Keep on getting a segmentation fault when running ...

# kotlin-native

[napperley]

Keep on getting a segmentation fault when running the Kotlin Native program, and wonder if the macro wrapper (in the def file) is correct:

# ...
---
static inline MQTTClient_connectOptions createMqttConnectionOptions() {
    MQTTClient_connectOptions options = MQTTClient_connectOptions_initializer;
    return options;
}

[napperley]

Below is the function where the segmentation fault occurs (on the line where the MQTTClient_connect function is called):

// ...
private fun setupMqttSubscription(settings: Map<String, String>): Unit = memScoped {
    val address = settings["address"] ?: ""
    val clientId = settings["clientId"] ?: ""
    val topic = settings["topic"] ?: ""
    val client = allocArray<MQTTClientVar>(1)
    val connOptions = createMqttConnectionOptions()
    val qos = 1

    MQTTClient_create(client, address, clientId, MQTTCLIENT_PERSISTENCE_NONE, null)
    connOptions.ptr.pointed.keepAliveInterval = 20
    connOptions.ptr.pointed.cleansession = 1
    MQTTClient_setCallbacks(client, null, null, null, staticCFunction(::onMessageReceived))
    println("Connecting to MQTT Broker...")
    val rc = MQTTClient_connect(client, connOptions)
    if (rc != MQTTCLIENT_SUCCESS) {
        println("Failed to connect (error code $rc)")
        exit(-1)
    }
    println("Subscribing to topic $topic for client $clientId using QoS $qos...")
    MQTTClient_subscribe(client, topic, qos)
}
// ...

[msink]

Of course it is incorrect - classical "use after free".
Add static before MQTTClient_connectOption

[napperley]

The MQTTClientVar type with the read only client property is the one I'am not sure about. Here are the relevant definitions in the knm file:

public typealias MQTTClient = kotlinx.cinterop.COpaquePointer

public typealias MQTTClientVar = kotlinx.cinterop.CPointerVarOf<paho_mqtt.MQTTClient>

[napperley]

Do wonder if the MQTTClient_create function is properly initialising client.

[napperley]

Here is the location in the Paho MQTT lib where the seg fault occurs: https://github.com/eclipse/paho.mqtt.c/blob/d631449f16ec78c37e063c220eb6cfc546a4facb/src/MQTTClient.c#L1503

Looks as though the client property isn't being properly initialised by the MQTTClient_create function.

[napperley]

Here is the truncated output from valgrind:

==16462== Using Valgrind-3.13.0 and LibVEX; rerun with -h for copyright info
==16462== Command: ./mqtt_client.kexe
==16462== 
Starting MQTT Client...
Settings:
* address: <tcp://xxx.xx.xx:xxxx>
* clientId: ExampleClientSub
* topic: MQTT Examples
* timeout: 10000
* qos: 1
Connecting to MQTT Broker...
==16462== Invalid read of size 4
==16462==    at 0x64731AA: MQTTClient_connect (MQTTClient.c:1503)

[napperley]

A bit puzzling when the client property's first element contains a pointer after a call to the MQTTClient_create function, eg: CPointer(raw=0x545d14)

[napperley]

Discovered what was really causing the segmentation fault. Was passing through a pointer to the array (MQTTClient_connect(client, connOptions)) instead of passing through a pointer to the first element in the array (MQTTClient_connect(client[0], connOptions)) 🤦‍♂️ .