# Segmentation fault in multi thread code

**URL:** <https://discourse.julialang.org/t/segmentation-fault-in-multi-thread-code/132562>\
**Category:** New to Julia\
**Created:** [September 22, 2025, 11:50am UTC](https://discourse.julialang.org/t/segmentation-fault-in-multi-thread-code/132562 "2025-09-22T11:50:51Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![liwa24](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/liwa24/32/218708_2.png) [@liwa24](https://discourse.julialang.org/u/liwa24)\
**Post date:** [September 22, 2025, 11:50am UTC](https://discourse.julialang.org/t/segmentation-fault-in-multi-thread-code/132562/1 "2025-09-22T11:50:51Z")

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Hello, I would appreciate some advice on a multi thread code causing a segmentation fault with my workstation Dell Inc. Precision 7960 Tower (Intel® Xeon® w9-3495X × 112), running Ubuntu 24.04.2 LTS. Here you find a simplified situation. I have a matrix, which I update inside a multi thread for cycle. I have also divided the total number of samples to process into chunks.

```julia-auto
# classify the trajectories. parallel version 
m = size(orbit_array,1)
output_report = zeros(m,6)
num_threads = Threads.nthreads() #I use 32 out of 112
chunk_size = 100
start_num = 1
selected_elements = range(start_num,m)
# Total number of elements to process

total_elements = length(selected_elements)

# the orbits are processed in chunks. At the end of the chunk, results are saved
num_chunks = ceil(Int, length(selected_elements) / chunk_size)

# Calculate chunk size (total elements divided by the number of chunks)
chunk_size = ceil(Int, total_elements / num_chunks)

# Create a list of ranges for each chunk
chunks = [(chunk_id - 1) * chunk_size + 1: min(chunk_id * chunk_size, total_elements) for chunk_id in 1:num_chunks]

npzwrite("output.npy", output_report)

for chunk_id in 1:num_chunks
    # Get the range of elements for this chunk
    range_start, range_end = chunks[chunk_id][1], chunks[chunk_id][end]

    # Load intermediate results
    output_report = npzread("output.npy")

    @threads for i in range_start:range_end
        j = selected_elements[i]
        println(" Thread $(threadid()). orbit $j, element $i of $total_elements")

        output_report[j,6] = classifier(orbit_array[j,:,:])
    end

    println("Finished chunk $chunk_id, saving intermediate results...\n")
    npzwrite("output.npy", output_report)
end

```

The classifier function just provides a float number, which I add at the last column at the corresponding row. The classifier (a complex function) is not important in this discussion: the error arises also when substituting it with a simpler ` output_report[j,6] = 1`.

When running this code on VSCode a segmentation fault is produced on this machine. I am not able to report the error output, as the julia terminal crashes right after. I have also tried another version of this code by using the `@spawn` method, but the result is the same. However, whenever I run this code on my Windows laptop, everything works as expected, without bugs.

What can be the issue with the workstation? Thank you for your help.

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**Author:** ![WalterMadelim](https://avatars.discourse-cdn.com/v4/letter/w/3e96dc/32.png) [@WalterMadelim](https://discourse.julialang.org/u/WalterMadelim)\
**Post date:** [September 22, 2025, 12:09pm UTC](https://discourse.julialang.org/t/segmentation-fault-in-multi-thread-code/132562/2 "2025-09-22T12:09:52Z")

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I think it’s very unlikely to crash.  
either add a `local` so that `local j = selected_elements[i]`,  
or add a `@lock` to `output_report[j,6] = classifier(orbit_array[j,:,:])`.

Otherwise I cannot figure out a reason to crash—you didn’t give the runnable code. And the only multithreading part is the `@threads for`.

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**Author:** ![eldee](https://avatars.discourse-cdn.com/v4/letter/e/b5a626/32.png) [@eldee](https://discourse.julialang.org/u/eldee)\
**Post date:** [September 22, 2025, 3:39pm UTC](https://discourse.julialang.org/t/segmentation-fault-in-multi-thread-code/132562/3 "2025-09-22T15:39:35Z")

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Hi, and welcome to the Julia community!

> [@liwa24](#):
>
> When running this code on VSCode a segmentation fault is produced on this machine. I am not able to report the error output, as the julia terminal crashes right after.

What happens if you run the code (or MWE thereof) in a different terminal than the VS Code one?

> [@liwa24](#):
>
> `classifier(orbit_array[j,:,:])`

I’m not sure that this would cause a segmentation error, but note that you’re (potentially) allocating a lot of memory here, as `orbit_array[j, :, :]` this makes a copy for each task. You could see if `@view(orbit_array[j, :, :])` fixes the problem. (Also, if possible, consider reordering the dimensions of `orbit_array` so that you get `orbit_array[:, :, j]`, which is a contiguous chunk of memory.)

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**Author:** ![sgaure](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/sgaure/32/14779_2.png) [@sgaure](https://discourse.julialang.org/u/sgaure)\
**Post date:** [September 23, 2025, 6:50pm UTC](https://discourse.julialang.org/t/segmentation-fault-in-multi-thread-code/132562/4 "2025-09-23T18:50:05Z")

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Hmm, I can’t see anything here which could cause a segfault, only several things which will slow things down.

Could it be that `npzread`/`npzwrite` does something dubious? Could you try `serialize`/`deserialize` instead (with `using Serialization`).

And what’s the output of `versioninfo()`?

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<div class="post-metadata">

**Author:** ![liwa24](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/liwa24/32/218708_2.png) [@liwa24](https://discourse.julialang.org/u/liwa24)\
**Post date:** [October 7, 2025, 2:53pm UTC](https://discourse.julialang.org/t/segmentation-fault-in-multi-thread-code/132562/5 "2025-10-07T14:53:23Z")

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Hello all, thank you for your feedback, greatly appreciated.

@sgaure this is the version I am using right now:  
Julia Version 1.11.6  
Commit 9615af0f269 (2025-07-09 12:58 UTC)  
Build Info:  
Official [https://julialang.org/](https://julialang.org/) release  
Platform Info:  
OS: Linux (x86\_64-linux-gnu)  
CPU: 112 × Intel(R) Xeon(R) w9-3495X  
WORD\_SIZE: 64  
LLVM: libLLVM-16.0.6 (ORCJIT, sapphirerapids)  
Threads: 32 default, 0 interactive, 16 GC (on 112 virtual cores)  
Environment:  
JULIA\_NUM\_THREADS = 32

I believe this crash is due to a bad habit of mine: I tend to write long scripts and then run them in separate blocks using the REPL (selecting a section and executing it with Shift + Enter). To isolate the issue, I created a clean, standalone script containing only the parallel code. When I ran it from VSCode (using the Run command), it kept crashing. However, it worked fine when executed directly from the terminal. This suggests the problem is likely related to VSCode. I will keep this in mind for future work.

Thank you again for your support