list level */ while (caml_global_roots.level > 0 && caml_global_roots.forward[caml_global_roots.level] == NULL) caml_global_roots.level--; } ...and memory.c says: CAMLexport void caml_stat_free (void * blk) { free (blk); } So... how could caml_remove_global_root() actually trigger a heap GC? -- best regards, Thomas Fischbacher t.fischbacher@soton.ac.uk