关于free的使用疑惑

 1 #include <stdio.h>
 2 #include <string.h>
 3 #include <stdlib.h>
 4 #include "mainc26.h"
 5 struct TelPhone{
 6     char name[20];
 7     char TelNumber[20];
 8     struct TelPhone* next;
 9 };
10 int mainc26()
11 {
12 
13 
14     setbuf(stdout, NULL);
15     //fflush(stdout);
16     struct TelPhone *head = NULL;
17     struct TelPhone *pre,*current;
18     char input[20];
19     printf("请输入联系人的姓名：\n");
20     fflush(stdout);
21     while(gets(input) != NULL && input[0] != '\0')
22     {
23         current = (struct TelPhone*)malloc(sizeof(struct TelPhone));
24         if(head == NULL)
25             head = current;
26         else
27             pre->next = current;
28         current->next = NULL;
29         strcpy(current->name,input);
30         printf("请输入联系人的电话号码：\n");
31         scanf("%s",current->TelNumber);
32         while(getchar() != '\n')
33             continue;
34         printf("请输入下一个联系人（空行则退出）：\n");
35         pre = current;
36     }
37     if(head == NULL)
38         printf("电话里面没有储存任何数据：");
39     else
40         puts("通讯录：");
41     current = head;
42     while(current != NULL)
43     {
44         printf("%s：%s\n",current->name,current->TelNumber);
45         current = current->next;
46     }
47     current = head;
48     int n=0;
49     while(current != NULL)
50     {
51         n++;
52         printf("this is %d time \n",n);
53         free(current);
54         current=NULL;
55         printf("cur.name %s   ,",current->name);
56         printf("cur.teln %s   ,",current->TelNumber);
57 //        printf("cur.name %p   ,",current->next);
58         printf("\n");
59         current = current->next;
60     }
61     return 0;
62 }

在 line 53中使用free(current) ,这里似乎是将current的内存释放，然后实际上，这里的释放后，其内存信息已经存在，

如果将line 54的current=null，注释，该程序仍能正确运行，并通过该链表将内存释放掉，而实际上这种问题时错误的。

通过百度，我查到

问题如下：

 1 代码如下：
 2 #include <stdio.h>
 3 #include <stdlib.h>
 4  
 5 struct test 
 6 {
 7 int a;
 8 struct test *next;
 9 };
10 int main()
11 {
12 struct test test0={10};
13 struct test *test1 = (struct test *)malloc(sizeof(struct test));
14 test1->a=20;
15 test1->next = &test0;
16 free(test1);
17 printf("%d %d",test1->a,(*test1->next).a); 
18 
19 }
20 输出test1->a为0  ，(*test1->next).a为10
21  
22 说明结构体里的变量a已经被free释放掉了，而test1的指针next却还可以被访问

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