Trouble figuring out syntax for pointer casting

Question

I was working today when I came across the following function:

void button_scheduler_event_handler(void *p_event_data, uint16_t event_size)
{
    button_handler(*((nrf_drv_gpiote_pin_t*)p_event_data));
}

with button handler defined as the following:

void button_handler(nrf_drv_gpiote_pin_t pin)

I'm not sure Í have encountered this syntax:

button_handler(*((nrf_drv_gpiote_pin_t*)p_event_data));

before and I am a bit bewildered. My first thought is that the syntax for calling button_handler should be:

button_handler(((*nrf_drv_gpiote_pin_t) *p_event_data));

I do however not have any good explanation for why.

Can anyone explain this?

Edit: after input from user UnholySheep

`*((nrf_drv_gpiote_pin_t*)p_event_data)` is a (pointer) cast followed by a dereferencing. This has nothing to do with function pointers — UnholySheep, Sep 03 '18 at 13:30
Since `nrf_drv_gpiote_pin_t` is a type, `nrf_drv_gpiote_pin_t*` is also a type, and `*nrf_drv_gpiote_pin_t` is nonsense. — molbdnilo, Sep 03 '18 at 14:00
With `void *p_event_data`, `*p_event_data` is invalid: attempt to de-reference a `void *`. — chux - Reinstate Monica, Sep 03 '18 at 18:57

score 1 · Accepted Answer · answered Sep 03 '18 at 14:10

1

The p_event_data parameter is declared as a void *, meaning it could point to anything. However, based on the fact that it calls button_handler, it suggests that p_event_data is actually pointing to a nrf_drv_gpiote_pin_t.

So first you need to cast the void * to a nrf_drv_gpiote_pin_t *:

(nrf_drv_gpiote_pin_t  *)p_event_data

Then you need to dereference that pointer:

*((nrf_drv_gpiote_pin_t  *)p_event_data)

And that's what gets passed to button_handler.

answered Sep 03 '18 at 14:10

dbush

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I got to the same conclusion after UnholySheep pointed me in the right direction! - Never the less thanks for taking the time to answer! – Casper Knudsen Sep 04 '18 at 09:47

Trouble figuring out syntax for pointer casting

1 Answers1