# Segfault using @cfunction in callback for Mosquitto MQTT client, no idea what could be wrong

**URL:** <https://discourse.julialang.org/t/segfault-using-cfunction-in-callback-for-mosquitto-mqtt-client-no-idea-what-could-be-wrong/70430>\
**Category:** General Usage\
**Tags:** question\
**Created:** [October 26, 2021, 7:22pm UTC](https://discourse.julialang.org/t/segfault-using-cfunction-in-callback-for-mosquitto-mqtt-client-no-idea-what-could-be-wrong/70430 "2021-10-26T19:22:44Z")\
**Posts on this page:** 11\
**Page:** 1

<div class="post-metadata">

**Author:** ![Christian\_Dengler](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/christian_dengler/32/20969_2.png) [@Christian\_Dengler](https://discourse.julialang.org/u/Christian_Dengler)\
**Post date:** [October 26, 2021, 7:22pm UTC](https://discourse.julialang.org/t/segfault-using-cfunction-in-callback-for-mosquitto-mqtt-client-no-idea-what-could-be-wrong/70430/1 "2021-10-26T19:22:44Z")

</div>

Hello, I have been stuck trying to wrap the Mosquitto C API v2.0.12 in Julia. While I managed to get connection and publishing messages done without major problems, I cannot seem to get any callback function to work. To define the callback I use the @cfunction macro, with the return and argument types as indicates in the c headers, but I receive a segfault when the callback is supposed to be called. The connect and loop\_start work fine, I also use them for the publishing, only the callback gives me headaches. Maybe someone can spot a mistake in this example (the code is also runnable as is, but requires a running MQTT service and libmosquitto)

```julia
# Define C structs and wrappers
struct Cmosquitto end

const libmosquitto = "libmosquitto.so.1" 

function mosquitto_new(id::String, clean_start::Bool, obj)
    return ccall((:mosquitto_new, libmosquitto), Ptr{Cmosquitto}, (Cstring, Bool, Ptr{Cvoid}), id, clean_start, obj)
end

function connect(client::Ptr{Cmosquitto}, host::String; port::Int = 1883, keepalive::Int = 60)
    msg_nr = ccall((:mosquitto_connect, libmosquitto), Cint, (Ptr{Cmosquitto}, Cstring, Cint, Cint), client, host, port, keepalive)
    return msg_nr
end

function loop_start(client::Ptr{Cmosquitto})
    msg_nr = ccall((:mosquitto_loop_start, libmosquitto), Cint, (Ptr{Cmosquitto},), client)
    return msg_nr
end

function connect_callback_set(client::Ptr{Cmosquitto}, cfunc)
    msg_nr = ccall((:mosquitto_connect_callback_set, libmosquitto), Cint, (Ptr{Cmosquitto}, Ptr{Cvoid}), client, cfunc)
    return msg_nr
end

# callback function
callback_connect_jl(mos::Ptr{Cmosquitto}, obj::Ptr{Cvoid}, msg_nr::Cint) = println("Connection ok")
callback_connect_c = @cfunction(callback_connect_jl, Cvoid, (Ptr{Cmosquitto}, Ptr{Cvoid}, Cint))

## Script
# init clib
ccall((:mosquitto_lib_init, libmosquitto), Cint, ())

# create mosquitto object
cobj = Ref{Cvoid}()
testclient = mosquitto_new("testclient", true, cobj)

#message_callback_set(testclient, cfunc_message)
connect_callback_set(testclient, callback_connect_c)

connect(testclient, "localhost")
loop_start(testclient) # segfault

```

(docs for Mosquitto:

- [https://github.com/eclipse/mosquitto/blob/master/include/mosquitto.h](https://github.com/eclipse/mosquitto/blob/master/include/mosquitto.h)
- [mosquitto.h](https://mosquitto.org/api/files/mosquitto-h.html) )

---

<div class="post-metadata">

**Author:** ![Sukera](https://avatars.discourse-cdn.com/v4/letter/s/ce7236/32.png) [@Sukera](https://discourse.julialang.org/u/Sukera)\
**Post date:** [October 26, 2021, 8:25pm UTC](https://discourse.julialang.org/t/segfault-using-cfunction-in-callback-for-mosquitto-mqtt-client-no-idea-what-could-be-wrong/70430/2 "2021-10-26T20:25:21Z")

</div>

> [@Christian\_Dengler](#):
>
> `callback_connect_c = @cfunction(callback_connect_jl, Cvoid, (Ptr{Cmosquitto}, Ptr{Cvoid}, Cint))`

I think this should have a return type of `Nothing` instead, as that’s what your function is returning. I don’t have MQTT set up though, so I can’t test it.

As an aside, have you considered using [BinaryBuilder](https://binarybuilder.org) to create the wrapper? It should make using this a bunch easier, though the callback will probably still need some fiddling. At least you won’t have to define the `ccall` stuff yourself. (And you’d also make it available for install via `Pkg`)

---

<div class="post-metadata">

**Author:** ![krrutkow](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/krrutkow/32/9138_2.png) [@krrutkow](https://discourse.julialang.org/u/krrutkow)\
**Post date:** [October 26, 2021, 8:53pm UTC](https://discourse.julialang.org/t/segfault-using-cfunction-in-callback-for-mosquitto-mqtt-client-no-idea-what-could-be-wrong/70430/3 "2021-10-26T20:53:48Z")

</div>

Not sure if it helps, but you might save some time by automatically generating the C bindings with [CBinding.jl](https://github.com/analytech-solutions/CBinding.jl).

```julia
module libmosquitto
  using CBinding

  c`-fparse-all-comments -I$(mosquitto_path)/include -L$(mosquitto_path)/lib -lmosquitto`

  const c"size_t" = Csize_t
  const c"uint8_t" = UInt8
  const c"uint16_t" = UInt16
  const c"uint32_t" = UInt32
  const c"uint64_t" = UInt64

  c"""
    #include <mosquitto.h>
  """ji
end

```

It would generate somewhat usable documentation too…

```julia
help?> libmosquitto.mosquitto_connect
  int mosquitto_connect(struct mosquitto *mosq, const char *host, int port, int keepalive)

  Defined at mosquitto.h:494 (file:///usr/include/mosquitto.h)

  ======================================================================

  Details
  =========

  Section: Connecting, reconnecting, disconnecting

  ======================================================================

  Function: mosquitto_connect

  Connect to an MQTT broker.

  It is valid to use this function for clients using all MQTT protocol versions. If you need to set MQTT v5 CONNECT properties, use
  <mosquitto_connect_bind_v5> instead.

  Parameters: mosq - a valid mosquitto instance. host - the hostname or ip address of the broker to connect to. port - the network port to
  connect to. Usually 1883. keepalive - the number of seconds after which the broker should send a PING message to the client if no other
  messages have been exchanged in that time.

  Returns: MOSQ_ERR_SUCCESS - on success. MOSQ_ERR_INVAL - if the input parameters were invalid, which could be any of: * mosq == NULL * host ==
  NULL * port < 0 * keepalive < 5 MOSQ_ERR_ERRNO - if a system call returned an error. The variable errno contains the error code, even on
  Windows. Use strerror_r() where available or FormatMessage() on Windows.

  See Also: <mosquitto_connect_bind>, <mosquitto_connect_async>, <mosquitto_reconnect>, <mosquitto_disconnect>, <mosquitto_tls_set>

```

---

<div class="post-metadata">

**Author:** ![giordano](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/giordano/32/2166_2.png) [@giordano](https://discourse.julialang.org/u/giordano)\
**Post date:** [October 26, 2021, 9:38pm UTC](https://discourse.julialang.org/t/segfault-using-cfunction-in-callback-for-mosquitto-mqtt-client-no-idea-what-could-be-wrong/70430/4 "2021-10-26T21:38:20Z")

</div>

> [@Sukera](#):
>
> I think this should have a return type of `Nothing` instead, as that’s what your function is returning.

```julia
julia> Cvoid
Nothing

```

---

<div class="post-metadata">

**Author:** ![thofma](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/thofma/32/1691_2.png) [@thofma](https://discourse.julialang.org/u/thofma)\
**Post date:** [October 27, 2021, 5:49am UTC](https://discourse.julialang.org/t/segfault-using-cfunction-in-callback-for-mosquitto-mqtt-client-no-idea-what-could-be-wrong/70430/5 "2021-10-27T05:49:17Z")

</div>

> [@Christian\_Dengler](#):
>
> `callback_connect_jl(mos::Ptr{Cmosquitto}, obj::Ptr{Cvoid}, Cint)`

This looks wrong. Surely you don’t want an argument named `Cint`, but an argument of type `Cint`? This would also match with the callback the line below.

---

<div class="post-metadata">

**Author:** ![Christian\_Dengler](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/christian_dengler/32/20969_2.png) [@Christian\_Dengler](https://discourse.julialang.org/u/Christian_Dengler)\
**Post date:** [October 27, 2021, 3:57pm UTC](https://discourse.julialang.org/t/segfault-using-cfunction-in-callback-for-mosquitto-mqtt-client-no-idea-what-could-be-wrong/70430/6 "2021-10-27T15:57:40Z")

</div>

Indeed, the argument was wrong, unfortunately correcting this mistake didnt change the result, still segfault ☹  
Ill update the code on top with the correction.

---

<div class="post-metadata">

**Author:** ![Christian\_Dengler](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/christian_dengler/32/20969_2.png) [@Christian\_Dengler](https://discourse.julialang.org/u/Christian_Dengler)\
**Post date:** [October 27, 2021, 4:37pm UTC](https://discourse.julialang.org/t/segfault-using-cfunction-in-callback-for-mosquitto-mqtt-client-no-idea-what-could-be-wrong/70430/7 "2021-10-27T16:37:27Z")

</div>

Thats a nice package! I tried a version using it, to make sure I didnt make a mistake when copying from the header, but still Segfault is all I get. I have no idea why this just doesnt want to work 😛

```julia
# C wrapper lib
module libmosquitto
  using CBinding
  mosquitto_path = raw"mypathtoadapt/mosquitto-2.0.12"
  c`-fparse-all-comments -I$(mosquitto_path)/include -L$(mosquitto_path)/lib -lmosquitto`

  const c"size_t" = Csize_t
  const c"uint8_t" = UInt8
  const c"uint16_t" = UInt16
  const c"uint32_t" = UInt32
  const c"uint64_t" = UInt64

  c"""
    #include <mosquitto.h>
  """ji
end

# Callback function
callback_connect_jl(mos::Ptr{libmosquitto.mosquitto}, obj::Ptr{Cvoid}, unused::Cint) = println("Connection ok")
callback_connect_c = @cfunction(callback_connect_jl, Cvoid, (Ptr{libmosquitto.mosquitto}, Ptr{Cvoid}, Cint))

## Script 
# Init
libmosquitto.mosquitto_lib_init()

# create mosquitto object
cobj = Ref{Cvoid}()
testclient = libmosquitto.mosquitto_new("testclient", true, cobj)

# set callback
libmosquitto.mosquitto_connect_callback_set(testclient, callback_connect_c)

# connect and start loop
libmosquitto.mosquitto_connect(testclient, "localhost", 1883, 60)
libmosquitto.mosquitto_loop_start(testclient) # segfault

```

---

<div class="post-metadata">

**Author:** ![krrutkow](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/krrutkow/32/9138_2.png) [@krrutkow](https://discourse.julialang.org/u/krrutkow)\
**Post date:** [October 27, 2021, 4:47pm UTC](https://discourse.julialang.org/t/segfault-using-cfunction-in-callback-for-mosquitto-mqtt-client-no-idea-what-could-be-wrong/70430/8 "2021-10-27T16:47:49Z")

</div>

Creating the `cobj` as a Ref could be a problem too. Does it segfault when you pass a null pointer for the context?

```julia
testclient = libmosquitto.mosquitto_new("testclient", true, C_NULL)

```

---

<div class="post-metadata">

**Author:** ![Christian\_Dengler](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/christian_dengler/32/20969_2.png) [@Christian\_Dengler](https://discourse.julialang.org/u/Christian_Dengler)\
**Post date:** [October 27, 2021, 4:52pm UTC](https://discourse.julialang.org/t/segfault-using-cfunction-in-callback-for-mosquitto-mqtt-client-no-idea-what-could-be-wrong/70430/9 "2021-10-27T16:52:46Z")

</div>

Unfortunately yes, I have tried that, and even a run with GC disabled.

---

<div class="post-metadata">

**Author:** ![Christian\_Dengler](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/christian_dengler/32/20969_2.png) [@Christian\_Dengler](https://discourse.julialang.org/u/Christian_Dengler)\
**Post date:** [October 28, 2021, 8:46pm UTC](https://discourse.julialang.org/t/segfault-using-cfunction-in-callback-for-mosquitto-mqtt-client-no-idea-what-could-be-wrong/70430/10 "2021-10-28T20:46:36Z")

</div>

I made a little progress. I found out that the “mosquitto\_loop\_forever” works just fine, so I assume it has something to do with the interaction of whatever mosquitto uses for threading and julia cfunctions. Ill see to just avoid “mosquitto\_loop\_start” in my wrapper then.

---

<div class="post-metadata">

**Author:** ![Christian\_Dengler](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/christian_dengler/32/20969_2.png) [@Christian\_Dengler](https://discourse.julialang.org/u/Christian_Dengler)\
**Post date:** [October 29, 2021, 10:11pm UTC](https://discourse.julialang.org/t/segfault-using-cfunction-in-callback-for-mosquitto-mqtt-client-no-idea-what-could-be-wrong/70430/11 "2021-10-29T22:11:11Z")

</div>

After some more trouble with @cfunction I now have a first working version, for those interested:[https://github.com/denglerchr/Mosquitto.jl](https://github.com/denglerchr/Mosquitto.jl)