用c语言编写图书馆管理系统中(3)图书的借阅和归还：包括图书的借阅记录 查询、借阅图书、归还图书代码及代码注释

在C语言中，创建一个简单的图书馆管理系统涉及到基本的数据结构，如链表用于存储图书信息和借阅记录。以下是图书借阅和归还功能的大致代码实现：

#include <stdio.h>
#include <stdlib.h> // 包含内存管理函数

// 定义图书结构体
typedef struct {
    char book_id[20];
    char title[50];
    int available; // 图书是否可供借阅，0表示已借出
} Book;

// 定义借阅记录结构体
typedef struct {
    Book* book;
    time_t borrow_date;
} BorrowRecord;

// 链表头结点
typedef struct Library {
    Book* books;
    BorrowRecord* borrow_records;
    int size;
} Library;

// 功能函数声明
void init_library(Library* lib);
Book* find_book_by_id(Library* lib, char id[]);
BorrowRecord* add_borrow_record(Library* lib, Book* book);
void return_book(Library* lib, BorrowRecord* record);

// 主程序入口
int main() {
    Library library;
    init_library(&library);

    // 示例操作：借书
    Book* book = find_book_by_id(&library, "001");
    if (book->available) {
        BorrowRecord* new_record = add_borrow_record(&library, book);
        printf("Book %s has been borrowed.\n", book->title);
        free(book); // 回收不再使用的图书内存
    } else {
        printf("Book is not available.\n");
    }

    // 示例操作：归还图书
    BorrowRecord* record = ...; // 获取需要归还的记录
    return_book(&library, record);
    printf("Book %s returned successfully.\n", record->book->title);

    return 0;
}

// 初始化库
void init_library(Library* lib) {
    lib->books = NULL;
    lib->borrow_records = NULL;
    lib->size = 0;
}

// 查找图书
Book* find_book_by_id(Library* lib, char id[]) {
    for (int i = 0; i < lib->size; i++) {
        if (!strcmp(lib->books[i].book_id, id)) {
            return &lib->books[i];
        }
    }
    return NULL;
}

// 添加借阅记录
BorrowRecord* add_borrow_record(Library* lib, Book* book) {
    BorrowRecord* new_record = (BorrowRecord*)malloc(sizeof(BorrowRecord));
    new_record->book = book;
    new_record->borrow_date = time(NULL);
    *lib->borrow_records = *new_record; // 将新记录添加到链表头部
    lib->borrow_records++;
    lib->size++;
    return new_record;
}

// 归还图书
void return_book(Library* lib, BorrowRecord* record) {
    // 更新书籍可用状态
    record->book->available = 1;

    // 移除借阅记录
    if (lib->borrow_records == record) { // 如果是最新的借阅记录
        lib->borrow_records--;
    } else {
        BorrowRecord** prev = &lib->borrow_records;
        while (*prev != record) {
            prev = &(*prev)->next;
        }
        *prev = record->next;
    }
    free(record); // 回收不再使用的记录内存
}