Linux之操作文件的系统调用

素材狗 2022-09-11 08:45:01 文章分类：系统/运维点击数：次

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目录1.打开文件参数介绍2.读文件3.写文件4.关闭分析题练习题系统调用和库函数的区别需要引入的头文件：#inlcudeunistd.h1.打开文件打开一个已存在的文件intopen(const...

1.打开文件

打开一个


int open(const char *pathname, int flags);


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

参数介绍

pathname：将要打开的文件路径和名称

flags：打开标志


The  argument  flags  must  include  one of the following access modes:
O_RDONLY, O_WRONLY, or O_RDWR.  These request opening  the  file  read-
only, write-only, or read/write, respectively.

O_RDONLY 只读打开

O_RDWR 读写打开

O_CREAT 文件不存在则创建

O_APPEND 文件末尾追加

O_TRUNC 清空文件，重新写入 mode


The following symbolic constants are provided for mode:

S_IRWXU  00700 user (file owner) has read,  write,  and  execute permission
                       

S_IRUSR  00400 user has read permission

S_IWUSR  00200 user has write permission

S_IXUSR  00100 user has execute permission

S_IRWXG  00070 group has read, write, and execute permission

S_IRGRP  00040 group has read permission

S_IWGRP  00020 group has write permission

S_IXGRP  00010 group has execute permission

S_IRWXO  00007 others have read, write, and execute permission

S_IROTH  00004 others have read permission

S_IWOTH  00002 others have write permission

S_IXOTH  00001 others have execute permission

返回值：文件描述符

2. 读文件


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

参数介绍

fd：对应打开的文件描述符buf ：存放数据的空间count：计划一次从文件中读多少字节数据返回值：实际读到的字节数

3. 写文件


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

参数介绍：

fd ：对应打开的文件描述符buf：存放待写入的数据count：计划一次向文件中写入多少数据

4.关闭


int close(int fd);

fd ：对应的文件描述符

分析题

文件内容


#include<stdio.h>
#include<unistd.h>
#include<assert.h>
#include<fcntl.h>
#include<stdlib.h>

int main()
{
    char buff[128] = {0};

    int fd = open("myfile.txt", O_RDONLY);

    pid_t pid = fork();
    assert(pid != -1);

    if (pid == 0)
    {
        read(fd, buff, 1);
        printf("child buff = %s\n", buff);

        sleep(1);
        read(fd, buff, 1);
        printf("child buff = %s\n", buff);

    }
    else
    {
        read(fd, buff, 1);
        printf("parent buff = %s\n", buff);

        sleep(1);
        read(fd, buff, 1);
        printf("parent buff = %s\n", buff);
    }

    close(fd);

    exit(0);
}

运行结果：

：

由于 fork 创建的子进程的 PCB 是，子进程的 PCB 中的文件表指向打开文件的指针只是拷贝了父进程 PCB 中的值，所以父子进程共享父进程 fork 之前打开的所有文件描述符。

练习题

原文件内容为：

代码：


#include<stdio.h>
#include<unistd.h>
#include<fcntl.h>
#include<stdlib.h>
#include<assert.h>

int main(void)
{
    char buff[128] = {0};

    int fdr = open("myfile.txt", O_RDONLY);
    assert(fdr != -1);

    int fdw = open("newfile.txt", O_WRONLY | O_CREAT, 0600);
    assert(fdw != -1);

    int n = 0;
    while (n = read(fdr, buff, 128) > 0)
    {
        write(fdw, buff, n);
    }

    close(fdr);
    close(fdw);
    
    exit(0);
}

运行示例：

可以看到newfile.txt创建成功