Doubly Linked List

Basic Concepts

A doubly linked list (DLL) is a linked data structure that consists of a set of sequentially linked records called nodes. Each node contains a pointer to the previous node and a pointer to the next node in the sequence of nodes. The pointer head is unique. A DLL allows access from a list node to its next node and also the previous node on the list. This data structure facilitates data search, especially traversal of a large amount of data. The symmetry of the DLL also makes operations, such as insertion and deletion, easy. However, pay attention to the pointer direction when performing operations.

Available APIs

The table below describes APIs available for the DLL. For more details about the APIs, see the API reference.

How to Develop

The typical development process of the DLL is as follows:

1. Call LOS_ListInit or LOS_DL_LIST_HEAD to initialize a DLL.

2. Call LOS_ListAdd to add a node into the DLL.

3. Call LOS_ListTailInsert to insert a node into the tail of the DLL.

4. Call LOS_ListDelete to delete the specified node.

5. Call LOS_ListEmpty to check whether the DLL is empty.

6. Call LOS_ListDelInit to delete the specified node and initialize the DLL based on the node.

NOTE

• Pay attention to the operations before and after the node pointer.

• The DLL APIs are underlying interfaces and do not check whether the input parameters are empty. You must ensure that the input parameters are valid.

• If the memory of a linked list node is dynamically allocated, release the memory when deleting the node.

Development Example

Example Description

This example implements the following:

1. Initialize a DLL.

2. Add nodes.

3. Delete nodes.

4. Check the operation result.

Sample Code

The sample code can be compiled and verified in ./kernel/liteos_a/testsuites/kernel/src/osTest.c. The ListSample function is called in TestTaskEntry.

The sample code is as follows:

#include "stdio.h"
#include "los_list.h"

static UINT32 ListSample(VOID)
{
    LOS_DL_LIST listHead = {NULL,NULL};
    LOS_DL_LIST listNode1 = {NULL,NULL};
    LOS_DL_LIST listNode2 = {NULL,NULL};

    // Initialize the DLL.
    PRINTK("Initial head\n");
    LOS_ListInit(&listHead);

    // Add node 1 and node 2 and verify their relationship.
    LOS_ListAdd(&listHead, &listNode1);
    if (listNode1.pstNext == &listHead && listNode1.pstPrev == &listHead) {
        PRINTK("Add listNode1 success\n");
    }

    LOS_ListTailInsert(&listHead, &listNode2);
    if (listNode2.pstNext == &listHead && listNode2.pstPrev == &listNode1) {
        PRINTK("Tail insert listNode2 success\n");
    }

    // Delete the two nodes.
    LOS_ListDelete(&listNode1);
    LOS_ListDelete(&listNode2);

    // Check whether the DLL is empty.
    if (LOS_ListEmpty(&listHead)) {
        PRINTK("Delete success\n");
    }

    return LOS_OK;
}

Verification

The development is successful if the return result is as follows:

Initial head 
Add listNode1 success 
Tail insert listNode2 success
Delete success