How can I use custom datatypes in my personal C libraries

I have implemented a linked list library in C. It has all the basic functions to quickly implement a linked list. I stored all that in a header file and an implementation file.

Now I want my linked list to be able to store more information than the standard datatypes like int, char, etc.

So I build everything around a custom datatype_t.

To give an example, this is part of the header file.

typedef int datatype_t;

typedef struct node_s
{
    datatype_t key;
    struct node_s *next_node_p;
    struct node_s *previous_node_p;
}node_t;

//add an element to the rear of the list
int         list_element_add(datatype_t new_key, list_t *list);

Now I have the problem that I want the custom datatype to be a datatype that is defined outside of the library. For example a struct. I think that is the usual case for a linked list. What options do I have to use the functions of my library.

Replace datatype_t with in void* library. This allows you to store pointers to any data type (e.g structs, ints, chars, etc.)

typedef struct node_s
{
    void* key;
    struct node_s *next_node_p;
    struct node_s *previous_node_p;
} node_t;

int list_element_add(void* new_key, list_t *list);

since you are making the program more generic so you may face runtime error obviously, and the one who is using the library must manage memory manually using malloc/free