Getting stuck with translating a C Macro (creates ...

# kotlin-native

[napperley]

Getting stuck with translating a C Macro (creates a Struct? - https://www.eclipse.org/paho/files/mqttdoc/MQTTClient/html/_m_q_t_t_client_8h.html#aefd7c865f2641c8155b763fdf3061c25) into a custom def declaration (https://github.com/JetBrains/kotlin-native/blob/master/INTEROP.md#adding-custom-declarations) that will create a instance of MQTTClient_connectOptions (generated class in the KLib). This is part of a Kotlin Native program which uses the Eclipse Paho MQTT Client library (https://www.eclipse.org/paho/clients/c/).

[msink]

Something like this, maybe:

static inline void MQTTClient_connectOptions_initialize(struct MQTTClient_connectOptions *value) {
    *value = MQTTClient_connectOptions_initializer;
}

[olonho]

yeah, Mike got the general pattern right - if you see no way to do smth in K/N, just add C function into .def file and do it there. However, we will investigate how to do structure initialization right

[napperley]

Especially tricky when it is not clear which order the properties in a struct are being initialised (when it comes to default values).

[napperley]

Is there a way to get more informative error messages from the cinterop tool?. End up with the following error after adding in the workaround in the def file:

Exception in thread "main" java.lang.Error: /tmp/tmp18083280284659516124.c:3:14: error: expected expression

[msink]

Well, yes - this was not valid C.
But this should work:

static inline void MQTTClient_connectOptions_initialize(MQTTClient_connectOptions *value) {
    MQTTClient_connectOptions initializer = MQTTClient_connectOptions_initializer;
    *value = initializer;
}

[svyatoslav.scherbina]

@napperley you may be interested in taking a look at this Kotlin/Native application using MQTT:
https://github.com/madhead/seaowl

For example, .def file:
https://github.com/madhead/seaowl/blob/master/sensor/src/main/c_interop/mqtt.def

[svyatoslav.scherbina]

By the way, this project is written by @madhead

[napperley]

This error is outputted by konanc when trying to get a pointer from a CVariable (MQTTClientVar):

error: unresolved reference. None of the following candidates is applicable because of receiver type mismatch: 
public final val <T : CVariable> CValues<MQTTClientVar /* = CPointerVarOf<MQTTClient /* = CPointer<out CPointed> */> */>.ptr: CPointer<MQTTClientVar /* = CPointerVarOf<MQTTClient /* = CPointer<out CPointed> */> */> defined in kotlinx.cinterop.MemScope
	createMqttClient(client.ptr, brokerUrl, clientId, MQTTCLIENT_PERSISTENCE_NONE, null)

[napperley]

Here is the Klib definition:

fun MQTTClient_create(handle: CValuesRef<MQTTClientVar /* = CPointerVarOf<MQTTClient /* = CPointer<out CPointed> */> */>?, serverURI: String?, clientId: String?, persistence_type: Int, persistence_context: CValuesRef<*>?): Int

[madhead]

How do you define client?

[napperley]

Like this:

memScoped {
  val client = alloc<MQTTClientVar>()
  //...
}

[olonho]

could you provide self-containing non-compiling source, please?

[napperley]

See the MQTT Sample snippet (https://kotlinlang.slack.com/files/U5R5FGVH7/FBEGLHWF6/mqtt_sample.kt).

[napperley]

MQTT_Sample.kt