Linux file status information and permissions

Posted by iceomnia on Sat, 12 Feb 2022 09:59:00 +0100

fstat,stat,lstat

Get file status information (status).

prototype

#include<unistd.h>
#include<sys/stat.h>
#include<sys/types.h>

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


Write the file status information into the stat structure pointed to by buf

stat structure

struct stat {
		mode_t    st_mode;        /* File type and mode(permission) */
		ino_t     st_ino;         /* Inode number */
    	dev_t     st_dev;         /* ID of device containing file */
    	dev_t     st_rdev;        /* Device ID (if special file) */
    	nlink_t   st_nlink;       /* Number of hard links */
    	uid_t     st_uid;         /* User ID of owner */
        gid_t     st_gid;         /* Group ID of owner */
        off_t     st_size;        /* Total size, in bytes */
        struct timespec st_atim;  /* Time of last access */
        struct timespec st_mtim;  /* Time of last modification */
        struct timespec st_ctim;  /* Time of last status change */
        blksize_t st_blksize;     /* Block size for filesystem I/O */
        blkcnt_t  st_blocks;      /* Number of 512B blocks allocated */
        };
  • st_mode
mode_t      st_mode; //unsigned int, only the lower 16 bits are used

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  1. file type

    Macro (the parameter is st_mode)file type
    S_ISREG()Regular file
    S_ISDIR()Directory file
    S_ISLNK()Symbolic link
    S_ISCHR()Character special file
    S_ISBLK()Block special file
    S_ISSOCK()Socket
    S_ISFIFO()Pipeline or FIFO

    Example:

    if(S_ISREG(buf.st_mode))
    printf("Files are ordinary files")；

  2. Permission attribute

  3. Saved text bit s_ ISVTX)

    If the adhesive bit of an executable program is set, a copy of the program body (machine instruction) is saved in the exchange area when the first execution of the program terminates.

  4. Set user ID and group ID when executing

//File type properties area
#define  S_ Bit mask for ifmt 0170000 file type
#define  S_IFSOCK    0140000     socket
#define  S_ Iflnk 012000 symbolic link
#define  S_ Ifreg 0100000 general documents
#define  S_ Ifblk 00600000 block device
#define  S_ Ifdir 0040000 directory
#define  S_ Ifchr 0020000 character device
#define  S_ IFO 0010000 first in first out (fifo)

#define S_ ISSOCK(m) (((m) & S_ IFMT) == S_ Ifsock) / / some macros are provided to help users perform & operations. If yes, 1 is returned
#define S_ISLNK(m)  (((m) & S_IFMT) == S_IFLNK)  
#define S_ISREG(m)  (((m) & S_IFMT) == S_IFREG)
#define S_ISBLK(m)  (((m) & S_IFMT) == S_IFBLK)
#define S_ISDIR(m)  (((m) & S_IFMT) == S_IFDIR)
#define S_ISCHR(m)  (((m) & S_IFMT) == S_IFCHR)
#define S_ISFIFO(m) (((m) & S_IFMT) == S_IFIFO)

//Special attribute area for permissions
#define  S_ (set user ID on execution) bit of isuid 0004000 file
#define  S_ (set group ID on execution) bit of isgid 0002000 file
#define  S_ sticky bit of isvtx 0001000 file

//Permission attribute area
//File owner
#define S_IRWXU 00700	/* mask for file owner permissions */
#define S_IRUSR 00400	/* owner has read permission */
#define S_IWUSR 00200	/* owner has write permission */
#define S_IXUSR 00100	/* owner has execute permission */
 //group user (group)
#define S_IRWXG 00070	/* mask for group permissions */
#define S_IRGRP 00040	/* group has read permission */
#define S_IWGRP 00020	/* group has write permission */
#define S_IXGRP 00010	/* group has execute permission */
 //other users
#define S_IRWXO 00007	/* mask for permissions for others (not in group) */
#define S_IROTH 00004	/* others have read permission */
#define S_IWOTH 00002	/* others have write permission */
#define S_IXOTH 00001	/* others have execute permission */

access and faccessat

Test access with actual user ID and actual group ID.

#include<unistd>

int access(const char *pathname,int mode);
int faccessat(int fd,const char *pathname,int mode,int flag);

access is consistent with faccessat in the following two cases

  1. The pathname parameter is an absolute path
  2. pathname parameter is relative path, and fd parameter is AT_FDCMD

Factessat tests the pathname relative to the open directory (fd)

  • mode parameter

    1. Whether the test file exists: F_OK
    2. Test access: R_ OK, W_ OK, X_ The bitwise or of OK (read, write, and execute permissions, respectively).

  • flag parameter

    Used to change the behavior of factessat.

Return value

It returns 0 on success and - 1 on error

umask system call

Controls the default permissions for files.

#include<sys/stat.h>

mode_t umask(mode_t cmask);

cmask is the bitwise or composition of the following 9 constants. If the bit is 1, the corresponding permission is closed.

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chmod, fchmod and fchmodet

Change access to existing files.

#include<sys/stat.h>

int chmod(const char *pathname,mode_t mode);
int fchmod(int fd,mode_t mode);
int fchmodat(int fd,const char *pathname,mode_t mode,int flag);

Return value

Return 0 when successful; Return - 1 in case of error

In the following two cases, fchmod is consistent with fchmodat

  1. The pathname parameter is an absolute path
  2. pathname parameter is relative path, and fd parameter is AT_FDCMD

fchmodat tests pathname. Relative to the open directory (fd)

flag parameter

Used to change the behavior of fchmodat.

When set to at_ SYMLINK_ When nofollow, dosen't follow symbolic links

chown, fchown, fchownat, and lchown

Change the user ID and group ID of the file

#include<unistd.h>

int chown(const char *pathname,uid_t owner,gid_t group);
int lchown(const char *pathname,uid_t owner,gid_t group);
int fchown(int fd,uid_t owner,gid_t group);
int fchownat(int fd,const char *pathname,uid_t owner,gid_t group,int flag);

If the owner or group parameter is - 1, the corresponding ID remains unchanged.

  • When the referenced file is a symbolic link, the following function is used to change the symbolic link itself.
    • lchown
    • Set the parameter to hofcat, flag_ SYMLINK_ NOFOLLOW

gid_t group);
int fchownat(int fd,const char *pathname,uid_t owner,gid_t group,int flag);

owner or group Parameter is-1，Then the corresponding ID unchanged.

- When the referenced file is a symbolic link, the following function is used to change the symbolic link itself.
  - lchown
  - fchownat, flag The parameter is set to AT_SYMLINK_NOFOLLOW

Topics: Linux Operation & Maintenance server

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