Shulou(Shulou.com)06/01 Report--

Xiaobian to share with you the example analysis of Linux file operation, I believe most people still do not know how, so share this article for everyone's reference, I hope you have a lot of harvest after reading this article, let's go to understand it together!

create

int creat(const char *filename, mode_t mode);

The parameter mode specifies the access permissions of the new file, which together with umask determines the final permissions of the file (mode&umask), where umask represents some access permissions that need to be removed when the file is created, it only affects read, write and execute permissions, and the call function is int umask(int newmask).

open

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

pathname is the name of the file we want to open (including the path name, default under the current path)

flags Open flags

O_RDONLY opens files read-only

O_WRONLY Open file in write-only mode

O_RDWR Open file read-write

O_APPEND Open file by appending

O_CREAT creates a file

O_EXEC If O_CREAT is used and the file already exists, an error occurs

O_NOBLOCK opens a file in a non-blocking manner

O_TRUNC If the file already exists, delete its contents

int open（const char *pathname,int flag，mode_t mode）

When flag is O_CREATE, specifies the mode flag used to indicate file access rights

S_IRUSR users can read

S_IWUSR Users can write

S_IXUSR user can execute

S_IRWXU Users can read, write and execute

S_IRGRP group can be read

S_IWGRP group can be written

S_IXGRP group can execute

S_IRWXG group can read, write and execute

S_IROTH Others can read

S_IWOTH Others can write

S_IXOTH Others can perform

S_IRWXO Others can read, write and execute

S_ISUID Sets the user's execution ID

S_ISGID Execution ID of Settings Group

The mode flag can also be used to indicate file permissions numerically:

Each number can take 1 (execute permission), 2 (write permission), 4 (read permission), 0 (none), or the sum of these values.

The first digit indicates the setting user ID

The second digit indicates the setting group ID

The third digit represents the user's own permission bit

The fourth digit indicates group permissions

The fifth digit indicates the authority of others.

open("test", O_CREAT, 10705);

The above statement is equivalent to:

open("test", O_CREAT, S_IRWXU | S_IROTH | S_IXOTH | S_ISUID );

read and write

int read(int fd, const void *buf, size_t length);int write(int fd, const void *buf, size_t length);

Parameter fd File descriptor, buf is a pointer to the buffer, length is the size of the buffer in bytes, and the return value is the actual number of bytes read and written.

read( ) reads length bytes from the file specified by the file descriptor fd into the buffer pointed to by buf, and the return value is the actual number of bytes read

The write( ) implementation writes length bytes from the buffer pointed to by buf to the file pointed to by the file descriptor fd, and returns the number of bytes actually written.

positioning

For random files, we can randomly specify where to read and write:

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

lseek() Moves the file read-write pointer offset (negative) bytes relative to whence. On success, returns the position of the file pointer relative to the file header.

The parameter whence can take the following values:

SEEK_SET: Relative to the beginning of the file.

SEEK_CUR: Current position relative to the file read/write pointer.

SEEK_END: Relative to the end of the file.

closed

int close(int fd);

File Operation of C Library Functions--Independent of Specific Operating System Platform

create and open

FILE *fopen(const char *path, const char *mode);

fopen() to open the specified file filename, where the mode is open mode, Linux system does not distinguish between binary files and text files.

Value of mode

r, rb open in read-only mode

W, WB open in write-only mode. If the file does not exist, create it, otherwise the file is truncated

A, AB is opened by appending. If the file does not exist, create it.

r+, r+b, rb+ open in read-write mode

w+, w+b, wh+ open in read-write mode. If the file does not exist, a new file is created, otherwise the file is truncated

A+, a+b, ab+ are opened in read and append mode. If the file does not exist, create a new file

read and write

int fgetc(FILE *stream);int fputc(int c, FILE *stream);char *fgets(char *s, int n, FILE *stream);int fputs(const char *s, FILE *stream);int fprintf(FILE *stream, const char *format, ...); int fscanf (FILE *stream, const char *format, ...); size_t fread(void *ptr, size_t size, size_t n, FILE *stream);size_t fwrite (const void *ptr, size_t size, size_t n, FILE *stream);int fsetpos(FILE *stream, fpos_t *pos);nt fsetpos(FILE *stream, const fpos_t *pos);int fseek(FILE *stream, long offset, int whence);

fread() reads n fields from stream, each field is size bytes, and puts the read fields into the character array referred to by ptr, returning the actual number of fields read.

write() implements writing n fields from the array referred to by buffer ptr to stream, each field is size bytes long, and returns the actual number of fields written.

closed

int fclose (FILE *stream);

Linux file system directory structure

/bin---Holds the most frequently used basic commands, such as ls, cp, mkdir, etc. The files in this directory are executable.

/boot---Some core files used when booting Linux, including some connection files and image files, such as vmlinuz, initrd.img

/dev---The directory where the device files are stored, and the application can access the actual device by reading, writing, and controlling these files.

/etc---Configuration files and subdirectories required for system administration, such as user account and password configuration files.

/home---home directory of ordinary users, each user has its own directory, usually named after the user's account.

/lib---Library file storage directory, the system's most basic dynamic connection shared library, similar to Windows DLL files.

/lost+found---Normally empty, some file fragments will be placed here when the system crashes unexpectedly or the machine shuts down unexpectedly.

/mnt---Convenient for users to temporarily mount other file systems, such as mounting the optical drive on/mnt/, enter the directory to view the contents of the optical drive

media---Automatically identifies some devices mounted in this directory, such as USB flash drives, CD-ROMs, etc.

/opt---Directory where additional installation software is stored for the host

/proc---When the operating system is running, process and kernel information (such as CPU, hard disk partition, memory information, etc.) is stored here. It is a mapping of system memory, stored in memory, and system information is obtained by directly accessing this directory.

/root----Home directory of super user

/sbin---The executable commands of super privilege users are stored in the directory, and ordinary users have no permission to execute the commands in this directory.

/tmp----Store temporary files.

/usr----Directory where system applications and files (such as commands and help files) are stored, similar to the program files directory under Windows.

/var----directories that are frequently modified are placed in this directory, such as log files

/sys---A visual representation of the kernel device tree. When a kernel object is created, the corresponding files and directories are also created in the kernel object subsystem.

/initrd--If the initrd image is used as a temporary root file system during boot, the original initial RAM file system is mapped to the/initrd directory after the/linuxrc mount to the real root file system is performed.

The above is "Linux file operation example analysis" all the content of this article, thank you for reading! I believe that everyone has a certain understanding, hope to share the content to help everyone, if you still want to learn more knowledge, welcome to pay attention to the industry information channel!

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