Linux内核（三）对文件的操作（读取、光标）

读取完整代码

#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
#include <string.h>
#include <stdlib.h> //开辟空间malloc需要这个头文件

int main(){
    //定义要写入的内容
    char *buf = "pjy!yyds!by鸭鸭";
    int fd;
        fd = open("./file1",O_RDWR);
            if(fd == -1){
                printf("file open error!\n");
                //如果file1不存在，创建一个file1
                fd = open("./file1",O_RDWR|O_CREAT,0600);
                    if(fd > 0)
                    {
                        printf("create file1 sucessfully!\n");
                    }
            }
        //如果已经存在file1，打印一句话并写入*buf的内容
        printf ("open file1 successfully!&fd = %d\n",fd);
        int n_write = write(fd,buf,strlen(buf));
        //打印写入了多少字节
        if (n_write != -1){
            printf("write%d byte to file\n",n_write);
        }
        //关闭后再打开，若无以下两行代码，将无法读取文件内容，因为光标停在最后，后面没有内容了
        close(fd);
        fd = open("./file1",O_RDWR);
        //读取文本内容
        char *readBuf;
        readBuf = (char*)malloc(sizeof(char)*n_write+1);
        int n_read = read(fd,readBuf,n_write);
        printf("read %d,context%s\n",n_read,readBuf);
        //关闭上述文件
        close(fd);
        return 0;
}

运行效果图

时隔几个月后复习，加了一些内容及思路

思路：

//1.打开（什么方式）

//2.打印文件描述符

//3.如果打开失败（fd=-1）以可读写的方式再来一次

//4.记得加上或操作方便创建新文件，新文件的权限可以写0600

//5.内循环判断文件描述符大于0，打印创建成功

//6.定义n_write

//7.如果fd!=-1,打印（已经写入了多少byte）

//8.暂时关闭文件

//9.读写方式打开

//10.char指针readbuf

//11.赋值readbuf

//12.赋值nread read（）api

//13.打印nread和readbuf，nread统计字数，readbuf记录内容

//14.关闭文件

#include<stdio.h>
#include<sys/types.h>
#include<sys/stat.h>
#include<fcntl.h>
#include<unistd.h>
#include<string.h>
#include<stdlib.h>

int main(){
    int fd;
    char *buf = "pjy yyds!";
    open("./file1",O_RDWR);
    printf("fd = %d\n",fd);
    if (fd = -1)
    {
       fd = open("./file1",O_RDWR|O_CREAT,0600);
        printf("file not exit!\n");
        if(fd > 0){
            printf("create file successfully!\n");
        }
    }   
//6.定义n_write
    int n_write = write(fd,buf,strlen(buf));
//7.如果fd!=-1,打印（已经写入了多少byte）
    if(fd!=-1){
        printf("write %d byte already\n",n_write);
    }
//8.暂时关闭文件
    close(fd);
//9.读写方式打开
     fd = open("./file1",O_RDWR);
//10.char指针readbuf
    char *readBuf;
//11.赋值readbuf
    readBuf = (char*)malloc(sizeof(char)*n_write+1);
//12.赋值nread read（）api
    int n_read = read(fd,readBuf,n_write);
//13.打印nread和readbuf，nread统计字数，readbuf记录内容
    printf("write %d byte,and the text is :%s\n",n_read,readBuf);
//14.关闭文件
    close(fd);
    return 0;
}

以下代码也可实现这个效果

光标移到最开始，偏移值为0

#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
#include <string.h>
#include <stdlib.h> //开辟空间malloc需要这个头文件

int main(){
    //定义要写入的内容
    char *buf = "pjy!yyds!by鸭鸭";
    int fd;
        fd = open("./file1",O_RDWR);
            if(fd == -1){
                printf("file open error!\n");
                //如果file1不存在，创建一个file1
                fd = open("./file1",O_RDWR|O_CREAT,0600);
                    if(fd > 0)
                    {
                        printf("create file1 sucessfully!\n");
                    }
            }
        //如果已经存在file1，打印一句话并写入*buf的内容
        printf ("open file1 successfully!&fd = %d\n",fd);
        int n_write = write(fd,buf,strlen(buf));
        //打印写入了多少字节
        if (n_write != -1){
            printf("write%d byte to file\n",n_write);
        }
        //读取文本内容
        char *readBuf;
        readBuf = (char*)malloc(sizeof(char)*n_write+1);
        lseek(fd,0,SEEK_SET);
        int n_read = read(fd,readBuf,n_write);
        printf("read %d,context%s\n",n_read,readBuf);
        //关闭上述文件
        close(fd);
        return 0;
}

光标停留在当下（最后），偏移值为【-所占字节】

#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
#include <string.h>
#include <stdlib.h> //开辟空间malloc需要这个头文件

int main(){
    //定义要写入的内容
    char *buf = "pjy!yyds!by鸭鸭";
    int fd;
        fd = open("./file1",O_RDWR);
            if(fd == -1){
                printf("file open error!\n");
                //如果file1不存在，创建一个file1
                fd = open("./file1",O_RDWR|O_CREAT,0600);
                    if(fd > 0)
                    {
                        printf("create file1 sucessfully!\n");
                    }
            }
        //如果已经存在file1，打印一句话并写入*buf的内容
        printf ("open file1 successfully!&fd = %d\n",fd);
        int n_write = write(fd,buf,strlen(buf));
        //打印写入了多少字节
        if (n_write != -1){
            printf("write%d byte to file\n",n_write);
        }
        //读取文本内容
        char *readBuf;
        readBuf = (char*)malloc(sizeof(char)*n_write+1);
        lseek(fd,-n_write,SEEK_CUR);
        int n_read = read(fd,readBuf,n_write);
        printf("read %d,context%s\n",n_read,readBuf);
        //关闭上述文件
        close(fd);
        return 0;
}

使用lseek（）函数计算已存在的文件大小

#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
#include <string.h>
#include <stdlib.h> //开辟空间malloc需要这个头文件

int main(){
    //定义要写入的内容
    char *buf = "pjy!yyds!by鸭鸭";
    int fd;
        fd = open("./file1",O_RDWR);
        //计算文件大小
        int fileSize = lseek(fd,0,SEEK_END);
        printf("this file's size is %d",fileSize);
        close(fd);
        return 0;
}

使用lseek（）函数计算已存在的文件大小的效果图

后记碎碎念

Linux是一个系列，可以点击专栏查看同系列的其他文章，希望能帮到屏幕前的每一位应届生往届生，该博文最初发表在CSDN上。