Basic IO (on): file management and descriptors

-----------------"如果说,Do not fight alone.Then let me go" 

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目录

 (1)复习回顾(C提供文件操作接口):

①File open and file generation location.

 问:我们常说,The file is generated in the current executable program directory,这对不对呢？

② Write and output

fputs: 

fgets:

(2)系统调用接口:

①open\close: 

flagVarious parameters designed according to the bits:

 ②write\read: 

(3)文件描述符:

①文件描述符从0开始,并默认0 1 2to third-rate

怎么验证呢？

 ②文件描述符分配规则:   

重定向的本质:Modify the file descriptor pointed toFILE* . 

redirection and buffers:

缓冲区:The peripheral files are based on the Von Neumann system,效率低.It's a balanced approach.

The stream is opened by default 与 文件标识符:

(4)文件管理

进程与文件的关系:

磁盘文件vs内存文件

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 (1)复习回顾(C提供文件操作接口):

我们先来回顾在C语言时期,用的C文件接口.

①File open and file generation location.

我们先看看fopen函数: 

 问:我们常说,The file is generated in the current executable program directory,这对不对呢？

At this point we let the process run.

//Find the process firstpid
ps ajx | grep test  


//We know that there is more than one viewing process aboveid的做法
ls /proc/+(进程id)

也就是,cwd和exe目录重叠了.

So we go back one level of directory, 

 Found that the file generated is no longer in the executable directory.所以,应当是进程运行路径as where the file is generated.

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② Write and output

fputs: 

Write to text.

The operating system will open three streams for us by default: stdin stdout stderr.

向stdout(显示器)输出:

当然,You must have heard it,linux下一切皆文件. 

So how to understand it？

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 fgets:

 从键盘获取

 

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(2)系统调用接口:

不管是C\C++Each has its own set of file calling interfaces.But they all basically call,interface at the system level.

//CLibraries provide interfaces

fopen fclose fwrite fread

//System file calls
open close write read

①open\close: 

打开文件的flag有多种方式,modeThe latter refers to the permissions for file creation(default & (~umask))

flag参数	说明
O_RDONLY	只读打开
O_WRONLY	只写打开 Does not have the ability to create files
O_RDWR	读写打开
O_CREAT	若文件不存在,will create and then open the usual sum(Open the connection),And requires named file permissionsmode
O_APPEND	对标fopen("a")

flagVarious parameters designed according to the bits:

宏判断,多种选项. 

 

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 ②write\read: 

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(3)文件描述符:

在open中,A successful opening of the file will returnint类型的值,这个就是文件描述符.

①文件描述符从0开始,并默认0 1 2to third-rate

Open several files consecutively.

And print out the file descriptor: 

can be seen from the phenomenon,File descriptors are incremented. 但为什么是从3 开始呢？ 

在前面已经说过,程序运行起来,C会默认打开三个流:

stdin\stdout\stderr,Their file descriptors are respectively0、1、2.

 

怎么验证呢？

首先我们让printf打印这个 Strings are on the screen.

close(1); 

 此时我们发现,什么也没打印.

 ②文件描述符分配规则:   

It was meant to bestdout 打印数据的,But because it's closed,而创建的fd assigned again1

So it is written directly to the file. 这也叫做输出重定向.

重定向的本质:Modify the file descriptor pointed toFILE* . 

 

redirection and buffers:

下面我们来看看这个代码: 

 关闭了stdout,Then it was meant to bestdout写入的,重定向到了log.txt当中.

After making some corrections to the code: 

 

 Why didn't you write itlog里面？

 

Because the data is written to the file,Not operating directly on memory,Instead, the data needs to be written first缓冲区:

 This is also known as input redirection;

缓冲区:The peripheral files are based on the Von Neumann system,效率低.It's a balanced approach.

1.无缓冲

2.行缓冲   以"\n"为标志 进行刷新

3.全缓冲   文件写入(磁盘)

缓冲区在哪里？？

由谁提供？

The OS has buffers？What does it have to do with it？

The code below will answer:

Print strings on the screen using different interfaces.

 

添加fork();

 Why redirect to log内,will become more？！

 

 所以,缓冲区的概念,It is provided by the language.

 

The stream is opened by default 与 文件标识符:

stdout(FILE* )-----> 1(fd).

FILE是一个结构体 ,封装了fd

所以stdout-->FILE对象(fd=1) Only accept bids,But don't care about the content~

 

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(4)文件管理

当进程运行,Call the file function,打开文件.Multiple processes can open multiple files.So we need an operating system,Manage these files. 

管理的本质: 先描述 再组织.

//文件信息(内容+属性)   
strcut file
{
  int _fd; 
}

进程与文件的关系:

磁盘文件vs内存文件

磁盘文件:内容+属性.

内存文件:More is attribute information.

 

 

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(5)使用dup2:

 简单来说,Just let the old descriptor 拷贝 新的文件描述符.即,对new重定向.

 

 把fd1的内容 Overwrite write to1里面.

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感谢你的阅读

祝你好运~