Параметр функции по умолчанию - C (СИ)

int add_after (int val, int pos = -1) {
struct list *current;
struct list *adding;
int i = 0;
 
    if (pos < 0 || pos > size) {
        return -1;
    }
 
    adding = (struct list *)malloc(sizeof(struct list));
    if (size == 0) {
        adding->previous = NULL;
        adding->val = val;
        adding->next = NULL;
        ++size;
        return size;
    }
    for (i = 0; i <  pos; i++) {
        current = current->next;
    }
    adding->previous = current;
    adding->next = current->next;
    current->next = adding;
    ++size;
    return size;
}

int add_after (int val, int pos = -1) {