在 C 中处理空指针

Question

I'm making a C code to handle sparse matrixes, and i want to modularize my code at its maximum.

I noticed that there are a couple times when i need to walk-until-i-find-the-previous-node (oh, i am using linked lists to build the matrix), and i want to make a reusable function.

I have 4 structs in my code, all of them have the vars first , last or next if is a child structure that have no header-role.

I thought a code like this:

void *iterateUntilNext(void *header, void *child)
{
    void *walker = header->first;
    while (walker->next != child)
    {
        walker = walker->next;
    }
    return walker;
}

But i got this error: expression must have pointer-to-struct-or-union type but it has type "void *"

Is there any way to make this work or am i dreaming too much?

Answer 1

OOP

Define a base struct like

#define list_item_interface \
  list_item *first; \
  list_item *next;

typedef struct list_item {
    list_item_interface
} list_item;

and 'derive' your custom items like

typedef struct custom_list_item {
    list_item_interface //needs to be the first entry
    //other members/fields
} custom_list_item;

then define your 'generic' functions like

list_item* iterate(list_item *header, list_item *child) { /*...*/ }

call it like

custom_list_item *header, *child;
iterate((list_item*) header, (list_item*) child);

---

GCC - Extension

Accepts any type, which provides the fields first and next of the same type.

#define LIST_ITEM_ITERATE(HEADER, CHILD) ({ \
    __auto_type HEADER_ = (HEADER); \
    __auto_type CHILD_  = (CHILD); \
    __auto_type WALKER  = HEADER_->first; \
    while (WALKER->next != CHILD_) \
        WALKER = WALKER->next; \
    WALKER; \
)}