文件编程（一）

1.文件编程概述
api　　open
　　　 read/write
　　　 lseek
　　　 close

2.文件打开和创建
#include<sys/types.h>
#include<sys/stat.h>
#include<fcntl.h>

int open(const char *pathname,int flags)
int open(const char *pathname,int flags,mode_t mode)
int creat(const char *pathname,mode_t mode)

pathname:要打开的文件名
flags： O_RDONLY只读打开 O_WRONLY只写打开 O_RDWR可读可写打开
下列常数是可以选择的：
O_CREAT若文件不存在则创建它，在使用此选项时，需要同时说明第三个参数mode
O_EXCL如果同时指定了O_CREAT，而文件已经存在，则出错
O_APPEND每次写时都加到文件的末端，不写会覆盖
O_TRUNC 属性去打开文件时，如果这个文件中本来是有内容的，而且为只读或只写成功打开，则将其长度截短为0

Mode：一定是在flags中使用O_CREAT标志，mode记录待创建的文件的访问权限

open的返回值就是文件描述符

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

int main(){
        int fd;

        fd = open("./file1",O_RDWR);
  
        if(fd == -1){
                printf("open file1 failed\n");
                fd = open("./file1",O_RDWR|O_CREAT,0600);
                if(fd > 0){
                        printf("creat file1 success\n");
                }
        }       
        printf("fd = %d\n",fd);

        return 0;
}

3.创建文件creat函数

int creat(const char *filename,mode_t mode)
filename:要创建的文件名
mode：创建模式
常见创建模式：
S_IRUSR 4 可读
S_IWUSR 2 可写
S_IXUSR 1 可执行
S_IRWXU 7 可读可写可执行

4.文件写入操作编程
#include<unistd.h>

ssize_t write(int fd,const void *buf,size_t count)

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


int main(){
        int fd;
        char *buf = "linqinlaoshi hen shuai!";

        fd = open("./file1",O_RDWR);

        if(fd == -1){
                printf("open file1 failed");
                fd = open("./file1",O_RDWR|O_CREAT,0600);
                if(fd > 0){
                        printf("creat file1 success");
                }
        }
        printf("open sucess fd = %d\n",fd);

        write(fd,buf,strlen(buf));

        close(fd);

        return 0;
}

5.文件读取操作
#include<unistd.h>

ssize_t read(int fd,void *buf,size_t count)

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


int main(){
    int fd;
    char *buf = "linqinlaoshi hen shuai!";

    fd = open("./file1",O_RDWR);

    if(fd == -1){
        printf("open file1 failed\n");
        fd = open("./file1",O_RDWR|O_CREAT,0600);
        if(fd > 0){
            printf("creat file1 success");
        }
    }    
    printf("open sucess fd = %d\n",fd);
    

    int n_write = write(fd,buf,strlen(buf));
    if(n_write !== -1){
        printf("write %d byte to file\n",n_write);
    }
    
    close(fd);
    fd = open("./file1",O_RDWR);    
    
    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);
    close(fd);

    return 0;
}

6.文件光标移动操作
文件“光标”位置
将文件读写指针相对whence移动offset个字节
#include<sys/types.h>
#include<unistd.h>

off_t lseek(int fd,off_t offset,int whence)
SEEK_SET 指向文件的头
SEEK_GND 指向文件的尾
SEEK_END 当前位置

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


int main(){
        int fd;
        char *buf = "linqinlaoshi hen shuai!";

        fd = open("./file1",O_RDWR);

        if(fd == -1){
                perror("open file1 failed\n");
                printf("open file1 failed\n");
                fd = open("./file1",O_RDWR|O_CREAT,0600);
                if(fd > 0){
                        printf("creat file1 success");
                }
        }
        printf("open sucess fd = %d\n",fd);


        int n_write = write(fd,buf,strlen(buf));
        if(n_write != -1){
                printf("write %d byte to file\n",n_write);
        }

        lseek(fd,0,SEEK_SET);

        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);
        close(fd);

        return 0;
}