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)
# 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:
