Linked list (list_head / queue) in C language

This is an example of implementing a list in C language.

Let's implement a bidirectional list using the following two macros.

--list_head, a list manipulation macro often used in the linux kernel --Queue, a BSD-based list manipulation macro

The execution environment is CentOS7 64bit, gcc 4.8.5.

List using list_head

The linux kernel has the following structure and some macros that use this structure to perform list operations.

struct list_head {
	struct list_head *next, *prev;
};

It's difficult to understand how it works, but it's a useful macro to learn how to use.

Get kernel source code

First, get the kernel source code.

# yum install kernel-devel
...
Installation:
  kernel-devel.x86_64 0:3.10.0-957.5.1.el7                                                                                              

Has completed!

Since the macro of list is linux / list.h, it will be the following file.

/usr/src/kernels/3.10.0-957.5.1.el7.x86_64/include/linux/list.h

Processing list.h

Since linux / list.h is a header file for kernel, if you try to use it as it is in a userland application, compilation errors will occur frequently, which is inconvenient. (~~ It's troublesome to resolve the error ~~)

So I will modify it a little.

$ cp /usr/src/kernels/3.10.0-957.5.1.el7.x86_64/include/linux/list.h .
$ vim list.h

Modification point

--Delete hlist (because it is not used this time) --Replace LIST_POISON1 and LIST_POISON2 with 0xdeadbeef --Replace typeof () with __typeof () --Copy and paste the container_of () macro from linux / kernel.h


Modified list.h
#ifndef _LINUX_LIST_H
#define _LINUX_LIST_H

struct list_head {
	struct list_head *next, *prev;
};

/**
 * container_of - cast a member of a structure out to the containing structure
 * @ptr:    the pointer to the member.
 * @type:   the type of the container struct this is embedded in.
 * @member: the name of the member within the struct.
 *
 */
#define container_of(ptr, type, member) ({          \
    const __typeof( ((type *)0)->member ) *__mptr = (ptr);    \
    (type *)( (char *)__mptr - offsetof(type,member) );})

/*
 * Simple doubly linked list implementation.
 *
 * Some of the internal functions ("__xxx") are useful when
 * manipulating whole lists rather than single entries, as
 * sometimes we already know the next/prev entries and we can
 * generate better code by using them directly rather than
 * using the generic single-entry routines.
 */

#define LIST_HEAD_INIT(name) { &(name), &(name) }

#define LIST_HEAD(name) \
	struct list_head name = LIST_HEAD_INIT(name)

static inline void INIT_LIST_HEAD(struct list_head *list)
{
	list->next = list;
	list->prev = list;
}

/*
 * Insert a new entry between two known consecutive entries.
 *
 * This is only for internal list manipulation where we know
 * the prev/next entries already!
 */
#ifndef CONFIG_DEBUG_LIST
static inline void __list_add(struct list_head *new,
			      struct list_head *prev,
			      struct list_head *next)
{
	next->prev = new;
	new->next = next;
	new->prev = prev;
	prev->next = new;
}
#else
extern void __list_add(struct list_head *new,
			      struct list_head *prev,
			      struct list_head *next);
#endif

/**
 * list_add - add a new entry
 * @new: new entry to be added
 * @head: list head to add it after
 *
 * Insert a new entry after the specified head.
 * This is good for implementing stacks.
 */
static inline void list_add(struct list_head *new, struct list_head *head)
{
	__list_add(new, head, head->next);
}


/**
 * list_add_tail - add a new entry
 * @new: new entry to be added
 * @head: list head to add it before
 *
 * Insert a new entry before the specified head.
 * This is useful for implementing queues.
 */
static inline void list_add_tail(struct list_head *new, struct list_head *head)
{
	__list_add(new, head->prev, head);
}

/*
 * Delete a list entry by making the prev/next entries
 * point to each other.
 *
 * This is only for internal list manipulation where we know
 * the prev/next entries already!
 */
static inline void __list_del(struct list_head * prev, struct list_head * next)
{
	next->prev = prev;
	prev->next = next;
}

/**
 * list_del - deletes entry from list.
 * @entry: the element to delete from the list.
 * Note: list_empty() on entry does not return true after this, the entry is
 * in an undefined state.
 */
#ifndef CONFIG_DEBUG_LIST
static inline void __list_del_entry(struct list_head *entry)
{
	__list_del(entry->prev, entry->next);
}

static inline void list_del(struct list_head *entry)
{
	__list_del(entry->prev, entry->next);
	entry->next = (struct list_head*)0xdeadbeef;
	entry->prev = (struct list_head*)0xdeadbeef;
}
#else
extern void __list_del_entry(struct list_head *entry);
extern void list_del(struct list_head *entry);
#endif

/**
 * list_replace - replace old entry by new one
 * @old : the element to be replaced
 * @new : the new element to insert
 *
 * If @old was empty, it will be overwritten.
 */
static inline void list_replace(struct list_head *old,
				struct list_head *new)
{
	new->next = old->next;
	new->next->prev = new;
	new->prev = old->prev;
	new->prev->next = new;
}

static inline void list_replace_init(struct list_head *old,
					struct list_head *new)
{
	list_replace(old, new);
	INIT_LIST_HEAD(old);
}

/**
 * list_del_init - deletes entry from list and reinitialize it.
 * @entry: the element to delete from the list.
 */
static inline void list_del_init(struct list_head *entry)
{
	__list_del_entry(entry);
	INIT_LIST_HEAD(entry);
}

/**
 * list_move - delete from one list and add as another's head
 * @list: the entry to move
 * @head: the head that will precede our entry
 */
static inline void list_move(struct list_head *list, struct list_head *head)
{
	__list_del_entry(list);
	list_add(list, head);
}

/**
 * list_move_tail - delete from one list and add as another's tail
 * @list: the entry to move
 * @head: the head that will follow our entry
 */
static inline void list_move_tail(struct list_head *list,
				  struct list_head *head)
{
	__list_del_entry(list);
	list_add_tail(list, head);
}

/**
 * list_is_last - tests whether @list is the last entry in list @head
 * @list: the entry to test
 * @head: the head of the list
 */
static inline int list_is_last(const struct list_head *list,
				const struct list_head *head)
{
	return list->next == head;
}

/**
 * list_empty - tests whether a list is empty
 * @head: the list to test.
 */
static inline int list_empty(const struct list_head *head)
{
	return head->next == head;
}

/**
 * list_empty_careful - tests whether a list is empty and not being modified
 * @head: the list to test
 *
 * Description:
 * tests whether a list is empty _and_ checks that no other CPU might be
 * in the process of modifying either member (next or prev)
 *
 * NOTE: using list_empty_careful() without synchronization
 * can only be safe if the only activity that can happen
 * to the list entry is list_del_init(). Eg. it cannot be used
 * if another CPU could re-list_add() it.
 */
static inline int list_empty_careful(const struct list_head *head)
{
	struct list_head *next = head->next;
	return (next == head) && (next == head->prev);
}

/**
 * list_rotate_left - rotate the list to the left
 * @head: the head of the list
 */
static inline void list_rotate_left(struct list_head *head)
{
	struct list_head *first;

	if (!list_empty(head)) {
		first = head->next;
		list_move_tail(first, head);
	}
}

/**
 * list_is_singular - tests whether a list has just one entry.
 * @head: the list to test.
 */
static inline int list_is_singular(const struct list_head *head)
{
	return !list_empty(head) && (head->next == head->prev);
}

static inline void __list_cut_position(struct list_head *list,
		struct list_head *head, struct list_head *entry)
{
	struct list_head *new_first = entry->next;
	list->next = head->next;
	list->next->prev = list;
	list->prev = entry;
	entry->next = list;
	head->next = new_first;
	new_first->prev = head;
}

/**
 * list_cut_position - cut a list into two
 * @list: a new list to add all removed entries
 * @head: a list with entries
 * @entry: an entry within head, could be the head itself
 *	and if so we won't cut the list
 *
 * This helper moves the initial part of @head, up to and
 * including @entry, from @head to @list. You should
 * pass on @entry an element you know is on @head. @list
 * should be an empty list or a list you do not care about
 * losing its data.
 *
 */
static inline void list_cut_position(struct list_head *list,
		struct list_head *head, struct list_head *entry)
{
	if (list_empty(head))
		return;
	if (list_is_singular(head) &&
		(head->next != entry && head != entry))
		return;
	if (entry == head)
		INIT_LIST_HEAD(list);
	else
		__list_cut_position(list, head, entry);
}

static inline void __list_splice(const struct list_head *list,
				 struct list_head *prev,
				 struct list_head *next)
{
	struct list_head *first = list->next;
	struct list_head *last = list->prev;

	first->prev = prev;
	prev->next = first;

	last->next = next;
	next->prev = last;
}

/**
 * list_splice - join two lists, this is designed for stacks
 * @list: the new list to add.
 * @head: the place to add it in the first list.
 */
static inline void list_splice(const struct list_head *list,
				struct list_head *head)
{
	if (!list_empty(list))
		__list_splice(list, head, head->next);
}

/**
 * list_splice_tail - join two lists, each list being a queue
 * @list: the new list to add.
 * @head: the place to add it in the first list.
 */
static inline void list_splice_tail(struct list_head *list,
				struct list_head *head)
{
	if (!list_empty(list))
		__list_splice(list, head->prev, head);
}

/**
 * list_splice_init - join two lists and reinitialise the emptied list.
 * @list: the new list to add.
 * @head: the place to add it in the first list.
 *
 * The list at @list is reinitialised
 */
static inline void list_splice_init(struct list_head *list,
				    struct list_head *head)
{
	if (!list_empty(list)) {
		__list_splice(list, head, head->next);
		INIT_LIST_HEAD(list);
	}
}

/**
 * list_splice_tail_init - join two lists and reinitialise the emptied list
 * @list: the new list to add.
 * @head: the place to add it in the first list.
 *
 * Each of the lists is a queue.
 * The list at @list is reinitialised
 */
static inline void list_splice_tail_init(struct list_head *list,
					 struct list_head *head)
{
	if (!list_empty(list)) {
		__list_splice(list, head->prev, head);
		INIT_LIST_HEAD(list);
	}
}

/**
 * list_entry - get the struct for this entry
 * @ptr:	the &struct list_head pointer.
 * @type:	the type of the struct this is embedded in.
 * @member:	the name of the list_struct within the struct.
 */
#define list_entry(ptr, type, member) \
	container_of(ptr, type, member)

/**
 * list_first_entry - get the first element from a list
 * @ptr:	the list head to take the element from.
 * @type:	the type of the struct this is embedded in.
 * @member:	the name of the list_struct within the struct.
 *
 * Note, that list is expected to be not empty.
 */
#define list_first_entry(ptr, type, member) \
	list_entry((ptr)->next, type, member)

/**
 * list_last_entry - get the last element from a list
 * @ptr:	the list head to take the element from.
 * @type:	the type of the struct this is embedded in.
 * @member:	the name of the list_struct within the struct.
 *
 * Note, that list is expected to be not empty.
 */
#define list_last_entry(ptr, type, member) \
	list_entry((ptr)->prev, type, member)

/**
 * list_first_entry_or_null - get the first element from a list
 * @ptr:	the list head to take the element from.
 * @type:	the type of the struct this is embedded in.
 * @member:	the name of the list_struct within the struct.
 *
 * Note that if the list is empty, it returns NULL.
 */
#define list_first_entry_or_null(ptr, type, member) \
	(!list_empty(ptr) ? list_first_entry(ptr, type, member) : NULL)

/**
 * list_next_entry - get the next element in list
 * @pos:	the type * to cursor
 * @member:	the name of the list_struct within the struct.
 */
#define list_next_entry(pos, member) \
	list_entry((pos)->member.next, __typeof(*(pos)), member)

/**
 * list_prev_entry - get the prev element in list
 * @pos:	the type * to cursor
 * @member:	the name of the list_struct within the struct.
 */
#define list_prev_entry(pos, member) \
	list_entry((pos)->member.prev, __typeof(*(pos)), member)

/**
 * list_for_each	-	iterate over a list
 * @pos:	the &struct list_head to use as a loop cursor.
 * @head:	the head for your list.
 */
#define list_for_each(pos, head) \
	for (pos = (head)->next; pos != (head); pos = pos->next)

/**
 * __list_for_each	-	iterate over a list
 * @pos:	the &struct list_head to use as a loop cursor.
 * @head:	the head for your list.
 *
 * This variant doesn't differ from list_for_each() any more.
 * We don't do prefetching in either case.
 */
#define __list_for_each(pos, head) \
	for (pos = (head)->next; pos != (head); pos = pos->next)

/**
 * list_for_each_prev	-	iterate over a list backwards
 * @pos:	the &struct list_head to use as a loop cursor.
 * @head:	the head for your list.
 */
#define list_for_each_prev(pos, head) \
	for (pos = (head)->prev; pos != (head); pos = pos->prev)

/**
 * list_for_each_safe - iterate over a list safe against removal of list entry
 * @pos:	the &struct list_head to use as a loop cursor.
 * @n:		another &struct list_head to use as temporary storage
 * @head:	the head for your list.
 */
#define list_for_each_safe(pos, n, head) \
	for (pos = (head)->next, n = pos->next; pos != (head); \
		pos = n, n = pos->next)

/**
 * list_for_each_prev_safe - iterate over a list backwards safe against removal of list entry
 * @pos:	the &struct list_head to use as a loop cursor.
 * @n:		another &struct list_head to use as temporary storage
 * @head:	the head for your list.
 */
#define list_for_each_prev_safe(pos, n, head) \
	for (pos = (head)->prev, n = pos->prev; \
	     pos != (head); \
	     pos = n, n = pos->prev)

/**
 * list_for_each_entry	-	iterate over list of given type
 * @pos:	the type * to use as a loop cursor.
 * @head:	the head for your list.
 * @member:	the name of the list_struct within the struct.
 */
#define list_for_each_entry(pos, head, member)				\
	for (pos = list_entry((head)->next, __typeof(*pos), member);	\
	     &pos->member != (head); 	\
	     pos = list_entry(pos->member.next, __typeof(*pos), member))

/**
 * list_for_each_entry_reverse - iterate backwards over list of given type.
 * @pos:	the type * to use as a loop cursor.
 * @head:	the head for your list.
 * @member:	the name of the list_struct within the struct.
 */
#define list_for_each_entry_reverse(pos, head, member)			\
	for (pos = list_entry((head)->prev, __typeof(*pos), member);	\
	     &pos->member != (head); 	\
	     pos = list_entry(pos->member.prev, __typeof(*pos), member))

/**
 * list_prepare_entry - prepare a pos entry for use in list_for_each_entry_continue()
 * @pos:	the type * to use as a start point
 * @head:	the head of the list
 * @member:	the name of the list_struct within the struct.
 *
 * Prepares a pos entry for use as a start point in list_for_each_entry_continue().
 */
#define list_prepare_entry(pos, head, member) \
	((pos) ? : list_entry(head, __typeof(*pos), member))

/**
 * list_for_each_entry_continue - continue iteration over list of given type
 * @pos:	the type * to use as a loop cursor.
 * @head:	the head for your list.
 * @member:	the name of the list_struct within the struct.
 *
 * Continue to iterate over list of given type, continuing after
 * the current position.
 */
#define list_for_each_entry_continue(pos, head, member) 		\
	for (pos = list_entry(pos->member.next, __typeof(*pos), member);	\
	     &pos->member != (head);	\
	     pos = list_entry(pos->member.next, __typeof(*pos), member))

/**
 * list_for_each_entry_continue_reverse - iterate backwards from the given point
 * @pos:	the type * to use as a loop cursor.
 * @head:	the head for your list.
 * @member:	the name of the list_struct within the struct.
 *
 * Start to iterate over list of given type backwards, continuing after
 * the current position.
 */
#define list_for_each_entry_continue_reverse(pos, head, member)		\
	for (pos = list_entry(pos->member.prev, __typeof(*pos), member);	\
	     &pos->member != (head);	\
	     pos = list_entry(pos->member.prev, __typeof(*pos), member))

/**
 * list_for_each_entry_from - iterate over list of given type from the current point
 * @pos:	the type * to use as a loop cursor.
 * @head:	the head for your list.
 * @member:	the name of the list_struct within the struct.
 *
 * Iterate over list of given type, continuing from current position.
 */
#define list_for_each_entry_from(pos, head, member) 			\
	for (; &pos->member != (head);	\
	     pos = list_entry(pos->member.next, __typeof(*pos), member))

/**
 * list_for_each_entry_safe - iterate over list of given type safe against removal of list entry
 * @pos:	the type * to use as a loop cursor.
 * @n:		another type * to use as temporary storage
 * @head:	the head for your list.
 * @member:	the name of the list_struct within the struct.
 */
#define list_for_each_entry_safe(pos, n, head, member)			\
	for (pos = list_entry((head)->next, __typeof(*pos), member),	\
		n = list_entry(pos->member.next, __typeof(*pos), member);	\
	     &pos->member != (head); 					\
	     pos = n, n = list_entry(n->member.next, __typeof(*n), member))

/**
 * list_for_each_entry_safe_continue - continue list iteration safe against removal
 * @pos:	the type * to use as a loop cursor.
 * @n:		another type * to use as temporary storage
 * @head:	the head for your list.
 * @member:	the name of the list_struct within the struct.
 *
 * Iterate over list of given type, continuing after current point,
 * safe against removal of list entry.
 */
#define list_for_each_entry_safe_continue(pos, n, head, member) 		\
	for (pos = list_entry(pos->member.next, __typeof(*pos), member), 		\
		n = list_entry(pos->member.next, __typeof(*pos), member);		\
	     &pos->member != (head);						\
	     pos = n, n = list_entry(n->member.next, __typeof(*n), member))

/**
 * list_for_each_entry_safe_from - iterate over list from current point safe against removal
 * @pos:	the type * to use as a loop cursor.
 * @n:		another type * to use as temporary storage
 * @head:	the head for your list.
 * @member:	the name of the list_struct within the struct.
 *
 * Iterate over list of given type from current point, safe against
 * removal of list entry.
 */
#define list_for_each_entry_safe_from(pos, n, head, member) 			\
	for (n = list_entry(pos->member.next, __typeof(*pos), member);		\
	     &pos->member != (head);						\
	     pos = n, n = list_entry(n->member.next, __typeof(*n), member))

/**
 * list_for_each_entry_safe_reverse - iterate backwards over list safe against removal
 * @pos:	the type * to use as a loop cursor.
 * @n:		another type * to use as temporary storage
 * @head:	the head for your list.
 * @member:	the name of the list_struct within the struct.
 *
 * Iterate backwards over list of given type, safe against removal
 * of list entry.
 */
#define list_for_each_entry_safe_reverse(pos, n, head, member)		\
	for (pos = list_entry((head)->prev, __typeof(*pos), member),	\
		n = list_entry(pos->member.prev, __typeof(*pos), member);	\
	     &pos->member != (head); 					\
	     pos = n, n = list_entry(n->member.prev, __typeof(*n), member))

/**
 * list_safe_reset_next - reset a stale list_for_each_entry_safe loop
 * @pos:	the loop cursor used in the list_for_each_entry_safe loop
 * @n:		temporary storage used in list_for_each_entry_safe
 * @member:	the name of the list_struct within the struct.
 *
 * list_safe_reset_next is not safe to use in general if the list may be
 * modified concurrently (eg. the lock is dropped in the loop body). An
 * exception to this is if the cursor element (pos) is pinned in the list,
 * and list_safe_reset_next is called after re-taking the lock and before
 * completing the current iteration of the loop body.
 */
#define list_safe_reset_next(pos, n, member)				\
	n = list_entry(pos->member.next, __typeof(*pos), member)

#endif

Sample program

Sample list creation, display, rotation, and deletion.

Source code
#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <limits.h>
#include <string.h>
#include <stddef.h>
#include "list.h"

/*Sea fish*/
#define FISH_SARDINE  "sardine"  /*sardine*/
#define FISH_MACKEREL "mackerel" /*mackerel*/
#define FISH_TUNA     "tuna"     /*Tuna*/

/*River fish*/
#define FISH_SALMON  "salmon"  /*salmon*/
#define FISH_SEABASS "seabass" /*Suzuki*/
#define FISH_EEL     "eel"     /*eel*/

typedef struct {
    struct list_head head;
    uint32_t count;
} fishlist_t;

typedef struct {
    struct list_head list;
    uint32_t id;
    char name[64];
} fish_t;

static fishlist_t FishSea;
static fishlist_t FishRiver;

/*ID issuance*/
static uint32_t get_id(void)
{
    static uint32_t id = 0;
    return (++id % UINT32_MAX);
}

/*List initialization*/
static void init_fish_list(void)
{
    INIT_LIST_HEAD(&FishSea.head);
    FishSea.count = 0;
    INIT_LIST_HEAD(&FishRiver.head);
    FishRiver.count = 0;
}

/*Add sea fish to list*/
static void add_sea_fish_entry(void)
{
    char *fish_name[] = {
        FISH_SARDINE,
        FISH_MACKEREL,
        FISH_TUNA
    };
    fish_t *fish = NULL;
    
    for (int i = 0; i < sizeof(fish_name)/sizeof(fish_name[0]); i++) {

        fish = (fish_t *)malloc(sizeof(fish_t));
        if (NULL != fish) {
            memset(fish, 0, sizeof(*fish));
            snprintf(fish->name, sizeof(fish->name)-1, "%s", fish_name[i]);
            fish->id = get_id();

            list_add_tail(&fish->list, &FishSea.head);
            FishSea.count++;
        }
    }
}

/*Add river fish to list*/
static void add_river_fish_entry(void)
{
    char *fish_name[] = {
        FISH_SALMON,
        FISH_SEABASS,
        FISH_EEL
    };
    fish_t *fish = NULL;
    
    for (int i = 0; i < sizeof(fish_name)/sizeof(fish_name[0]); i++) {

        fish = (fish_t *)malloc(sizeof(fish_t));
        if (NULL != fish) {
            memset(fish, 0, sizeof(*fish));
            snprintf(fish->name, sizeof(fish->name)-1, "%s", fish_name[i]);
            fish->id = get_id();

            list_add_tail(&fish->list, &FishRiver.head);
            FishRiver.count++;
        }
    }
}

/*Show list of sea fish*/
static void show_sea_fish_entry(void)
{
    fish_t *fish = NULL;
    fish_t *n = NULL;

    printf("----------- show sea fish entry (cnt:%u) ------------\n", 
           FishSea.count);
    list_for_each_entry_safe(fish, n, &(FishSea.head), list) {
        printf("id:name = %u:%s\n", fish->id, fish->name);
    }
}

/*View a list of river fish*/
static void show_river_fish_entry(void)
{
    fish_t *fish = NULL;
    fish_t *n = NULL;

    printf("----------- show river fish entry (cnt:%u) ------------\n",
           FishRiver.count);
    list_for_each_entry_safe(fish, n, &(FishRiver.head), list) {
        printf("id:name = %u:%s\n", fish->id, fish->name);
    }
}

/*Rotate the list of river fish counterclockwise*/
static void rotate_left_sea_fish(void)
{
    list_rotate_left(&(FishSea.head));
}

/*Combine river fish with sea fish*/
static void splice_river_to_sea_fish(void)
{
    /*Combined with Fish River and joined with Fish Sea*/
    list_splice(&(FishRiver.head), &(FishSea.head));
    FishSea.count += FishRiver.count;

    /*After that, I'm not good at using the list of Fish River*/
    INIT_LIST_HEAD(&(FishRiver.head));
    FishRiver.count = 0;
}

/*Delete all entries in the saltwater fish list*/
static void del_sea_fish_all(void)
{
    fish_t *e = NULL;

    while(!list_empty(&(FishSea.head))) {
        e = list_first_entry((&FishSea.head), fish_t, list);
        list_del(&e->list);
        free(e);
        e = NULL;
        FishSea.count--;
    }
}


int main(void)
{
    /*List initialization*/
    init_fish_list();

    /*List of sea fish*/
    add_sea_fish_entry();
    show_sea_fish_entry();

    /*List of river fish*/
    add_river_fish_entry();
    show_river_fish_entry();

    /*Join list(river -> sea) */
    splice_river_to_sea_fish();
    show_sea_fish_entry();

    /*List rotation*/
    rotate_left_sea_fish();
    show_sea_fish_entry();

    /*Delete list*/
    del_sea_fish_all();
    show_sea_fish_entry();

    return 0;
}
Execution result
$ gcc list.c  -std=c99 -o fish_list
$ ./fish_list 
----------- show sea fish entry (cnt:3) ------------
id:name = 1:sardine
id:name = 2:mackerel
id:name = 3:tuna
----------- show river fish entry (cnt:3) ------------
id:name = 4:salmon
id:name = 5:seabass
id:name = 6:eel
----------- show sea fish entry (cnt:6) ------------
id:name = 4:salmon
id:name = 5:seabass
id:name = 6:eel
id:name = 1:sardine
id:name = 2:mackerel
id:name = 3:tuna
----------- show sea fish entry (cnt:6) ------------
id:name = 5:seabass
id:name = 6:eel
id:name = 1:sardine
id:name = 2:mackerel
id:name = 3:tuna
id:name = 4:salmon
----------- show sea fish entry (cnt:0) ------------

List using queue

queue is a macro that manipulates BSD lists, but it can also be used on linux.

This is also a quirk in usage, but it is a useful macro function when implementing a list in a userland application.

However, there are three types of lists that can be used on linux: lists, tail queues, and circular queues. Lists and tails are unidirectional, and circular queues are bidirectional lists.

Let's use the circular queue to replace the list_head macro used in the previous sample program with CIRCLEQ.

Sample program

Source code
#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <limits.h>
#include <string.h>
#include <stddef.h>
#include <sys/queue.h>

/*Sea fish*/
#define FISH_SARDINE  "sardine"  /*sardine*/
#define FISH_MACKEREL "mackerel" /*mackerel*/
#define FISH_TUNA     "tuna"     /*Tuna*/

/*River fish*/
#define FISH_SALMON  "salmon"  /*salmon*/
#define FISH_SEABASS "seabass" /*Suzuki*/
#define FISH_EEL     "eel"     /*eel*/

typedef struct {
    CIRCLEQ_HEAD(circleq, fish) head;
    uint32_t count;
} fishlist_t;
struct circleq *headp;              /* Circular queue head. */


typedef struct fish {
    uint32_t id;
    CIRCLEQ_ENTRY(fish) list;
    char name[64];
} fish_t;

static fishlist_t FishSea;
static fishlist_t FishRiver;

/*ID issuance*/
static uint32_t get_id(void)
{
    static uint32_t id = 0;
    return (++id % UINT32_MAX);
}

/*List initialization*/
static void init_fish_list(void)
{
    CIRCLEQ_INIT(&FishSea.head);
    FishSea.count = 0;
    CIRCLEQ_INIT(&FishRiver.head);
    FishRiver.count = 0;
}

/*Add sea fish to list*/
static void add_sea_fish_entry(void)
{
    char *fish_name[] = {
        FISH_SARDINE,
        FISH_MACKEREL,
        FISH_TUNA
    };
    fish_t *fish = NULL;
    
    for (int i = 0; i < sizeof(fish_name)/sizeof(fish_name[0]); i++) {

        fish = (fish_t *)malloc(sizeof(fish_t));
        if (NULL != fish) {
            memset(fish, 0, sizeof(*fish));
            snprintf(fish->name, sizeof(fish->name)-1, "%s", fish_name[i]);
            fish->id = get_id();

            CIRCLEQ_INSERT_TAIL(&FishSea.head, fish, list);
            FishSea.count++;
        }
    }
}

/*Add river fish to list*/
static void add_river_fish_entry(void)
{
    char *fish_name[] = {
        FISH_SALMON,
        FISH_SEABASS,
        FISH_EEL
    };
    fish_t *fish = NULL;
    
    for (int i = 0; i < sizeof(fish_name)/sizeof(fish_name[0]); i++) {

        fish = (fish_t *)malloc(sizeof(fish_t));
        if (NULL != fish) {
            memset(fish, 0, sizeof(*fish));
            snprintf(fish->name, sizeof(fish->name)-1, "%s", fish_name[i]);
            fish->id = get_id();

            CIRCLEQ_INSERT_TAIL(&FishRiver.head, fish, list);
            FishRiver.count++;
        }
    }
}

/*Show list of sea fish*/
static void show_sea_fish_entry(void)
{
    fish_t *fish = NULL;
    fish_t *n = NULL;

    printf("----------- show sea fish entry (cnt:%u) ------------\n", 
           FishSea.count);
    for (fish = (fish_t *)FishSea.head.cqh_first; 
         fish != (void *)&FishSea.head; 
         fish = (fish_t *)fish->list.cqe_next) {

        printf("id:name = %u:%s\n", fish->id, fish->name);
    }
}

/*View a list of river fish*/
static void show_river_fish_entry(void)
{
    fish_t *fish = NULL;
    fish_t *n = NULL;

    printf("----------- show river fish entry (cnt:%u) ------------\n",
           FishRiver.count);
    for (fish = (fish_t *)FishRiver.head.cqh_first; 
         fish != (void *)&FishRiver.head; 
         fish = (fish_t *)fish->list.cqe_next) {

        printf("id:name = %u:%s\n", fish->id, fish->name);
    }
}

/*Rotate the list of river fish counterclockwise*/
static void rotate_left_sea_fish(void)
{
    fish_t *first = (fish_t *)FishSea.head.cqh_first;
    CIRCLEQ_REMOVE(&FishSea.head, first, list);
    CIRCLEQ_INSERT_TAIL(&FishSea.head, first, list);
}

/*Combine river fish with sea fish*/
static void splice_river_to_sea_fish(void)
{
    fish_t *fish = NULL;

    /*Combined with Fish River and joined with Fish Sea*/
    while (FishRiver.head.cqh_first != (void *)&FishRiver.head) {
        fish = (fish_t *)FishRiver.head.cqh_last;
        CIRCLEQ_REMOVE(&FishRiver.head, fish, list);
        CIRCLEQ_INSERT_HEAD(&FishSea.head, fish, list);
    }
    FishSea.count += FishRiver.count;
    FishRiver.count = 0;
}

/*Delete all entries in the saltwater fish list*/
static void del_sea_fish_all(void)
{
    fish_t *fish = NULL;

    while (FishSea.head.cqh_first != (void *)&FishSea.head) {
        fish = FishSea.head.cqh_first;
        CIRCLEQ_REMOVE(&FishSea.head, fish, list);
        free(fish);
        fish = NULL;
        FishSea.count--;
    }
}


int main(void)
{
    /*List initialization*/
    init_fish_list();

    /*List of sea fish*/
    add_sea_fish_entry();
    show_sea_fish_entry();

    /*List of river fish*/
    add_river_fish_entry();
    show_river_fish_entry();

    /*Join list(river -> sea) */
    splice_river_to_sea_fish();
    show_sea_fish_entry();

    /*List rotation*/
    rotate_left_sea_fish();
    show_sea_fish_entry();

    /*Delete list*/
    del_sea_fish_all();
    show_sea_fish_entry();

    return 0;
}
Execution result
$ gcc queue.c  -std=c99 -o fish_queue
$ ./fish_queue 
----------- show sea fish entry (cnt:3) ------------
id:name = 1:sardine
id:name = 2:mackerel
id:name = 3:tuna
----------- show river fish entry (cnt:3) ------------
id:name = 4:salmon
id:name = 5:seabass
id:name = 6:eel
----------- show sea fish entry (cnt:6) ------------
id:name = 4:salmon
id:name = 5:seabass
id:name = 6:eel
id:name = 1:sardine
id:name = 2:mackerel
id:name = 3:tuna
----------- show sea fish entry (cnt:6) ------------
id:name = 5:seabass
id:name = 6:eel
id:name = 1:sardine
id:name = 2:mackerel
id:name = 3:tuna
id:name = 4:salmon
----------- show sea fish entry (cnt:0) ------------

reference

[Small story] I want to use List or data structure easily in C somehow https://qiita.com/chromabox/items/ea9720422d7a974f6ced

A small sample note of list_head https://qiita.com/kure/items/71057470322b1b636c57

How to use the list provided by the linux kernel http://d.hatena.ne.jp/mmitou/20120626/1340731801

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