C链接列表添加新节点问题

Question

最终的完整程序在下面的链接下面。谢谢每一个回复我的人。

已完成程序：http://pastebin.com/1Qrs11JE

问题：

我有一个恼人的问题，链接列表在C语言。我明天要提交一份作业。我从昨天开始就一直试图解决这个问题，但是我做不到。我认为问题是在链接列表中添加新的节点，但是当我试图查看链接列表中的记录时，我会得到错误。我面对的是有意义的结果，而这个程序最终导致一个错误。

您可以在下面的链接中看到更易读的代码或从下面看到的代码。链接：http://pastebin.com/Ugtue6JT

这是我的代码：

#include <stdio.h>
#include <stdlib.h>
#include <conio.h>
#include <malloc.h>

// structure definition
 struct node {
char name[50];
int age;
int student_no;
char nationality[50];
struct node *next;
};
typedef struct node *NODE_T;

// function declarations
void enter_new_data();
void remove_existing_data();
void display_all_data();
void show_options();
NODE_T getnode();
void freenode(NODE_T p);
void free_buffer();
void continue_f(char message[100]);

// global variables
char ch;

// create linked-list's main holder variable
NODE_T students,conductor;

int main(){

// allocate memory for the node link-list holder
students = NULL;
//students->next = 0; // initialize next to 0 for first node of linked-list

// show the options that user has
show_options();

return 1;
}

// this function will list options that user can apply
void show_options(){

system("cls"); // clear screen

int opt,opt_bool=0;

printf("%s\n\n%s\n%s\n%s\n%s\n\n","The Options You Have","1. Add New Student's Record","2. Delete An Existing Student's Record","3. Display The List Of Students","4. Exit");


while(opt_bool != 1){
    printf("Operation:  ");
    scanf("%d",&opt);

    if(opt == 1 || opt == 2 || opt == 3 || opt == 4){
        opt_bool = 1;
    }

}

// check the operation and go to the operation
if(opt == 1){ // adding record
    enter_new_data();
} else if(opt == 2){ // removing record

} else if(opt == 3){ // displaying records
    display_all_data();
} else if(opt == 4){ // exit the program
    exit(0);
}
}

// enter new student data into linked-list
void enter_new_data(){

system("cls"); // clear screen

// get a new node
NODE_T p = getnode();

printf("You are entering a new student's record\n");

// take student's name
printf("Student's Name: ");
scanf("%s",p->name);
free_buffer();

// take student's age
printf("Student's Age: ");
scanf("%d",&p->age);
free_buffer();

// take student's number
printf("Student's Number: ");
scanf("%d",&p->student_no);
free_buffer();

// take student's nationality
printf("Student's Nationality: ");
scanf("%s",p->nationality);
free_buffer();

// set p->next next value of last node of linked-list, which is equal to 0
p->next = 0;

printf("%s, %d, %d, %s",p->name,p->age,p->student_no,p->nationality);


// if there is no any node yet, add node p as first node
if(students == NULL) {
    students = p;
} else {

    conductor = students; // assign linked-list to the conductor to traverse

    // reach the last node
    while (conductor->next != 0)
    {
        conductor = conductor->next;
    }

    conductor->next = p; // append the node p to the linked list
}

freenode(p); // set free node p

continue_f("Adding new record is done."); // ask press any key to continue

show_options(); // show options

}

// to display all data of linked list
void display_all_data(){

system("cls"); // clear screen

printf("The Student Records\n\n");

printf("%s%7s%18s%15s","STUDENT'S NAME","AGE","STUDENT NUMBER","NATIONALITY"); // captions

freenode(conductor);
conductor = getnode();
conductor = students; // assign linked-list to the conductor to traverse

if (conductor != NULL ) { /* Makes sure there is a place to start */
    // traverse untill last node
    while ( conductor->next != 0)
    {

        printf("\n%s%7d%18d%15s",conductor->name,conductor->age,conductor->student_no,conductor->nationality); // record

        conductor = conductor->next;
    }
  } else {
    printf("\n\n There is not any record yet.");
  }

  continue_f("Listing records is done."); // ask press any key to continue

  show_options(); // show options
}

// create new node
NODE_T getnode(void){
   NODE_T p;
   p = (struct node *) malloc(sizeof(struct node));
   return p;
}

// set free a node
void freenode(NODE_T p){
    free(p);
}

// clear the buffer if there are any extra data in it
void free_buffer(){
    while (getchar() != '\n') { }   
}

void continue_f(char message[100]){
     printf("\n\n%s\nPress any key to continue...",message);
     getch(); // wait for pushing any key from user
}

Answer 1

在查找和检查所提供的源代码后，由于对链接列表中节点的malloc()/free()的误解，存在多个问题。

1. 若要将节点添加到链接列表中，应先对其进行分配和初始化，
2. 当使用指向已分配节点的指针时，既不分配也不释放该指针，
3. 添加到链接列表中的节点只能在删除或删除链接列表后才能释放。

按照这些规则，以下是检测到的错误：

错误1：在enter_new_data()中，一个意外的freenode(p);。

节点p已被链接..。别解放它。

// if there is no any node yet, add node p as first node
if(students == NULL) {
    students = p;
}
else {
    conductor = students; // assign linked-list to the conductor to traverse
    // reach the last node
    while (conductor->next != NULL)
    {
        conductor = conductor->next;
    }
    conductor->next = p; // append the node p to the linked list
}
// NO !!!!
freenode(p); // set free node p

Error 2：在display_all_data()中，conductor的意外空闲/malloc。

变量conductor只是一个临时指针，用于浏览链接列表(未分配)。

// NO !!!!
freenode(conductor);
// NO !!!!
conductor = getnode();

conductor = students; // assign linked-list to the conductor to traverse

if (conductor != NULL ) { /* Makes sure there is a place to start */

错误3：display_all_data()中的小错误，要探索链接列表中的所有项，请使用节点指针而不是下一个指针。

通过( conductor != NULL)的测试也探讨了最后一项。

    while ( conductor != NULL) //->next != 0)
    {
        printf("\n%s%7d%18d%15s",conductor->name,conductor->age,
            conductor->student_no,conductor->nationality); // record
        conductor = conductor->next;
    }