Detailed explanation of Linux file operation knowledge points

Related system calls for file operations

create

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

The parameter mode specifies the access permissions of the newly created file. It and umask together determine the final permissions of the file (mode & umask). The umask represents some access permissions that need to be removed when the file is created. It only affects the read, write and execute permissions. The calling function is int umask (int newmask).

Open

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

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

flagsOpen flag

O_RDONLY Open the file as read-only

O_WRONLY Open the file for writing only

O_RDWR Open the file for reading and writing

O_APPEND Open the file in append mode

O_CREAT Create a file

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

O_NOBLOCK Open a file in non-blocking mode

O_TRUNC If the file already exists, delete the contents of the file.

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

When flag is O_CREATE, specify the mode flag to indicate the access rights of the file.

S_IRUSR User can read

S_IWUSR user can write

S_IXUSR user can execute

S_IRWXU User can read, write, and execute

The S_IRGRP group can read

The S_IWGRP group can write

The S_IXGRP group can execute

The S_IRWXG group can read, write, and execute

S_IROTH Others can read

S_IWOTH Others can write

S_IXOTH Others can execute

S_IRWXO Others can read, write, and execute

S_ISUID Set the user's execution ID

S_ISGID Set the execution ID of the group

The mode flag can also use numbers to represent file permissions:

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

The first digit indicates setting the user ID

The second digit indicates the group ID

The third bit indicates the user's own permission bit

The fourth digit indicates the group's permissions

The fifth digit indicates the permissions 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);

The parameter fd is the file descriptor, buf is the pointer to the buffer, length is the size of the buffer (in bytes), and the return value is the number of bytes actually 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 number of bytes actually read

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

position

For random files, we can randomly specify locations to read and write:
int lseek(int fd, offset_t offset, int whence);

lseek() moves the file read/write pointer by offset (which can be negative) bytes relative to whence. If the operation is successful, the file pointer is returned relative to the file header.

The whence parameter can use the following values:

SEEK_SET: Relative to the beginning of the file.
SEEK_CUR: The current position of the relative file read and write pointer.
SEEK_END: ​​Relative to the end of the file.

closure

int close(int fd);

C library function file operations - independent of the specific operating system platform

Create and Open

FILE *fopen(const char *path, const char *mode);
fopen() opens the specified file filename, where mode is the opening mode. The Linux system does not distinguish between binary files and text files.

The value of mode

r, rb Open in read-only mode

w, wb Open for writing only. If the file does not exist, it is created, otherwise it is truncated.

a, ab are opened in append mode. If the file does not exist, create it

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

w+, w+b, wh+ Open for reading and writing. If the file does not exist, a new file is created, otherwise the file is truncated.

a+, a+b, ab+ Open for reading and appending. 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, puts the read fields into the character array pointed to by ptr, and returns the number of fields actually read.

write() writes n fields from the array pointed to by buffer ptr to stream, where each field is size bytes long, and returns the number of fields actually written.

closure

int fclose (FILE *stream);

Linux file system directory structure

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

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

/dev----Device file storage directory. Applications can access actual devices by reading, writing and controlling these files.

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

/home----The home directory of ordinary users. Each user has his or her own directory, which is usually named after the user's account.

/lib----The directory where library files are stored, the system's most basic dynamic link shared library, similar to the DLL file in Windows.

/lost+found----Usually it is empty. When the system crashes unexpectedly or the machine shuts down unexpectedly, some file fragments will be generated and placed here.

/mnt----It is convenient for users to temporarily mount other file systems. For example, if you mount the CD-ROM drive on /mnt/, you can enter this directory to view the contents of the CD-ROM drive.

Media----Automatically identify some devices and mount them in this directory, such as USB flash drives, optical drives, etc.

/opt----Directory for storing additional software installed on the host

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

/root----Home directory of super-privileged users

/sbin----The directory where executable commands of super-privileged users are stored. Ordinary users do not have permission to execute commands in this directory.

/tmp-----Store temporary files.

/usr-----The 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 reflection 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 the temporary root file system during the startup process, after the /linuxrc on it is executed to mount the real root file system, the original initial RAM file system is mapped to the /initrd directory.

Linux file system and device driver