Question

Can anyone please tell me,

• what exactly is meant by registering a callback function in C (with some examples)?
• what are Notify callbacks ?
• what are Asynchronous call backs?

Answer 1

Registering a callback function simply means that you are arranging for an external entity to call your function.

It might happen at a later time, or it might happen straight away. A straightforward example is qsort. It is declared like this:

void qsort(void *base, size_t nel, size_t width,
       int (*compar)(const void *, const void *));

In order to use it, you must pass a pointer to a function that compares elements – the callback.

That was a simple example but generally “registering a callback” means passing a function pointer to someone who will call it for you in the future.