How can I casting void * to void **?

Question

I make some project to using nRF52833 by CPP.

I need to add NUS(Nordic Uart Service) in my project. But I have some problem.

Nrf library NUS using ble_nus.c file. I try to build by CPP compiler but some error occur.

Error message is this. Error[Pe167]: argument of type "void *" is incompatible with parameter of type "void **" C:\git\heden_ble\src\components\ble\ble_services\ble_nus\ble_nus.c 139

ble_nus.c file has four error points.

How can I casting void * to void **

ble_nus_client_context_t * p_client = NULL;

err_code = blcm_link_ctx_get(p_nus->p_link_ctx_storage, p_ble_evt->evt.gap_evt.conn_handle, (void *) &p_client);

ret_code_t blcm_link_ctx_get(blcm_link_ctx_storage_t const * const p_link_ctx_storage, uint16_t const conn_handle, void ** const pp_ctx_data)

Using CPP static_cast, reinterpret_cast. I expect to build ble_nus.c

p_client is already a pointer. When you takes it address, e.g. &p_client the type is ble_nus_client_context_t**. Your cast with (void *) &p_client is casting to void* (legal in C), not ble_nus_client_context_t** - so you must account for the additional level of indirection when you cast back from void*. — David C. Rankin
– David C. Rankin, Commented Dec 26, 2023 at 9:09
@DavidC.Rankin Is it not ble_nus_client_context_t**? Or is that an alias? — Passer By
– Passer By, Commented Dec 26, 2023 at 9:10
@PasserBy - yes, sorry, but the additional level of indirection holds. This is made a bit more troubling by the mixing of C++ and ble_nus.c. Please provide A Minimal Complete Reproducable Example to help clear up the confusion. — David C. Rankin
– David C. Rankin, Commented Dec 26, 2023 at 9:12
Why are you trying to compile c code with a c++ compiler? Why not just use a c compiler, they're separate languages, c++ isn't a superset of c — Alan Birtles
– Alan Birtles, Commented Dec 26, 2023 at 9:49
Why not show us a snippet of code that will reproduce the error so that we don't have to guess? — Ted Lyngmo
– Ted Lyngmo, Commented Dec 26, 2023 at 9:51

SHR · Accepted Answer · 2023-12-26 11:45:09Z

0

&p_client is a pointer to a pointer (two-levels).

You cast a pointer to a pointer (two-levels) to a (one-level) pointer.

If you remove the casting of the last parameter, it should work just fine.

Like:

ble_nus_client_context_t * p_client = NULL;
err_code = blcm_link_ctx_get(p_nus->p_link_ctx_storage, p_ble_evt->evt.gap_evt.conn_handle, &p_client);

Another option is to cast to void** as expected.

ble_nus_client_context_t * p_client = NULL;
err_code = blcm_link_ctx_get(p_nus->p_link_ctx_storage, p_ble_evt->evt.gap_evt.conn_handle, (void**)&p_client);

answered Dec 26, 2023 at 11:45

SHR

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