指標的值改變而不修改

我正在研究鏈表并嘗試實作一些基本功能。

我的代碼：

#include <stdio.h>
#include <string.h>
#include <stdlib.h>

#define MAX_LEN_NAME 5

#define INSERT_NODE 1
#define APPEND_NODE 2
#define DEL_HEAD    3
#define DEL_TAIL    4
#define SHOW_LIST   5

struct people {
    int id;
    char name[MAX_LEN_NAME];
    struct people *next;
};

typedef struct people people_list;

static void node_insert(people_list** head_ref, int id, const char* name);
static void node_append(people_list** head_ref, int id, const char* name);
static void node_del_head(people_list** head_ref);
static void node_del_tail(people_list** head_ref);
static void node_show(people_list** head_ref);

static void node_insert(people_list** head_ref, int id, const char* name)
{
    people_list* new_node = NULL;
    new_node = malloc(sizeof(people_list));
    memset(new_node, 0, sizeof(people_list));

    if (new_node == NULL) {
        printf("Fail to allocate memory for new node\n");
        exit(1);
    }

    new_node->id = id;
    memcpy(new_node->name, name, sizeof(name));

    new_node->next = *head_ref;
    *head_ref = new_node;
}

static void node_append(people_list** head_ref, int id, const char* name)
{
    return;
}

static void node_del_head(people_list** head_ref)
{
    return;
}

static void node_del_tail(people_list** head_ref)
{
    return;
}

static void node_show(people_list** head_ref)
{
    people_list* current = NULL;
    current = *head_ref;

    if (current == NULL) {
        printf("Empty list, nothing to show\n");
        return;
    }

    printf("Elements in the list: \n");
    while (current != NULL) {
        printf("id = %d, name = %s\n", current->id, current->name);
        current = current->next;
        if (current == NULL) {
            printf("This is the last element\n");
        }
    }
}

int main(void)
{
    people_list* list = NULL;
#if 0
    people_list* second = NULL;
    people_list* third = NULL;

    list = malloc(sizeof(people_list));
    memset(list, 0, sizeof(people_list));
    second = malloc(sizeof(people_list));
    memset(second, 0, sizeof(people_list));
    third = malloc(sizeof(people_list));
    memset(third, 0, sizeof(people_list));

    list->id = 1;
    memcpy(list->name, "duc", sizeof("duc"));
    list->next = second;

    second->id = 2;
    memcpy(second->name, "hy", sizeof("hy"));
    second->next = third;

    third->id = 3;
    memcpy(third->name, "bo", sizeof("bo"));
    third->next = NULL;
#endif
    char id;
    char name[MAX_LEN_NAME] = {0};
    int option = 0;
    while (1) {
        printf("******************\n");
        printf("1: Insert new node\n");
        printf("2: Append new node\n");
        printf("3: Delete head node\n");
        printf("4: Delete tail node\n");
        printf("5: Show list\n");
        printf("Insert your choice: ");
        scanf("%d", &option);
        switch (option) {
            case INSERT_NODE:
                printf("debug: list=Xh\n", list);
                printf("Enter id's value: ");
                scanf("%d", &id);
                printf("Enter name: ");
                scanf("%s", name);
                printf("debug: list=Xh\n", list);
                node_insert(&list, id, name);
                memset(name, 0, MAX_LEN_NAME);
                break;
            case APPEND_NODE:
                printf("Enter id's value: ");
                scanf("%d", &id);
                printf("Enter name: ");
                scanf("%s", name);
                node_insert(&list, id, name);
                memset(name, 0, MAX_LEN_NAME);
                break;
            case DEL_HEAD:
                node_del_head(&list);
                break;
            case DEL_TAIL:
                node_del_tail(&list);
                break;
            case SHOW_LIST:
                node_show(&list);
                break;
            default:
                printf ("Invalid input, closing program...\n");
                exit(0);
                break;
        }
    }
}

我如何測驗：

• 插入first節點 (1, jon)
• 插入second節點（2，可能）
• 顯示串列 => 僅列印第二個節點。

我發現，second->next不指向first節點，它指向一個 NULL 節點。我繼續除錯如下

            case INSERT_NODE:
                printf("debug: list=Xh\n", list);
                printf("Enter id's value: ");
                scanf("%d", &id);
                printf("Enter name: ");
                scanf("%s", name);
                printf("debug: list=Xh\n", list);
                node_insert(&list, id, name);
                memset(name, 0, MAX_LEN_NAME);
                break;

控制臺日志是

Insert your choice: 1
debug: list=001F29A8h => This is the address holding value of the first node
Enter id's value: 2
Enter name: may  
debug: list=00000000h => The value of list is clear ???

如您所見，ponter 的值list無需修改即可更改。請幫我解決這個問題。謝謝你。

uj5u.com熱心網友回復：

正如評論中指出的那樣，使用scanf("%d", &id);,%d期待int *. 改變

char id;
char name[MAX_LEN_NAME] = {0};

至

int id;
char name[MAX_LEN_NAME] = {0};

否則，您通過將意外型別傳遞給 來呼叫未定義行為scanf。

通過良好的編譯器警告，我們得到以下資訊：

warning: 'memcpy' call operates on objects of type 'const char'
while the size is based on a different type 'const char *'
      [-Wsizeof-pointer-memaccess]
        memcpy(new_node->name, name, sizeof(name));

當sizeof運算子用于指標（在本例中為 a const char *）時，結果是指標的大小（以位元組為單位），而不是它指向的物件。

將此函式呼叫更改為

memcpy(new_node->name, name, sizeof new_node->name);
/* OR memcpy(new_node->name, name, MAX_LEN_NAME); */

或更恰當地說，使用strcpy

strcpy(new_node->name, name);

建議改成#define MAX_LEN_NAME 5更合理的，比如#define MAX_LEN_NAME 128.

同時，scanf("%s", name);應更改為具有限制輸入讀取長度的欄位寬度說明符，以避免緩沖區溢位。這應該是緩沖區大小減一（例如，scanf("7s", name);）。

此外，scanf不應忽略 的回傳值。確保它是預期的成功轉換次數，否則處理失敗。例如，

if (2 != scanf("%d%u", &si, &us)) {
    fprintf(stderr, "Failed to read inputs.\n");
    exit(EXIT_FAILURE);
}

旁白：這些函式呼叫

people_list* new_node = NULL;
new_node = malloc(sizeof(people_list));
memset(new_node, 0, sizeof(people_list));

可以簡化為calloc

people_list *new_node = calloc(1, sizeof *new_node);

標籤：C链表

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