如何将链表传递给C中的函数

Question

所以我已经制作了一个工作程序，用于从文件中获取数据并根据您希望在 main 中的文件的哪一部分打印出文件。但我的目标是通过创建用户界面并从文件中附加到链接列表或根据要求打印出该链接列表的一部分（如果有）来使这个模块化。 只有一个问题：我似乎无法找到一种方法来成功地将链表传递给函数，这样当你在函数中创建新节点（附加）时，它也可以在 main 中工作，然后在 (打印）。

这是工作代码：

#include <stdio.h>
#include <stdlib.h>      /* has the malloc prototype      */
#define TSIZE 100        /* size of array to hold title   */
struct book {
    char title[TSIZE], author[TSIZE],year[6];
    struct book * next;  /* points to next struct in list */
};
//typedef struct book ITEM;
typedef struct book * Node;     // struct book *
void Append(Node *List, Node *Lcurrent,char filename[]);
void ClearGarbage(void);
int main(void){
    Node head=NULL;
    Node current;
    current=head;
    char fname[]="HW15Data.txt";
    char op;
    do{
        puts("Select operation from the following list:");
        puts("a. Append    p. Print q. quit");
        op = getchar();
        ClearGarbage();
        if (op=='a'){
            /* Gather  and store information          */
            Append(&head,&current,fname);
        }
        else if (op=='q'){
            /* User quit, so free allocated memory */
            current = head;
            while (current != NULL)
            {
                free(current);
                current = current->next;
            }
            printf("Bye!\n");
            return 0;
        }
        else{
            /* Program done, so free allocated memory */
            current = head;
            while (current != NULL)
            {
            free(current);
            current = current->next;
            }
            printf("Invalid characted entered. Bye!\n");
            return 0;
        }
    } while (op!='q');
    return 0;
}
void Append(Node *List, Node * Lcurrent,char filename[TSIZE]){
    FILE * fp;
    Node head=*List;
    Node current=*Lcurrent;
    int loops=0;
    fp=fopen(filename,"r");
    if (head==NULL){
        head=(Node) malloc(sizeof(struct book));
        current=head;
        current->next=NULL;
    }
    do{
        current->next = (Node) malloc(sizeof(struct book));
        current=current->next;
        loops++;
    } while(fgets(current->title,sizeof(current->title),fp) && fgets(current->author,sizeof(current->title),fp) && fgets(current->year,sizeof(current->year),fp));;
    free(current);
    printf("Number of records written: %d\n",loops);
//Same as Print function in the nonworking code
    int num;
    int i=0;
    if (head == NULL){
        printf("No data entered. ");
    }
    else{
        printf("Enter record # to print: ");
        scanf("%d",&num);
        ClearGarbage();
        num=num+1;
        current = head;
        while (current != NULL && i<num)
        {
            for (i=0;i<num;i++)
                current = current->next;
            printf("Book: %sAuthor: %sYear: %s\n",
            current->title, current->author, current->year);
            }
    }
}
void ClearGarbage(void){
    while (getchar()!='\n');
}

好吧，这很有效，但我的猜测是，一旦 Append 完成，在 Append 中创建的节点在 main 中就没有用了，因为它们现在已经消失了。所以当我尝试在下面的代码中创建一个打印函数时，没有什么可以打印的。

#include <stdio.h>
#include <stdlib.h>      /* has the malloc prototype      */
#define TSIZE 100        /* size of array to hold title   */
struct book {
    char title[TSIZE], author[TSIZE],year[6];
    struct book * next;  /* points to next struct in list */
};
//typedef struct book ITEM;
typedef struct book * Node;     // struct book *
void Append(Node *List, Node *Lcurrent,char filename[]);
int Print(Node *List,Node *Lcurrent);
void ClearGarbage(void);
int main(void){
    Node head=NULL;
    Node current;
    current=head;
    char fname[]="HW15Data.txt";
    char op;
    do{
        puts("Select operation from the following list:");
        puts("a. Append    p. Print q. quit");
        op = getchar();
        ClearGarbage();
        if (op=='a'){
            /* Gather  and store information          */
            Append(&head,&current,fname);
        }
        else if (op=='p'){
            /*Print book record of user's choice*/
            Print(&head,&current);
        }
        else if (op=='q'){
            /* User quit, so free allocated memory */
            current = head;
            while (current != NULL)
            {
                free(current);
            current = current->next;
            }
            printf("Bye!\n");
            return 0;
        }
        else{
            /* Program done, so free allocated memory */
            current = head;
            while (current != NULL)
            {
            free(current);
            current = current->next;
            }
            printf("Invalid characted entered. Bye!\n");
            return 0;
        }
    } while (op!='q');
    return 0;
}
void Append(Node *List, Node * Lcurrent,char filename[TSIZE]){
    FILE * fp;
    Node head=*List;
    Node current=*Lcurrent;
    int loops=0;
    fp=fopen(filename,"r");
    if (head==NULL){
        head=(Node) malloc(sizeof(struct book));
        current=head;
        current->next=NULL;
    }
    do{
        current->next = (Node) malloc(sizeof(struct book));
        current=current->next;
        loops++;
    } while(fgets(current->title,sizeof(current->title),fp) && fgets(current->author,sizeof(current->title),fp) && fgets(current->year,sizeof(current->year),fp));
    free(current);
    printf("Number of records written: %d\n",loops);
}
int Print(Node *List,Node *Lcurrent){
    int num;
    int i=0;
    Node head=*List;
    Node current=*Lcurrent;
    if (head == NULL){
        printf("No data entered.\n");
        return -1;
    }
    printf("Enter record # to print: ");
    scanf("%d",&num);
    ClearGarbage();
    num=num+1;
    current = head;
    while (current != NULL && i<num)
    {
        for (i=0;i<num;i++){
            current = current->next;
        }
        printf("Book: %sAuthor: %sYear: %s\n",
        current->title, current->author, current->year);
        }
    return 0;
}
void ClearGarbage(void){
    while (getchar()!='\n');
}

感谢任何人的帮助！

编辑：为了清楚起见，去掉了一个未使用的 typedef

Answer 1

似乎大多数人都在关注缺乏组织（这也让我很困扰）而不是您的实际问题。

似乎问题的根源在于您分配变量头的位置。 当您定义“Node head = *List”并且 List 为 NULL 时，因为它第一次初始化，它失去了与您从 main 发送的原始列表的连接，您只需创建一个仅具有本地引用的链表。

我刚刚在 Append 和 Print 函数中将“head”的用法更改为“*List”，它似乎已经解决了。

这是我更改后的代码：

#include <stdio.h>
#include <stdlib.h>      /* has the malloc prototype      */
#define TSIZE 100        /* size of array to hold title   */
struct book {
    char title[TSIZE], author[TSIZE], year[6];
    struct book * next;  /* points to next struct in list */
};
//typedef struct book ITEM;
typedef struct book * Node;     // struct book *
void Append(Node *List, Node *Lcurrent, char filename[]);
int Print(Node *List, Node *Lcurrent);
void ClearGarbage(void);
int main(void) {
    Node head = NULL;
    Node current;
    current = head;
    char fname[] = "HW15Data.txt";
    char op;
    do {
        puts("Select operation from the following list:");
        puts("a. Append    p. Print q. quit");
        op = getchar();
        ClearGarbage();
        if (op == 'a') {
            /* Gather  and store information          */
            Append(&head, &current, fname);
        }
        else if (op == 'p') {
            /*Print book record of user's choice*/
            Print(&head, &current);
        }
        else if (op == 'q') {
            /* User quit, so free allocated memory */
            current = head;
            while (current != NULL)
            {
                free(current);
                current = current->next;
            }
            printf("Bye!\n");
            return 0;
        }
        else {
            /* Program done, so free allocated memory */
            current = head;
            while (current != NULL)
            {
                free(current);
                current = current->next;
            }
            printf("Invalid characted entered. Bye!\n");
            return 0;
        }
    } while (op != 'q');
    return 0;
}
void Append(Node *List, Node * Lcurrent, char filename[TSIZE]) {
    FILE * fp;
    Node head = *List;
    Node current = *Lcurrent;
    int loops = 0;
    char line[256];
    fp = fopen(filename, "r");
    if (*List == NULL) {
        *List = (Node)malloc(sizeof(struct book));
        current = *List;
        current->next = NULL;
    }
    do {
        current->next = (Node)malloc(sizeof(struct book));
        current = current->next;
        loops++;
    } while (fgets(current->title, sizeof(line), fp) && fgets(current->author, sizeof(line), fp) && fgets(current->year, sizeof(line), fp));
    free(current);
    printf("Number of records written: %d\n", loops);
}
int Print(Node *List, Node *Lcurrent) {
    int num;
    int i = 0;
    Node head = *List;
    Node current = *Lcurrent;
    if (*List == NULL) {
        printf("No data entered.\n");
        return -1;
    }
    printf("Enter record # to print: ");
    scanf("%d", &num);
    ClearGarbage();
    num = num + 1;
    current = *List;
    while (current != NULL && i<num)
    {
        for (i = 0; i<num; i++) {
            current = current->next;
        }
        printf("Book: %sAuthor: %sYear: %s\n",
            current->title, current->author, current->year);
    }
    return 0;
}
void ClearGarbage(void) {
    while (getchar() != '\n');
}

还有很多逻辑错误和一些错误，但它解决了您寻求帮助的问题。