# Segfault when invoking Julia callback function from C

**URL:** <https://discourse.julialang.org/t/segfault-when-invoking-julia-callback-function-from-c/79541>\
**Category:** General Usage\
**Tags:** question, ccall, debug, struct, c\
**Created:** [April 15, 2022, 4:38pm UTC](https://discourse.julialang.org/t/segfault-when-invoking-julia-callback-function-from-c/79541 "2022-04-15T16:38:23Z")\
**Posts on this page:** 11\
**Page:** 1

<div class="post-metadata">

**Author:** ![Octogonapus](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/octogonapus/32/9489_2.png) [@Octogonapus](https://discourse.julialang.org/u/Octogonapus)\
**Post date:** [April 15, 2022, 4:38pm UTC](https://discourse.julialang.org/t/segfault-when-invoking-julia-callback-function-from-c/79541/1 "2022-04-15T16:38:24Z")

</div>

I am trying to pass a Julia function to a C library (aws-c-mqtt) which will use it as a callback when an asynchronous event occurs. I am running in to trouble because some of the parameters passed to this callback appear corrupted on the Julia side of the execution, but appear correct on the C side. This `rr` backtrace illustrates what I mean:

```julia
(rr) bt
#0 0x00007efcec134770 in julia_on_connection_complete_479 (connection=139624673185440, error_code=0, return_code=1, session_present=0 '\000', userdata=139624665866896)
    at /home/salmon/Documents/code/aws-c-mqtt-generator/test/runtests.jl:23
#1 0x00007efcec14051b in jlcapi_on_connection_complete_485 ()
#2 0x00007efce6c27213 in s_packet_handler_connack (connection=0x19e43c0, message_cursor=...) at /workspace/srcdir/aws-c-mqtt/source/client_channel_handler.c:165
#3 0x00007efce6c27de6 in s_process_mqtt_packet (connection=0x19e43c0, packet_type=AWS_MQTT_PACKET_CONNACK, packet=...) at /workspace/srcdir/aws-c-mqtt/source/client_channel_handler.c:459

```

In frames 2 and 3, you can see that `connection=0x19e43c0`. The `connection` pointer is originally given to the C library from Julia, and if you printed `connection` from Julia, you would see this address. Therefore, this is the correct address. Further inspection of the memory at this address in `rr` confirms that this is correct. The problem arises when passing this pointer to my callback. In frame 0, you can see that `connection` has changed: it is now `connection=139624673185440`.

> **Julia Code**
>
> This is the full Julia code (except the JLL wrapper). Relevant points are the definitions of `on_connection_complete` and `on_connection_complete_cb`, and the call to `aws_mqtt_client_connection_connect`.
> 
> ```julia
> # set substitute-path /workspace/srcdir ../local_sources
> 
> GC.enable(false)
> GC.enable_finalizers(false)
> 
> using Test, LibAWSMQTT, CountDownLatches
> 
> const held_refs = Vector{Ref}()
> const received_on_connection_complete = CountDownLatch(1)
> 
> function on_connection_interrupted(connection, error_code, userdata)
> @warn "Connection Interrupted" error_code
> end
> 
> function on_connection_resumed(connection, return_code, session_present, userdata)
> @info "Connection Resumed" return_code session_present
> if !session_present
> @info "Resubscribing..."
> packet_id = aws_mqtt_resubscribe_existing_topics(connection, s_on_resubscribed, C_NULL)
> if packet_id == 0
> error("Failed to resubscribe aws_last_error=$(aws_last_error())")
> end
> end
> end
> 
> function on_connection_complete(connection::Ptr{aws_mqtt_client_connection}, error_code::Cint, return_code::Cint, session_present::Cuchar, userdata::Ptr{Cvoid})
> count_down(received_on_connection_complete)
> println("on_connection_complete on_connection_complete on_connection_complete on_connection_complete")
> return nothing
> end
> 
> struct UserData
> connection::Ptr{aws_mqtt_client_connection}
> end
> 
> function aws_iot_client_test_main()
> endpoint = "a19vzmsjwkvm63-ats.iot.us-east-1.amazonaws.com"
> client_id_name = "test-client-id"
> subscribe_topic = "mytopic"
> will_payload = "The client has gone offline!"
> ca_filepath = joinpath(@ __DIR__ , "certs", "AmazonRootCA1.pem")
> cert_filepath = joinpath(@ __DIR__ , "certs", "5909390f9eff261d9cf92331e0e6dfc3e0e673b0005c272ee258dd3cb192fffb-certificate.pem.crt")
> pri_key_filepath = joinpath(@ __DIR__ , "certs", "5909390f9eff261d9cf92331e0e6dfc3e0e673b0005c272ee258dd3cb192fffb-private.pem.key")
> 
> allocator = aws_default_allocator()
> 
> aws_mqtt_library_init(allocator)
> 
> el_group = aws_event_loop_group_new_default(allocator, 1, C_NULL)
> 
> resolver_options = Ref(aws_host_resolver_default_options(8, el_group, C_NULL, C_NULL))
> resolver = aws_host_resolver_new_default(allocator, resolver_options)
> 
> bootstrap_options = Ref(aws_client_bootstrap_options(el_group, resolver, C_NULL, C_NULL, C_NULL))
> bootstrap = aws_client_bootstrap_new(allocator, bootstrap_options)
> 
> tls_ctx_opt = Ref(aws_tls_ctx_options(ntuple(_ -> UInt8(0), 200)))
> @test AWS_OP_SUCCESS ==
> aws_tls_ctx_options_init_client_mtls_from_path(tls_ctx_opt, allocator, cert_filepath, pri_key_filepath)
> @test tls_ctx_opt[].allocator == allocator
> @test AWS_OP_SUCCESS == aws_tls_ctx_options_set_alpn_list(tls_ctx_opt, "x-amzn-mqtt-ca")
> @test AWS_OP_SUCCESS == aws_tls_ctx_options_override_default_trust_store_from_path(tls_ctx_opt, C_NULL, ca_filepath)
> 
> tls_ctx = aws_tls_client_ctx_new(allocator, tls_ctx_opt)
> @test tls_ctx != C_NULL
> 
> aws_tls_ctx_options_clean_up(tls_ctx_opt)
> 
> tls_connection_options =
> Ref(aws_tls_connection_options(C_NULL, C_NULL, C_NULL, C_NULL, C_NULL, C_NULL, C_NULL, false, 0))
> aws_tls_connection_options_init_from_ctx(tls_connection_options, tls_ctx)
> 
> client = aws_mqtt_client_new(allocator, bootstrap)
> @show client
> connection = aws_mqtt_client_connection_new(client)
> @show connection
> ud = UserData(connection)
> 
> GC.@preserve connection begin
> socket_options = Ref(aws_socket_options(AWS_SOCKET_STREAM, AWS_SOCKET_IPV6, 3000, 0, 0, 0, false))
> 
> host_name_cur = Ref(aws_byte_cursor_from_c_str(endpoint))
> @show host_name_cur
> client_id_cur = Ref(aws_byte_cursor_from_c_str(client_id_name))
> 
> on_connection_interrupted_cb =
> @cfunction on_connection_interrupted Cvoid (Ptr{aws_mqtt_client_connection}, Int, Ptr{Cvoid})
> on_connection_resumed_cb = @cfunction on_connection_resumed Cvoid (Ptr{aws_mqtt_client_connection},aws_mqtt_connect_return_code,Bool,Ptr{Cvoid},)
> @test AWS_OP_SUCCESS == aws_mqtt_client_connection_set_connection_interruption_handlers(
> connection,
> on_connection_interrupted_cb,
> C_NULL,
> on_connection_resumed_cb,
> C_NULL,
> )
> 
> subscribe_topic_cur = Ref(aws_byte_cursor_from_c_str(subscribe_topic))
> will_cur = Ref(aws_byte_cursor_from_c_str(will_payload))
> aws_mqtt_client_connection_set_will(connection, subscribe_topic_cur, AWS_MQTT_QOS_AT_LEAST_ONCE, false, will_cur)
> 
> on_connection_complete_cb = @cfunction(on_connection_complete, Cvoid, (Ptr{aws_mqtt_client_connection},Cint,Cint,Cuchar,Ptr{Cvoid}))
> GC.@preserve allocator el_group resolver bootstrap client connection socket_options tls_connection_options ud begin
> @show connection
> @show on_connection_complete_cb
> push!(held_refs, Ref(on_connection_complete_cb))
> conn_options = Ref(
> aws_mqtt_connection_options(
> host_name_cur[],
> UInt16(8883),
> Base.unsafe_convert(Ptr{aws_socket_options}, socket_options),
> Base.unsafe_convert(Ptr{aws_tls_connection_options}, tls_connection_options),
> client_id_cur[],
> 0,
> 0,
> 0,
> on_connection_complete_cb,
> Base.unsafe_convert(Ptr{Cvoid}, Ref(ud)), # user_data
> true,
> ),
> )
> push!(held_refs, conn_options)
> aws_mqtt_client_connection_connect(connection, conn_options)
> await(received_on_connection_complete)
> println("connected connected connected connected connected connected")
> end
> end
> end
> 
> @testset "LibAWSMQTT" begin
> @testset "aws_iot_client_test" begin
> aws_iot_client_test_main()
> end
> end
> 
> ```

> **C Code**
>
> mqtt/source/client\_channel\_handler.c:165
> 
> ```nohighlight
> MQTT_CLIENT_CALL_CALLBACK_ARGS(
> connection, on_connection_complete,
> AWS_OP_SUCCESS, connack.connect_return_code, connack.session_present);
> 
> ```
> 
> `MQTT_CLIENT_CALL_CALLBACK_ARGS` is a variadic macro defined like so:
> 
> ```nohighlight
> #define MQTT_CLIENT_CALL_CALLBACK_ARGS(client_ptr, callback, ...) \
> do { \
> if ((client_ptr)->callback) { \
> (client_ptr)->callback((client_ptr), __VA_ARGS__ , (client_ptr)->callback##_ud); \
> } \
> } while (false)
> 
> ```

Looking at the C code, `connection` is passed from the `s_packet_handler_connack` function to the callback function without modification. I don’t understand what is causing its value to change when invoking `julia_on_connection_complete_479`.

Ultimately, the error I get when the callback is invoked is a segfault:

```julia
Thread 3 received signal SIGSEGV, Segmentation fault.
[Switching to Thread 70533.70674]
0x00007efcec134770 in julia_on_connection_complete_479 (connection=139624673185440, error_code=0, return_code=1, session_present=0 '\000', userdata=139624665866896) at /home/salmon/Documents/code/aws-c-mqtt-generator/test/runtests.jl:23

```

Debugging steps I’ve tried so far that are worth noting:

- I can `ccall` the callback just fine. The segfault occurs when it’s called from the C library.
- I have tried combinations of various Julia types and C types in the `cfunction` argument tuple and return type to debug possible argument passing errors.
- I have tried creating the `cfunction` in the global scope.
- I have tried preserving (`GC.@preserve`) all relevant data and even disabled the GC. This has no effect on the error, so this doesn’t appear to be a GC problem.

I have also uploaded a zip of the entire project [here](https://drive.google.com/file/d/1ik5I5oxJqhEuOdZDxcFqj6sL1YoaG-g-/view?usp=sharing) if anyone wants to reproduce it locally.

---

<div class="post-metadata">

**Author:** ![vchuravy](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/vchuravy/32/8_2.png) [@vchuravy](https://discourse.julialang.org/u/vchuravy)\
**Post date:** [April 15, 2022, 4:49pm UTC](https://discourse.julialang.org/t/segfault-when-invoking-julia-callback-function-from-c/79541/2 "2022-04-15T16:49:25Z")

</div>

Julia functions being called from C must adhere to a few rules.  
They must either be called on a Julia worker thread (e.g. the backtrace needs to go back into Julia),  
or they must **not** interact with the Julia runtime. The only excempt function is `uv_async_send` and  
`AsyncConditions`.

In your code you are trying to do arbitrary IO and from a “foreign” thread.

I wrote a package [ForeignCallbacks.jl](https://github.com/vchuravy/ForeignCallbacks.jl) that aims to make the latter use-case less error prone.

Edit:  
I am actually not sure if you are running into this use-case since it is not immediately clear if the backtrace you supplied is just really short or you cut it off and it does reach back into Julia.

---

<div class="post-metadata">

**Author:** ![Octogonapus](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/octogonapus/32/9489_2.png) [@Octogonapus](https://discourse.julialang.org/u/Octogonapus)\
**Post date:** [April 15, 2022, 4:56pm UTC](https://discourse.julialang.org/t/segfault-when-invoking-julia-callback-function-from-c/79541/3 "2022-04-15T16:56:05Z")

</div>

Here is the full backtrace for you:

```julia
(rr) c
Continuing.
client = Ptr{aws_mqtt_client} @0x0000000001a0dbf0
connection = Ptr{aws_mqtt_client_connection} @0x00000000019e43c0
host_name_cur = Base.RefValue{aws_byte_cursor}(aws_byte_cursor(0x000000000000002e, Ptr{UInt8} @0x00007efd05553ad8))
connection = Ptr{aws_mqtt_client_connection} @0x00000000019e43c0
on_connection_complete_cb = Ptr{Nothing} @0x00007efcec140490
[New Thread 70533.70675]
[New Thread 70533.70674]
[New Thread 70533.70534]

Thread 3 received signal SIGSEGV, Segmentation fault.
[Switching to Thread 70533.70674]
0x00007efcec134770 in julia_on_connection_complete_479 (connection=139624673185440, error_code=0, return_code=1, session_present=0 '\000', userdata=139624665866896) at /home/salmon/Documents/code/aws-c-mqtt-generator/test/runtests.jl:23
23 end
(rr) bt
#0 0x00007efcec134770 in julia_on_connection_complete_479 (connection=139624673185440, error_code=0, return_code=1, session_present=0 '\000', userdata=139624665866896)
    at /home/salmon/Documents/code/aws-c-mqtt-generator/test/runtests.jl:23
#1 0x00007efcec14051b in jlcapi_on_connection_complete_485 ()
#2 0x00007efce6c27213 in s_packet_handler_connack (connection=0x19e43c0, message_cursor=...) at /workspace/srcdir/aws-c-mqtt/source/client_channel_handler.c:165
#3 0x00007efce6c27de6 in s_process_mqtt_packet (connection=0x19e43c0, packet_type=AWS_MQTT_PACKET_CONNACK, packet=...) at /workspace/srcdir/aws-c-mqtt/source/client_channel_handler.c:459
#4 0x00007efce6c2830a in s_process_read_message (handler=0x19e43e8, slot=0x7efce002d3b0, message=0x7efce000dac0) at /workspace/srcdir/aws-c-mqtt/source/client_channel_handler.c:573
#5 0x00007efce6c33416 in aws_channel_handler_process_read_message (handler=0x19e43e8, slot=0x7efce002d3b0, message=0x7efce000dac0) at /workspace/srcdir/aws-c-io/source/channel.c:1000
#6 0x00007efce6c32761 in aws_channel_slot_send_message (slot=0x7efce0001770, message=0x7efce000dac0, dir=AWS_CHANNEL_DIR_READ) at /workspace/srcdir/aws-c-io/source/channel.c:760
#7 0x00007efce6c4c2b6 in s_s2n_handler_process_read_message (handler=0x7efce0011f20, slot=0x7efce0001770, message=0x7efce00099c0)
    at /workspace/srcdir/aws-c-io/source/s2n/s2n_tls_channel_handler.c:538
#8 0x00007efce6c33416 in aws_channel_handler_process_read_message (handler=0x7efce0011f20, slot=0x7efce0001770, message=0x7efce00099c0) at /workspace/srcdir/aws-c-io/source/channel.c:1000
#9 0x00007efce6c32761 in aws_channel_slot_send_message (slot=0x7efce0011b60, message=0x7efce00099c0, dir=AWS_CHANNEL_DIR_READ) at /workspace/srcdir/aws-c-io/source/channel.c:760
#10 0x00007efce6c4fe3d in s_do_read (socket_handler=0x7efce0011e10) at /workspace/srcdir/aws-c-io/source/socket_channel_handler.c:164
#11 0x00007efce6c5013c in s_on_readable_notification (socket=0x7efce0000ca0, error_code=0, user_data=0x7efce0011e10) at /workspace/srcdir/aws-c-io/source/socket_channel_handler.c:221
#12 0x00007efce6c49252 in s_on_socket_io_event (event_loop=0x21f0810, handle=0x7efce0000d98, events=3, user_data=0x7efce0000ca0) at /workspace/srcdir/aws-c-io/source/posix/socket.c:1618
#13 0x00007efce6c41f20 in s_main_loop (args=0x21f0810) at /workspace/srcdir/aws-c-io/source/linux/epoll_event_loop.c:624
#14 0x00007efce6f2bada in thread_fn (arg=0x1dcb4c0) at /workspace/srcdir/aws-c-common/source/posix/thread.c:137
#15 0x00007efd195bf2a5 in start_thread (arg=0x7efce69c7640) at pthread_create.c:481
#16 0x00007efd194e7323 in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:95

```

---

<div class="post-metadata">

**Author:** ![vchuravy](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/vchuravy/32/8_2.png) [@vchuravy](https://discourse.julialang.org/u/vchuravy)\
**Post date:** [April 15, 2022, 5:05pm UTC](https://discourse.julialang.org/t/segfault-when-invoking-julia-callback-function-from-c/79541/4 "2022-04-15T17:05:50Z")

</div>

So yes this is a “foreign” thread and the segmentation fault you are seeing is because you are trying to interact with the runtime.

Also I am curious you define:

```julia
on_connection_complete_cb = @cfunction(
    on_connection_complete, 
    Cvoid,
    (Ptr{aws_mqtt_client_connection},Cint,Cint,Cuchar,Ptr{Cvoid}))

```

and the library defines it as:

```julia
typedef void(aws_mqtt_client_on_connection_complete_fn)(
    struct aws_mqtt_client_connection *connection,
    int error_code,
    enum aws_mqtt_connect_return_code return_code,
    bool session_present,
    void *userdata);

```

Are you sure that enum has the correct underlying type?

---

<div class="post-metadata">

**Author:** ![Octogonapus](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/octogonapus/32/9489_2.png) [@Octogonapus](https://discourse.julialang.org/u/Octogonapus)\
**Post date:** [April 15, 2022, 5:13pm UTC](https://discourse.julialang.org/t/segfault-when-invoking-julia-callback-function-from-c/79541/5 "2022-04-15T17:13:20Z")

</div>

> Are you sure that enum has the correct underlying type?

No, I’m not sure. `Cuchar` is my best guess from reading the Julia documentation. Please do let me know if you think it should be something else.

> So yes this is a “foreign” thread and the segmentation fault you are seeing is because you are trying to interact with the runtime.

Okay, good to know why this happens. It is also the case that if I don’t do much of anything in the callback (like simply `return nothing` and nothing else), the segfault does not occur. (even with `-O0`). I guess this lines up with your explanation.

I guess my question now becomes, how else I should be implementing this functionality? With your ForeignCallbacks package, it’s my understanding that:

1. I create a `ForeignCallback` instance.
2. Inside my `on_connection_complete` callback function, I `ccall` the `notify!` function.
3. All the logic I want to put into `on_connection_complete` is moved into the foreign callback closure.

Do I understand that correctly?

---

<div class="post-metadata">

**Author:** ![vchuravy](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/vchuravy/32/8_2.png) [@vchuravy](https://discourse.julialang.org/u/vchuravy)\
**Post date:** [April 15, 2022, 5:29pm UTC](https://discourse.julialang.org/t/segfault-when-invoking-julia-callback-function-from-c/79541/6 "2022-04-15T17:29:09Z")

</div>

> [@Octogonapus](#):
>
> Do I understand that correctly?

Yes, except for the point that you don’t need to `ccall` the notify function you can directly call it from your callback. In the demo I should probably use Julia’s `@threadcall` 🙂 to a custom callback which then constructs the message type and calls notify from there. I sadly got distracted from the project where I needed it for.

---

<div class="post-metadata">

**Author:** ![Octogonapus](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/octogonapus/32/9489_2.png) [@Octogonapus](https://discourse.julialang.org/u/Octogonapus)\
**Post date:** [April 15, 2022, 5:42pm UTC](https://discourse.julialang.org/t/segfault-when-invoking-julia-callback-function-from-c/79541/7 "2022-04-15T17:42:19Z")

</div>

Ok, I now have:

```julia
struct Message
    error_code::Cint
    return_code::Cint
    session_present::Cuchar
end

callback = ForeignCallbacks.ForeignCallback{Message}() do msg
    @show msg
    count_down(received_on_connection_complete)
    return nothing
end
token = ForeignCallbacks.ForeignToken(callback)

function on_connection_complete(connection::Ptr{aws_mqtt_client_connection}, error_code::Cint, return_code::Cint, session_present::Cuchar, userdata::Ptr{Cvoid})
    ForeignCallbacks.notify!(token, Message(error_code, return_code, session_present))
    return nothing
end

```

Which results in this segfault:

```julia
(rr) c
Continuing.
client = Ptr{aws_mqtt_client} @0x00000000046ab6a0
connection = Ptr{aws_mqtt_client_connection} @0x00000000047143e0
host_name_cur = Base.RefValue{aws_byte_cursor}(aws_byte_cursor(0x000000000000002e, Ptr{UInt8} @0x00007f0d761ebe98))
connection = Ptr{aws_mqtt_client_connection} @0x00000000047143e0
on_connection_complete_cb = Ptr{Nothing} @0x00007f0d5cdd8d70
[New Thread 180422.180467]
[New Thread 180422.180466]
[New Thread 180422.180423]

Thread 3 received signal SIGSEGV, Segmentation fault.
[Switching to Thread 180422.180466]
0x00007f0d5cdcceea in julia_on_connection_complete_488 (connection=16, error_code=0, return_code=80, session_present=0 '\000', userdata=1) at /home/salmon/Documents/code/aws-c-mqtt-generator/test/runtests.jl:40
40 function on_connection_complete(connection::Ptr{aws_mqtt_client_connection}, error_code::Cint, return_code::Cint, session_present::Cuchar, userdata::Ptr{Cvoid})
(rr) bt
#0 0x00007f0d5cdcceea in julia_on_connection_complete_488 (connection=16, error_code=0, return_code=80, session_present=0 '\000', userdata=1)
    at /home/salmon/Documents/code/aws-c-mqtt-generator/test/runtests.jl:40
#1 0x00007f0d5cdd8dfb in jlcapi_on_connection_complete_493 ()
#2 0x00007f0d578bf213 in s_packet_handler_connack (connection=0x47143e0, message_cursor=...) at /workspace/srcdir/aws-c-mqtt/source/client_channel_handler.c:165
#3 0x00007f0d578bfde6 in s_process_mqtt_packet (connection=0x47143e0, packet_type=AWS_MQTT_PACKET_CONNACK, packet=...) at /workspace/srcdir/aws-c-mqtt/source/client_channel_handler.c:459
#4 0x00007f0d578c030a in s_process_read_message (handler=0x4714408, slot=0x7f0d4802d3b0, message=0x7f0d4800dac0) at /workspace/srcdir/aws-c-mqtt/source/client_channel_handler.c:573
#5 0x00007f0d578cb416 in aws_channel_handler_process_read_message (handler=0x4714408, slot=0x7f0d4802d3b0, message=0x7f0d4800dac0) at /workspace/srcdir/aws-c-io/source/channel.c:1000
#6 0x00007f0d578ca761 in aws_channel_slot_send_message (slot=0x7f0d48001770, message=0x7f0d4800dac0, dir=AWS_CHANNEL_DIR_READ) at /workspace/srcdir/aws-c-io/source/channel.c:760
#7 0x00007f0d578e42b6 in s_s2n_handler_process_read_message (handler=0x7f0d48011f20, slot=0x7f0d48001770, message=0x7f0d480099c0)
    at /workspace/srcdir/aws-c-io/source/s2n/s2n_tls_channel_handler.c:538
#8 0x00007f0d578cb416 in aws_channel_handler_process_read_message (handler=0x7f0d48011f20, slot=0x7f0d48001770, message=0x7f0d480099c0) at /workspace/srcdir/aws-c-io/source/channel.c:1000
#9 0x00007f0d578ca761 in aws_channel_slot_send_message (slot=0x7f0d48011b60, message=0x7f0d480099c0, dir=AWS_CHANNEL_DIR_READ) at /workspace/srcdir/aws-c-io/source/channel.c:760
#10 0x00007f0d578e7e3d in s_do_read (socket_handler=0x7f0d48011e10) at /workspace/srcdir/aws-c-io/source/socket_channel_handler.c:164
#11 0x00007f0d578e813c in s_on_readable_notification (socket=0x7f0d48000ca0, error_code=0, user_data=0x7f0d48011e10) at /workspace/srcdir/aws-c-io/source/socket_channel_handler.c:221
#12 0x00007f0d578e1252 in s_on_socket_io_event (event_loop=0x38aa850, handle=0x7f0d48000d98, events=3, user_data=0x7f0d48000ca0) at /workspace/srcdir/aws-c-io/source/posix/socket.c:1618
#13 0x00007f0d578d9f20 in s_main_loop (args=0x38aa850) at /workspace/srcdir/aws-c-io/source/linux/epoll_event_loop.c:624
#14 0x00007f0d57bc3ada in thread_fn (arg=0x468c210) at /workspace/srcdir/aws-c-common/source/posix/thread.c:137
#15 0x00007f0d8a2572a5 in start_thread (arg=0x7f0d4f657640) at pthread_create.c:481
#16 0x00007f0d8a17f323 in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:95

```

I guess I misunderstand something about how to use ForeignCallbacks. Do you see the error?

It is very odd to me that `connection=16` in the Julia callback invocation.

Thank you for your help so far.

---

<div class="post-metadata">

**Author:** ![vchuravy](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/vchuravy/32/8_2.png) [@vchuravy](https://discourse.julialang.org/u/vchuravy)\
**Post date:** [April 15, 2022, 5:51pm UTC](https://discourse.julialang.org/t/segfault-when-invoking-julia-callback-function-from-c/79541/8 "2022-04-15T17:51:18Z")

</div>

Yes that is very weird. That smells like an ABI disagreement between your `@cfunction` and what C expects.

You can’t pass in `token` as a global. Do you have control of `userdata`? If so that would be the place to pass it through.

---

<div class="post-metadata">

**Author:** ![Octogonapus](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/octogonapus/32/9489_2.png) [@Octogonapus](https://discourse.julialang.org/u/Octogonapus)\
**Post date:** [April 15, 2022, 5:52pm UTC](https://discourse.julialang.org/t/segfault-when-invoking-julia-callback-function-from-c/79541/9 "2022-04-15T17:52:53Z")

</div>

> That smells like an ABI disagreement between your `@cfunction` and what C expects.

Indeed. I’ve spent a while debugging this, so if you have any insights or debugging tips, I would very much appreciate it.

> You can’t pass in `token` as a global. Do you have control of `userdata` ? If so that would be the place to pass it through.

Yes, I do. I’ll go that route.

---

<div class="post-metadata">

**Author:** ![Octogonapus](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/octogonapus/32/9489_2.png) [@Octogonapus](https://discourse.julialang.org/u/Octogonapus)\
**Post date:** [April 19, 2022, 1:34pm UTC](https://discourse.julialang.org/t/segfault-when-invoking-julia-callback-function-from-c/79541/10 "2022-04-19T13:34:30Z")

</div>

@vchuravy I am quite stuck on this problem now. If you have any ideas on how I can continue debugging this, I would appreciate hearing them.

---

<div class="post-metadata">

**Author:** ![Octogonapus](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/octogonapus/32/9489_2.png) [@Octogonapus](https://discourse.julialang.org/u/Octogonapus)\
**Post date:** [April 20, 2022, 3:51pm UTC](https://discourse.julialang.org/t/segfault-when-invoking-julia-callback-function-from-c/79541/11 "2022-04-20T15:51:50Z")

</div>

Thanks to Valentin for meeting with me offline. We arrived at a final solution to this problem:

Declare a callback this way:

```julia
struct Message
	error_code::Cint
	return_code::Cint
	session_present::Cuchar
end

callback = ForeignCallbacks.ForeignCallback{Message}() do msg
	# actual callback impl in here
	return nothing
end

function on_connection_complete(connection::Ptr{aws_mqtt_client_connection}, error_code::Cint, return_code::Cint, session_present::Cuchar, userdata::Ptr{Cvoid})
	# convert and then load the pointer because this function must be type-stable
	token = Base.unsafe_load(Base.unsafe_convert(Ptr{ForeignCallbacks.ForeignToken}, userdata))
	ForeignCallbacks.notify!(token, Message(error_code, return_code, session_present))
	return nothing
end

```

Store the `user_data` this way:

```julia
token = Ref(ForeignCallbacks.ForeignToken(callback))
Base.unsafe_convert(Ptr{Cvoid}, token) # this is the user_data

```

Then you can create the callback C function:

```julia
on_connection_complete_cb = @cfunction(on_connection_complete, Cvoid, (Ptr{aws_mqtt_client_connection},Cint,Cint,Cuchar,Ptr{Cvoid}))

```

Relevant variables like `token` and `on_connection_complete_cb` must be `GC.@preserve`-ed while in use.