On Sat, Jun 29, 2019 at 7:44 PM Greg Troxel <g...@lexort.com> wrote: > Isaac Jurado <dipto...@gmail.com> writes: > > > On Thu, Jun 27, 2019 at 9:52 PM Greg Troxel <g...@lexort.com> wrote: > > > >> I have been down this path before, with guile and with lua. Basically, > >> if C (or non-scheme) has a pointer to a scheme object, then you need to > >> hold a logical reference for it and protect the scheme object, and when > >> the C pointer is dropped decrease the refcnt. > >> > >> I am unclear on the details of how you have a ref that gc is made aware > >> of. One way is to have a scheme array of the object and a count, and > >> have the code null out the object when the count goes to zero or > >> something like that. But the point is that you need to have a proxy in > >> the scheme world, visible to gc, when a pointer to a scheme object is > >> held outside of the scheme world. > >> > > > > That's more or less what I had in mind, although instead of an array I > > would use a hash table indexed by a fundamental type (e.g. integer) which > > can be converted painlessly between Scheme and C. > > Sure - it just needs to be something the gc will find. >
My intention was quite the opposite, use something that the GC will NOT find, like 32/64 bit integers. I assumed that (pointer->procedure) and its inverse would convert from/to uint32, uint64, etc. natively, without creating GC objects. This way I can safely give a way such a value to the C code, while using it as a key to a Scheme hashmap. > Just because something wasn't collected doesn't mean it is safe. You > don't actually know that the closure wasn't garbage collected, just that > when you used it the bits were still there. You might try running under > valgrind. Or modify guile to clear all objects that are garbage > collected, which I'm guessing it doesn't do. > Agreed, just like the old use after free(). > In my experience, it's definitely not ok to capture a pointer to a > scheme object and store it for later use without protecting the scheme > object from gc by holding a reference. > It's the sort of advice I was seeking to confirm, so thanks :-) Best regards. -- Isaac Jurado "The noblest pleasure is the joy of understanding" Leonardo da Vinci
