# Memory management for C bindings

**URL:** <https://discourse.julialang.org/t/memory-management-for-c-bindings/118822>\
**Category:** General Usage\
**Tags:** cwrap\
**Created:** [August 30, 2024, 7:07pm UTC](https://discourse.julialang.org/t/memory-management-for-c-bindings/118822 "2024-08-30T19:07:00Z")\
**Posts on this page:** 2\
**Page:** 1

<div class="post-metadata">

**Author:** ![bthorne93](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/bthorne93/32/19570_2.png) [@bthorne93](https://discourse.julialang.org/u/bthorne93)\
**Post date:** [August 30, 2024, 7:07pm UTC](https://discourse.julialang.org/t/memory-management-for-c-bindings/118822/1 "2024-08-30T19:07:00Z")

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When writing bindings for a C library which performs memory allocations, how can we perform automatic memory management in the style of Julia with the external free functions? We want something similar to the defer keyword, like the following:

```julia
my_ptr = @ccall mylib.myAlloc(alloc_bytes)::Ptr{UInt8}
defer @ccall mylib.myFree(my_ptr)

```

Ideally we would like to plug into the garbage collector to make this kind of C call memory safe.

---

<div class="post-metadata">

**Author:** ![stevengj](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/stevengj/32/71_2.png) [@stevengj](https://discourse.julialang.org/u/stevengj)\
**Post date:** [August 30, 2024, 7:22pm UTC](https://discourse.julialang.org/t/memory-management-for-c-bindings/118822/2 "2024-08-30T19:22:33Z")

</div>

The usual approach is to wrap the output of your `ccall` in a Julia `mutable struct` that hides the raw pointer, and has a `finalizer` that calls the C destructor.

For example, ZMQ.jl does this to wrap the raw `void*` returned by [`zmq_socket`](https://libzmq.readthedocs.io/en/zeromq3-x/zmq_socket.html) in the C library with a [Julia `Socket` type with a finalizer](https://github.com/JuliaInterop/ZMQ.jl/blob/873f35fefe640e41b65f35d727d3c95613dd0fe2/src/socket.jl#L21) that [calls `zmq_close` on the pointer](https://github.com/JuliaInterop/ZMQ.jl/blob/873f35fefe640e41b65f35d727d3c95613dd0fe2/src/socket.jl#L61).

In general, you rarely want to expose a raw C pointer-based API to the user; always wrap it.