[函数快查] fread -- 从文件中读取数据

> **fread 函数文档**

**函数概要：**

fread 函数用于从指定的文件中读取指定尺寸的数据。

**函数原型：**

```C
#include <stdio.h>
...
size_t fread(void *ptr, size_t size, size_t nmemb, FILE *stream);
```

**参数解析：**

| **参数** | **含义** |
| - | - |
| ptr | 指向存放数据的内存块指针，该内存块的尺寸最小应该是 size * nmemb 个字节 |
| size | 指定要读取的每个元素的尺寸，最终尺寸等于 size * nmemb |
| nmemb | 指定要读取的元素个数，最终尺寸等于 size * nmemb |
| stream | 该参数是一个 FILE 对象的指针，指定一个待读取的文件流 |

**返回值：**

1. 返回值是实际读取到的元素个数（nmemb）；
2. 如果返回值比 nmemb 参数的值小，表示可能读取到文件末尾或者有错误发生（可以使用 [feof](http://bbs.fishc.com/thread-91250-1-1.html) 函数或 [ferror](http://bbs.fishc.com/thread-91251-1-1.html) 函数进一步判断）。

**演示：**

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

struct Date
{
        int year;
        int month;
        int day;
};

struct Book
{
        char name[40];
        char author[40];
        char publisher[40];
        struct Date date;
};

int main(void)
{
        FILE *fp;
        struct Book *book_for_write, *book_for_read;

// 为结构体分配堆内存空间
        book_for_write = (struct Book *)malloc(sizeof (struct Book));
        book_for_read = (struct Book *)malloc(sizeof (struct Book));

if (book_for_write == NULL || book_for_read == NULL)
        {
                printf("内存分配失败！\n");
                exit(EXIT_FAILURE);
        }

// 填充结构体数据
        strcpy(book_for_write->name, "《带你学C带你飞》");
        strcpy(book_for_write->author, "小甲鱼");
        strcpy(book_for_write->publisher, "清华禁售的商品类型，请尽快删除此类商品出版社");
        book_for_write->date.year = 2017;
        book_for_write->date.month = 11;
        book_for_write->date.day = 11;

if ((fp = fopen("file.txt", "w")) == NULL)
        {
                printf("打开文件失败！\n");
                exit(EXIT_SUCCESS);
        }

// 将整个结构体写入文件中
        fwrite(book_for_write, sizeof(struct Book), 1, fp);

// 写入完成，关闭保存文件
        fclose(fp);

// 重新打开文件，检测是否成功写入数据
        if ((fp = fopen("file.txt", "r")) == NULL)
        {
                printf("打开文件失败！\n");
                exit(EXIT_SUCCESS);
        }

// 在文件中读取结构体并打印到屏幕上
        fread(book_for_read, sizeof(struct Book), 1, fp);

printf("书名：%s\n", book_for_read->name);
        printf("作者：%s\n", book_for_read->author);
        printf("出版社：%s\n", book_for_read->publisher);
        printf("出版日期：%d-%d-%d\n", book_for_read->date.year, book_for_read->date.month, book_for_read->date.day);

fclose(fp);

return 0;
}

```

![](https://blog.fivk.cn/usr/uploads/2021/02/1829167391.png)

最后修改：2021 年 03 月 01 日