Stat

• This is from chapter 15.1

•        #include <sys/types.h>
         #include <sys/stat.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;        /* protection */
                 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 calls provide access to file statistics.
• stat deals with files but not symbolic links,
  • Symbolic links are followed.
• For lstat symbolic links are reported on.
• Permission on the file is not needed, but execute permission on the directory containing the file is.
• Return is -1 on error or 0 otherwise.
• The stat struct
  • st_dev contains the major and minor numbers of the device on which the file is located.
    • There are macros major() and minor() which report these.
  • st_ino is the inode number.
    • The author says that the combination of these two uniquely identify a file.
  • st_size is the size of a regular file in bytes, or the length of the path stored in a symbolic link without null termination.
  • st_blocks the number of blocks allocated to the file.
    • This is a way to detect files with holes if the holes are bigger than a block. (ie st_blocks != ceil(st_size/512)) In 512 bye blocks
  • st_blksize - this is the preferred block size.
  • st_uid/st_gid user and group id of the file owner.
  • st_nlink - the number of hard links to the file.
  • st_mode - encodes file type and permissions
    • S_ISREG(), S_ISDIR(), S_ISCHR(), S_ISBLK(), S_ISFIFO(), S_ISLNK(), S_ISSOCK()
    • S_IRWXU, S_IRUSR, S_IWUSR, S_IXUSR, ... S_IRGRP, ...S_IXOTH
    • S_ISUID, S_ISGID, S_ISVTX
  • Set GID bit
    • On directories, new files get the group id from the director, not from the group id of the creating process.
    • On files, where group execution is not set, mandatory file locking is enforced.
  • The Sticky bit (S_ISVTX) on a directory means that the owner of a file or by the owner of the directory.
    • This is for directories like /tmp where permissions are usually rwx for all. (777)
    • Normally anyone could rename or remove anything in the directory.
    • On HP-UX this keeps executable files in swap (for gcc, vi, bash ...)