File Attributes

• This is chapter 15, and you should read it.
• A process sometimes needs information about files.
  • #include <sys/stat.h>
  • #include <sys/types.h>
  • #include <unistd.h>
  • int stat(const char *pathname, struct stat *buf);
  • int fstat(int fd, struct stat *buf);
  • int lstat(const char *pathname, struct stat *buf);

  •  struct stat {
                   dev_t     st_dev;         /* ID of device containing file */
                   ino_t     st_ino;         /* inode number */
                   mode_t    st_mode;        /* file type and mode */
                   nlink_t   st_nlink;       /* number of hard links */
                   uid_t     st_uid;         /* user ID of owner */
                   gid_t     st_gid;         /* group ID of owner */
                   dev_t     st_rdev;        /* device ID (if special file) */
                   off_t     st_size;        /* total size, in bytes */
                   blksize_t st_blksize;     /* blocksize for filesystem I/O */
                   blkcnt_t  st_blocks;      /* number of 512B blocks allocated */
                   /* Since Linux 2.6, the kernel supports nanosecond
                      precision for the following timestamp fields.
                      For the details before Linux 2.6, see NOTES. */
    
                   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 */
    
               #define st_atime st_atim.tv_sec      /* Backward compatibility */
               #define st_mtime st_mtim.tv_sec
               #define st_ctime st_ctim.tv_sec
               };
    	

  • These functions take a path or file descriptor and provide information about a file.
  • since stat follows symbolic links, lstat is used to obtain symbolic link information.
  • stat and lstat require only execute permission on the parent directory.
  • See myls.C
  • A timespec is

    struct timespec {
        time_t tv_sec;
        long tv_nsec;
    }
    	

• You can change various values for files.
  • time
    • Access, modification and change dimes are described on page 286.

    •        #include <sys/types.h>
             #include <utime.h>
      
             int utime(const char *filename, const struct utimbuf *times);
      
             #include <sys/time.h>
      
             int utimes(const char *filename, const struct timeval times[2]);
      
             struct utimbuf {
                time_t actime;       /* access time */
                time_t modtime;      /* modification time */
             };
      
      
      
      	    

    • utime sets the access time and modtime fillds of the file, but only for the second.
    • If it is NULL, the time for both is set to the current time.
    • utimes uses an array of timeval structs to do this but only updates to the microsecond.
    • times[0] is the access time, times[1] is the modification time.

    •  
             #include  /* Definition of AT_* constants */
             #include 
      
             int utimensat(int dirfd, const char *pathname,
                           const struct timespec times[2], int flags);
      
             int futimens(int fd, const struct timespec times[2]);
      

    • Give nanosecond precision
    • dirfd
      • if dirfd is AT_FDCWD the pathname is assumed to be a filename
      • Otherwise it is dirfd should be a file descriptor of a directory containing the file.
      • the values of tv_nsec
        • UTIME_OMIT - don't change this time.
        • UTIME_NOW - use the current time.
      • flags:
        • AT_SYMLINK_NOFOLLOW
        • 0
    • Ownership

      •        #include 
        
               int chown(const char *pathname, uid_t owner, gid_t group);
               int fchown(int fd, uid_t owner, gid_t group);
               int lchown(const char *pathname, uid_t owner, gid_t group);
        	    

      • Need permission.
      • Stickey bits are cleared.
    • Permissions

      •        #include 
        
               int chmod(const char *pathname, mode_t mode);
               int fchmod(int fd, mode_t mode);
        	   

      • uses S_ISGID , S_IWUSR, S_IRGRP, ...
  • One more thing: access

    •        #include 
      
             int access(const char *pathname, int mode);
             

    • mode:
      • F_OK - file existance check
      • or bitwise or of R_OK, W_OK,X_OK - check for existance and corresponding permission.
      • uses real UID and GID
      • Returns 0 on success