C语言之二进制读写文件

来源：我的博客站 OceanicKang |《C语言之二进制读写文件》

一、二进制读写

• size_t fread(void *ptr, size_t size, size_t nmemb, FILE *stream) - 读
• size_t fwrite(void *ptr, size_t size, size_t nmemb, FILE *stream) - 写

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：

• 返回值是实际读取到的元素个数（nmemb）
• 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函数或ferror函数进一步判断）

【注】文件是否以二进制形式打开并不影响二进制读写，或者说影响很小

二、例子

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

// 定义时间结构体
struct Date
{
    int year;
    int month;
    int day;
};

// 定义书籍结构体
struct Book
{
    char name[40];
    char author[40];
    struct Date date;
};

int main(void)
{
    struct Book *book_for_write = (struct Book *)malloc(sizeof(struct Book));
    struct Book *book_for_read = (struct Book *)malloc(sizeof(struct Book));
    if (book_for_write == NULL || book_for_read == NULL) {

        perror("申请内存失败！");
        exit(EXIT_FAILURE);

    }

    FILE *fp;

    // 写入 ************************************/
    struct tm *ptime;
    time_t t;
    time(&t);
    ptime = localtime(&t);

    strcpy(book_for_write -> name, "《书籍名称》");
    strcpy(book_for_write -> author, "OceanicKang");
    book_for_write -> date.year = 1900 + ptime -> tm_year;
    book_for_write -> date.month = 1 + ptime -> tm_mon;
    book_for_write -> date.day = ptime -> tm_mday;

    if ((fp = fopen("./test.txt", "w")) == NULL) {

        printf("打开文件失败！");
        exit(EXIT_FAILURE);

    }

    fwrite(book_for_write, sizeof(struct Book), 1, fp);

    fclose(fp);

    // 读取 ************************************/
    if ((fp = fopen("./test.txt", "r")) == NULL) {

        perror("打开文件失败！");
        exit(EXIT_FAILURE);

    }

    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("时间：%d-%d-%d\n", book_for_read -> date.year, book_for_read -> date.month, book_for_read -> date.day);

    fclose(fp);

    free(book_for_write);
    free(book_for_read);

    return 0;
}