双链表，空指针[find方法] [英] Double linked list and void pointers [find method]

问题描述

我写了空指针这个双向链表

i wrote this double linked list with void pointers

 typedef struct list_el
 { 
    void *data;            
    struct list_el *prev;  
    struct list_el *next; 

 } list_el;


typedef struct linked_list
{
   int n_el;            /*number of elements*/      
   list_el * head;      /*pointer to the head*/ 
   list_el * tail;      /*pointer to the head*/ 

 } linked_list;

和我写这些功能用它来处理。

and i wrote those functions to handle with it.

/*for list_el allocation*/
list_el * new_el ( void )
{
    return (list_el *) malloc(sizeof(list_el));
}

/*list initialization*/
void init_list(linked_list **l_ptr)
{
    (*l_ptr) = (linked_list * )malloc(sizeof(linked_list)); 
    (*l_ptr)->n_el = 0;
    (*l_ptr)->head = NULL;
    (*l_ptr)->tail = NULL;
}

/*head insertion*/
void append(void *data , linked_list **l_ptr)
{
    list_el *nv;
    nv = new_el();

    nv->data = data;

    if((*l_ptr)->n_el == 0 )
    {
        nv->next = nv->prev = NULL;
        (*l_ptr)->head = (*l_ptr)->tail = nv;
        (*l_ptr)->n_el += 1;
    }
    else
    {
       nv->next = (*l_ptr)->head;
       (*l_ptr)->head->prev = nv;
       (*l_ptr)->head = nv;
       (*l_ptr)->n_el += 1;
    }
}

我想写这样一个查找功能。

I'm trying to write a find function in this way.

void * find(void * el , linked_list ** l_ptr);

，其中** l_ptr是指针列表以搜索和埃尔是要搜索的元素。
因为我想比较两个无效*（无效* E1和无效*数据），我不知道如何实现这种类型的比较。

where **l_ptr is the pointer to the list to search in and el is the element to search. since I'm trying to compare two void * (void * el and void *data) I do not know how to implement a comparison of this type.

感谢。

推荐答案

要求用户提供一个回调（指针的函数定义的用户），比较他的数据。看看 的qsort 例如

Ask the user to provide a callback (a pointer to a function the user defined) for comparing his data. Look at qsort for example.

typedef int (*linked_list_compare)(void*, void*);

typedef struct linked_list
{
   int n_el;            /*number of elements*/      
   list_el * head;      /*pointer to the head*/ 
   list_el * tail;      /*pointer to the head*/ 

   linked_list_compare data_compare_func;

} linked_list;

void init_list(linked_list **l_ptr, linked_list_compare compare_func)
{
    if (!l_ptr || !compare_func)
      return; /* You should do error checking and error reporting */
    (*l_ptr) = (linked_list * )malloc(sizeof(linked_list)); 
    (*l_ptr)->n_el = 0;
    (*l_ptr)->head = NULL;
    (*l_ptr)->tail = NULL;
    (*l_ptr)->data_compare_func = compare_func;
}

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