Passing functions as parameters to button handlers in C

Question

I am working on a smartwatch app written in C and I am trying to pass a function pointer as a parameter into my button handlers so it can be called from in there. So far I ve done the following but i get an error saying that the object I am calling is not a function.

void func(){
    ...
}

static void select_click_handler(ClickRecognizerRef recognizer, void *context){
    context();
}

static void click_config_provider(void *context) {
    window_single_click_subscribe(BUTTON_ID_SELECT, select_click_handler);
    window_set_click_context(BUTTON_ID_SELECT, context);
}

int main(void){
    ...
    action_bar = action_bar_layer_create();
    action_bar_layer_set_click_config_provider(action_bar, click_config_provider);
    action_bar_layer_set_context(action_bar, &func);
}

Documentation:

action_bar_layer_create

action_bar_layer_set_click_config_provider

action_bar_layer_set_context

Answer 1

In select_click_handler() function context is void * so you need to typecast it appropriately before calling it.

Assuming context is func you can do this

static void select_click_handler(ClickRecognizerRef recognizer, void *context){
    ((void (*)(void)) context)();
}

Also in this line action_bar_layer_set_context(action_bar, &func); you don't need & just pass func not &func.

Answer 2

static void select_click_handler(ClickRecognizerRef recognizer, void *context){
context();
}

In this, context is only pointer variable, not a function. so, You have to typecast or define it properly to use it for function calling.

But you are passing function pointer as argument. so, i hope these links help you

Function pointer as an argument

Answer 3

In C, void* is used in many places to stand in for a generic pointer which can either be a pointer to a variable or a pointer to a function. You are making use of the latter.

At the point of use you need to cast it to the correct type else the runtime will not know what to do. In your case, the correct type is a function that takes no arguments and returns a void.

To do that, use ((void (*)(void *))context)() at the calling site; within select_click_handler