Linked Lists in the Linux Kernel - An example in userspace

Hareesh Nagarajan (hnagaraj AT cs uic edu)
Sat Nov 5 22:47:56 CST 2005

For a good introduction to the implementation of linked lists API inside the Linux Kernel check this link.

If you are weary of programming with linked lists inside the Linux kernel, download the list.h file which has been stripped away of kernel specific stuff from this link. Then, all you have to do is, add this file into your userspace code with a:

#include "list.h"

and you are all set.

You can also download the following simple example (which appears below) from this link. The compile flags are the usual:

gcc link-list-test.c -olink-list-test

Note: In your kernel code use list_for_each instead of __list_for_each

The Example

#include <stdio.h>
#include <assert.h>
#include <cstdlib.h>

#include "list.h"

struct foo {
	int info;
	struct list_head list_member;
};

void add_node(int arg, struct list_head *head)
{
    struct foo *fooPtr = (struct foo *)malloc(sizeof(struct foo));
    assert(fooPtr != NULL);
    
    fooPtr->info = arg;
    INIT_LIST_HEAD(&fooPtr->list_member);
    list_add(&fooPtr->list_member, head);
}

void display(struct list_head *head)
{
    struct list_head *iter;
    struct foo *objPtr;

    __list_for_each(iter, head) {
        objPtr = list_entry(iter, struct foo, list_member);
        printf("%d ", objPtr->info);
    }
    printf("\n");
}

void delete_all(struct list_head *head)
{
    struct list_head *iter;
    struct foo *objPtr;
    
  redo:
    __list_for_each(iter, head) {
        objPtr = list_entry(iter, struct foo, list_member);
        list_del(&objPtr->list_member);
        free(objPtr);
        goto redo;
    }
}

int find_first_and_delete(int arg, struct list_head *head)
{
    struct list_head *iter;
    struct foo *objPtr;

    __list_for_each(iter, head) {
        objPtr = list_entry(iter, struct foo, list_member);
        if(objPtr->info == arg) {
            list_del(&objPtr->list_member);
            free(objPtr);
            return 1;
        }
    }

    return 0;
}

main() 
{
    LIST_HEAD(fooHead);
    
    add_node(10, &fooHead);
    add_node(20, &fooHead);
    add_node(25, &fooHead);
    add_node(30, &fooHead);
    
    display(&fooHead);
    find_first_and_delete(20, &fooHead);
    display(&fooHead);
    delete_all(&fooHead);
    display(&fooHead);
}