# Doubly Linked List ## Basic Concepts A doubly linked list 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 doubly linked list 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 doubly linked list also makes operations, such as insertion and deletion, easy. However, pay attention to the pointer direction when performing operations. ## Available APIs The doubly linked list module provides the following APIs. For more details about the APIs, see the API reference.

Category	API	Description
Initializing a linked list	LOS_ListInit	Initializes a specified doubly linked list node as a doubly linked list.
Initializing a linked list	LOS_DL_LIST_HEAD	Defines a doubly linked list node and initializes the node as a doubly linked list.
Adding a node	LOS_ListAdd	Inserts the specified node to the head of a doubly linked list.
Adding a node	LOS_ListTailInsert	Inserts the specified node to the end of a doubly linked list.
Deleting a node	LOS_ListDelete	Deletes the specified node from a doubly linked list.
Deleting a node	LOS_ListDelInit	Deletes the specified node from the linked list and uses the node to initialize the linked list.
Checking whether a doubly linked list is empty	LOS_ListEmpty	Checks whether a linked list is empty.
Obtaining structure information	LOS_DL_LIST_ENTRY	Obtains the address of the structure that contains the linked list. The first input parameter of the API indicates a node in the list, the second input parameter indicates the name of the structure to be obtained, and the third input parameter indicates the name of the linked list in the structure.
Obtaining structure information	LOS_OFF_SET_OF	Obtains the offset of a member in a specified structure relative to the start address of the structure.
Traversing a doubly linked list	LOS_DL_LIST_FOR_EACH	Traverses a doubly linked list.
Traversing a doubly linked list	LOS_DL_LIST_FOR_EACH_SAFE	Traverses a doubly linked list, and stores the next node of the current node for security verification.
Traversing the structure that contains the doubly linked list	LOS_DL_LIST_FOR_EACH_ENTRY	Traverses the specified doubly linked list and obtains the address of the structure that contains the linked list node.
	LOS_DL_LIST_FOR_EACH_ENTRY_SAFE	Traverses the specified doubly linked list, obtains the structure address of the node that contains the linked list, and stores the structure address that contains the next node of the current node.

## How to Develop The typical development process of the doubly linked list is as follows: 1. Call **LOS\_ListInit/LOS\_DL\_LIST\_HEAD** to initialize a doubly linked list. 2. Call **LOS\_ListAdd** to insert a node to the list. 3. Call **LOS\_ListTailInsert** to insert a node to the end of the list. 4. Call **LOS\_ListDelete** to delete the specified node. 5. Call **LOS\_ListEmpty** to check whether a linked list is empty. 6. Call **LOS\_ListDelInit** to delete a specified node, and initialize the linked list based on this node. >![](../public_sys-resources/icon-note.gif) **NOTE:** >- Pay attention to the operations of the front and back pointer of the node. >- The linked list operation APIs are underlying APIs 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 requested, release the memory after deleting the node. ## Development Example ### Example Description This example implements the following: 1. Initialize a doubly linked list. 2. Add nodes. 3. Delete a node. 4. Check whether the operation is performed successfully. ### Sample Code 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 linked list. printf("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) { printf("Add listNode1 success\n"); } LOS_ListTailInsert(&listHead, &listNode2); if (listNode2.pstNext == &listHead && listNode2.pstPrev == &listNode1) { printf("Tail insert listNode2 success\n"); } // Delete the two nodes. LOS_ListDelete(&listNode1); LOS_ListDelete(&listNode2); // Check that the linked list is empty. if (LOS_ListEmpty(&listHead)) { printf("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 ```