# Memory Errors at Julia-C interface and Safe Pointer Usage

**URL:** https://discourse.julialang.org/t/memory-errors-at-julia-c-interface-and-safe-pointer-usage/124484
**Category:** General Usage
**Created:** [January 6, 2025, 10:18pm UTC](https://discourse.julialang.org/t/memory-errors-at-julia-c-interface-and-safe-pointer-usage/124484 "2025-01-06T22:18:09Z")
**Posts on this page:** 1
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### Author: ![luke-kiernan](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/luke-kiernan/32/214309_2.png) [@luke-kiernan](https://discourse.julialang.org/u/luke-kiernan)
#### Post date: [January 9, 2025, 3:59pm UTC](https://discourse.julialang.org/t/memory-errors-at-julia-c-interface-and-safe-pointer-usage/124484/7 "2025-01-09T15:59:24Z")

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I’ve discovered and fixed one issue: my strategy of using a `Cshort` to emulate a C struct of packed bitfields was messing with field alignment. Changing that to a `Cuint` makes it _usually_ run successfully, but it still gives memory errors every now and then. Checking the offsets of the struct fields (as in the help entry for `fieldoffset`), they now all match between the C and the Julia, so that’s not the problem.

Useful things learned while debugging: `ccall(:jl_, Cvoid, (Any,), x)` is a nice way to get low-level info about `x`. Julia’s `-g` debug flag made the memory error more consistent. `lldb` is a neat tool, but not very useful in this case (no debug symbols in the compiled C library).

I’ve updated my minimum working example to incorporate the above comments and a few other things: eg to compare apples to apples, I ought to heap allocate the arrays in the C. You may need to run the Julia file a couple times in order to get the memory error (`-g` flag recommended).

[mwe.jl](https://discourse.julialang.org/uploads/short-url/A511DjNJ6RQSRybH767EZPxQw8V.jl) (3.7 KB)  
[mwe.c.jl](https://discourse.julialang.org/uploads/short-url/dxHBtYKUYLzHMITHdf7MkuLIBs2.jl) (1.8 KB)

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