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File IO (1)

2022-06-23 10:03:00 【Snail is also persistent】

（1） file IO：

Open file

function ：open

The header file ：

#include <sys/types.h>

#include <sys/stat.h>

#include <fcntl.h>

The function prototype ：

int open(const char *pathname, int flags); ------》 Suitable for opening files in read-only mode

function ： Used to open a file

Parameters ：

Parameters 1： Open file name （ Can include path ）

Parameters 2： How to open a file

There are three ways ： Main sign （ Mutually exclusive ）

O_RDONLY

O_WRONLY

O_RDWR

Secondary sign ：O_CREAT O_TRUNC

Return value ：

Successfully returned a file descriptor （ Nonnegative integers ）

Failure returns -1（errno is set）

int open(const char *pathname, int flags, mode_t mode);---》 Suitable for opening files in write only mode

function ： Used to open a file

Parameters ：

Parameters 1： Open file name （ Can include path ）

Parameters 2： How to open a file

There are three ways ： Main sign （ Mutually exclusive ）

O_RDONLY

O_WRONLY

O_RDWR

Secondary sign ：O_CREAT O_TRUNC

Parameters 3： jurisdiction -----》 It can be expressed in octal

Return value ：

Successfully returned a file descriptor （ Nonnegative integers ）

Failure returns -1（errno is set）

Operation file

2-1 Writing documents ：

#include <unistd.h>

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

function ：

Used to write data to the specified open file

Parameters ：

Parameters 1： The file descriptor after successfully opening the file

Parameters 2： A buffer that stores data to be written

Parameters 3： The number of data elements to be written

Return value ：

Success returns the number of successfully written data

Failure returns -1（ It's set up errno Value ）

2-2 Reading documents ：

#include <unistd.h>

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

function ：

Read data from the specified open file

Parameters ：

Parameters 1： The file descriptor after successfully opening the file

Parameters 2： A buffer that stores the read content

Parameters 3： The number of data elements to be read

Return value ：

Success returns the number of successfully read data

Failure returns -1（ It's set up errno Value ）

The return value is equal to 0, The representative has finished reading

Close file

#include <unistd.h>

int close(int fd);

Parameters ： The file descriptor after successfully opening the file

Case study 1： Realize the use of four functions in combination

Case study 2： Utilization of documents IO Realization CP command

location

The name of the function ：lseek

The header file ：

#include <sys/types.h>

#include <unistd.h>

The function prototype ：

off_t lseek(int fd, off_t offset, int whence);

function ： Realize jump

Parameters ：

Parameters 1： The file descriptor after successfully opening the file

Parameters 2： Offset

Parameters 3： Base point position relative to offset

SEEK_SET At the beginning of the file

SEEK_CUR File current location

SEEK_END End of file

Return value ： If successful, the current displacement of the file will be returned .

Failure returns -1;

Case study ： verification lseek What is the return value of ？

summary ：lseek The return value of represents the current number of bytes successfully offset ！

Empty files

5-1. What is an empty file ？

5-2. How to create an empty file ？

The code implementation process is as follows ：（ file IO Realized ）, Standards can also be used IO Realization

Catalog related

Open Directory

The header file ：

#include <sys/types.h>

#include <dirent.h>

The function prototype ：

DIR *opendir(const char *name);

function ： Open a directory

Parameters ：

Name of the directory to be opened （ You can take the path ）

Return value ：

Success returns a directory pointer

Failure returns NULL

Read the table of contents

#include <dirent.h>

The function prototype ：

struct dirent *readdir(DIR *dirp);

function ： Used to read the contents of the directory

Parameters ： The return value of successfully opening the directory

Return value ：

struct dirent {

ino_t d_ino; /* inode number */

off_t d_off; /* offset to the next dirent */

unsigned short d_reclen; /* length of this record */

unsigned char d_type; /* type of file; not supported

by all file system types */

char d_name[256]; /* filename */

};

Case study 1： Realization ls -a command

Case study 2： Realization ls command

Learn how to test file properties ？

function ：stat/lstat/fstat

The header file

#include <sys/types.h>

#include <sys/stat.h>

#include <unistd.h>

The function prototype ：

int stat(const char *path, struct stat *buf);

int fstat(int fd, struct stat *buf);

int lstat(const char *path, struct stat *buf);

Be careful ： The above three functions can test the properties of the file .

among ：

about fstat for ： The first parameter of this function is the file descriptor of a file that has been opened , We don't usually take this approach .

about stat and lstat for ： These two parameters are the same , But its nature is slightly different .

stat: It is called a trace function or a penetration function , It means if you use stat Function to test file properties , The first parameter passed in is a soft link file , This means that the function tests the source file pointed to by the linked file , Not the linked file itself .

lstat： Not tracking （ through ） function , This means that when the first parameter passed in is a linked file , The test attribute is also the attribute of the linked file itself .

Analysis function ： With lstat For example ：

int lstat(const char *path, struct stat *buf);

function ： The implementation can test the properties of the specified file

Parameters ：

Parameters 1： File name to be tested （ Path taking ）

Parameters 2： An address information structure for storing file attributes

Return value ： Successfully returns 0, Failure to return -1

Be careful ：lstat The address information structure of the second parameter is as follows ：

struct stat {

dev_t st_dev; /* ID of device containing file */

ino_t st_ino; /* inode number */

mode_t st_mode; /* protection */