Generic list manipulation function in C?

Question

What is a generic list manipulation function in C? (I saw this when I was going through some materials.)

What is the difference between this function and a function

Answer 1

The Linux kernel has an interesting implementation of a generic linked list in C on its linux/list.h header. It is a doubly-linked list with a head node, used like this:

struct mystruct {
    ...
    /* Contains the next and prev pointers */
    struct list_head mylist;
    ...
    /* A single struct can be in several lists */
    struct list_head another_list;
    ...
};

struct list_head mylist_head;
struct list_head another_list_head;

Some interesting things in this small example:

• The list node is embedded within the target struct, needing no data pointer.
• The list node does not have to come at the beginning of the target struct.
• A single struct can be in several linked lists at the same time.
• The next and prev pointers within the list point to the struct list_head, not to the target structure (on the example above, they point to &(foo->mylist) for the first list and &(foo->another_list) for the second list).

All the list manipulation functions take pointers to struct list_head (and most do not care at all whether it is the separate head node or one of the embedded nodes). To get from the struct list_head to the target struct, you use the list_entry macro (which is the same as the containter_of macro from the linux/kernel.h header), which expands into a simple pointer subtraction.

Since it is a doubly-linked list with a head node, you can in O(1):

• Check if the list is empty, or if a node is not on any list.
• Get the node after or before any other node (if the node is the head, you get the first or last element of the list).
• Insert a node after or before any other node (if the node is the head, you insert at the beginning or end of the list).
• Remove a node from the list (you need only the pointer to the node itself to do this).
• Several other operations.