# How to Obtain Efficient Garbage Collection of C-Ptrs

**URL:** <https://discourse.julialang.org/t/how-to-obtain-efficient-garbage-collection-of-c-ptrs/126307>\
**Category:** General Usage\
**Tags:** question\
**Created:** [February 25, 2025, 6:39pm UTC](https://discourse.julialang.org/t/how-to-obtain-efficient-garbage-collection-of-c-ptrs/126307 "2025-02-25T18:39:11Z")\
**Posts on this page:** 6\
**Page:** 1

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**Author:** ![ejmeitz](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/ejmeitz/32/45283_2.png) [@ejmeitz](https://discourse.julialang.org/u/ejmeitz)\
**Post date:** [February 25, 2025, 6:39pm UTC](https://discourse.julialang.org/t/how-to-obtain-efficient-garbage-collection-of-c-ptrs/126307/1 "2025-02-25T18:39:11Z")

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I am working with a C++ wrapper where the most common type is a wrapper around a shared pointer to memory. Essentially an array view. Lets call this type `MyArray`.

My general question is given function which returns `MyArray`, how will Julia’s GC operate on that object given that it knows nothing about the underlying heap allocation and when that will be freed. From what I understand Julia sees `MyArray` as taking only 8 bytes and the memory that pointer refers to in C belongs to another heap which Julia has no information about.

Julia’s GC seems like it would be incapable of understanding when GC needs to be run on an object like this since the heuristics are based heap allocations and from Julia’s POV the heap has very few allocations. We have run into such issues when generating lots of these array views inside of a loop which are not being GC’ed fast enough since Julia does not know they actually use a lot of RAM. (We can actually OOM). We can add `GC.gc` to each loop iteration but that is incredibly slow.

Is there some way around this or to give Julia extra information about the underlying allocation? Thanks!

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**Author:** ![nhz2](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/nhz2/32/44428_2.png) [@nhz2](https://discourse.julialang.org/u/nhz2)\
**Post date:** [February 25, 2025, 7:08pm UTC](https://discourse.julialang.org/t/how-to-obtain-efficient-garbage-collection-of-c-ptrs/126307/2 "2025-02-25T19:08:17Z")

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I recently fixed a similar issue in [CodecZlib.jl](https://github.com/JuliaIO/CodecZlib.jl/pull/97#discussion_r1943986518) by getting the C library to use `jl_malloc` and `jl_free` instead of `malloc` and `free` Ref: [C struct garbage collection not run frequently enough - #14 by vchuravy](https://discourse.julialang.org/t/c-struct-garbage-collection-not-run-frequently-enough/116919/14)

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**Author:** ![krasow](https://avatars.discourse-cdn.com/v4/letter/k/3ab097/32.png) [@krasow](https://discourse.julialang.org/u/krasow)\
**Post date:** [February 25, 2025, 7:16pm UTC](https://discourse.julialang.org/t/how-to-obtain-efficient-garbage-collection-of-c-ptrs/126307/3 "2025-02-25T19:16:33Z")

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To clarify, we are using c++ class constructors and destructors for memory managment. Forcing the garbage collection calls with `GC.gc` calls our deconstructor.

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**Author:** ![ejmeitz](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/ejmeitz/32/45283_2.png) [@ejmeitz](https://discourse.julialang.org/u/ejmeitz)\
**Post date:** [February 25, 2025, 7:28pm UTC](https://discourse.julialang.org/t/how-to-obtain-efficient-garbage-collection-of-c-ptrs/126307/4 "2025-02-25T19:28:03Z")

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The `jl_malloc` and `jl_free` is interesting. We are not necessarily able to modify the source code of the original library so I’m not sure this would work. Maybe I’m missing something about how you implemented this in Zlib though.

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**Author:** ![krasow](https://avatars.discourse-cdn.com/v4/letter/k/3ab097/32.png) [@krasow](https://discourse.julialang.org/u/krasow)\
**Post date:** [February 25, 2025, 7:33pm UTC](https://discourse.julialang.org/t/how-to-obtain-efficient-garbage-collection-of-c-ptrs/126307/5 "2025-02-25T19:33:30Z")

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These classes are **logical** representations of data. They can be placed on different target devices and have no explicit notion of using “malloc”. By calling the deconstructor, it notifies the runtime that the physical data can be free’d.

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**Author:** ![nhz2](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/nhz2/32/44428_2.png) [@nhz2](https://discourse.julialang.org/u/nhz2)\
**Post date:** [February 25, 2025, 8:52pm UTC](https://discourse.julialang.org/t/how-to-obtain-efficient-garbage-collection-of-c-ptrs/126307/6 "2025-02-25T20:52:22Z")

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Yes `jl_malloc` and `jl_free` with finalizers only work in this case because zlib has support for custom allocators, only uses CPU memory, and is a thread-safe library without its own runtime.

Using the Julia GC to clean up resources in general cases can get very tricky, so it might be better to just require C-style manual calls to the clean-up functions, with some functions/macros for common patterns.