c - C中的链表结构与读取txt

Question

我上传了上面的图片。我问问题，如何从 txt 文件中读取以添加带有结构的链接列表。我尝试这样做，但如何声明basket.txt并生成.txt

FILE *fp
char CustomerName[20];
char CustomerSurname[20];
int  CustomerId;
struct Customer *current = NULL, *head = NULL;
fp = fopen("customer.txt", "r");

while (fscanf(fp, "%d\t%s\t%s", &CustomerId, CustomerName, CustomerSurname) != EOF) {
    struct Customer *ReadList;
    ReadList = (struct Customer *)malloc(sizeof(struct Customer));
    ReadList->NextPtr = NULL;
    ReadList->CustomerId = CustomerId;
    ReadList->CustomerName = strdup(CustomerName);
    ReadList->CustomerSurname = strdup(CustomerSurname);
    if (head == NULL) {
        current = head = ReadList;
    } else {
        current = current->NextPtr = ReadList;
    }
}
fclose(fp);

Answer 1

您的代码中存在一个微妙的问题:解析输入文件的方式不正确:

while (fscanf(fp, "%d\t%s\t%s", &CustomerId, CustomerName, CustomerSurname) != EOF)

• 您应该通过告诉 fscanf 要存储到这些数组中的最大字符数来防止 CustomerName 和 CustomerSurname 中的缓冲区溢出:使用 format “%d\t%19s\t%19s” 。
• 您只在文件末尾检查 fscanf 是否完全失败，它会返回 EOF，但它会返回已解析的字段数，您应该检查这一点为3。如果fscanf无法解析第一个字段的整数，它将返回0并且所有字段的内容将保持不变。您将无限循环地向列表中添加相同的新记录，直到 malloc 最终返回 NULL(您不进行测试)时崩溃。
• 格式字符串中的 \t 字符将匹配文件中的任何空白字符序列，而不仅仅是 TAB 字符。这可能与您的预期不同。请注意，%d 和 %s 都忽略前导空白字符，因此格式字符串中的 \t 字符是完全多余的。<

还有其他问题，例如

• 您没有检查 fopen 失败
• 您不检查内存分配失败

这是一个改进的版本:

FILE *fp
char CustomerName[20];
char CustomerSurname[20];
int  CustomerId;
int res;
struct Customer *current = NULL, *head = NULL;
fp = fopen("customer.txt", "r");
if (fp == NULL) {
    fprintf(stderr, "cannot open input file\n");
    return 1;
}

while ((res = fscanf(fp, "%d\t%19s\t%19s", &CustomerId, CustomerName, CustomerSurname)) == 3) {
    struct Customer *ReadList = malloc(sizeof(*ReadList));
    if (ReadList == NULL) {
        fprintf(stderr, "cannot allocate memory for customer\n");
        fclose(fp);
        return 1;
    }
    ReadList->NextPtr = NULL;
    ReadList->CustomerId = CustomerId;
    ReadList->CustomerName = strdup(CustomerName);
    ReadList->CustomerSurname = strdup(CustomerSurname);
    if (ReadReadList->CustomerName == NULL || ReadList->CustomerSurname == NULL) {
        fprintf(stderr, "cannot allocate memory for customer fields\n");
        fclose(fp);
        return 1;
    }
    if (head == NULL) {
        current = head = ReadList;
    } else {
        current = current->NextPtr = ReadList;
    }
}
if (res != EOF) {
    fprintf(stderr, "input file has incorrect format\n");
    fclose(fp);
    return 1;
}
fclose(fp);