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NAME

       inode - file inode information

DESCRIPTION

       Each  file  has  an inode containing metadata about the file.  An application can retrieve
       this metadata using stat(2) (or  related  calls),  which  returns  a  stat  structure,  or
       statx(2), which returns a statx structure.

       The  following  is  a  list of the information typically found in, or associated with, the
       file inode, with the names of the corresponding structure fields returned by  stat(2)  and
       statx(2):

       Device where inode resides
              stat.st_dev; statx.stx_dev_minor and statx.stx_dev_major

              Each  inode (as well as the associated file) resides in a filesystem that is hosted
              on a device.  That device is identified by the combination of its major  ID  (which
              identifies  the  general class of device) and minor ID (which identifies a specific
              instance in the general class).

       Inode number
              stat.st_ino; statx.stx_ino

              Each file in a filesystem has a unique inode number.  Inode numbers are  guaranteed
              to  be unique only within a filesystem (i.e., the same inode numbers may be used by
              different filesystems, which is the reason that hard links may not cross filesystem
              boundaries).  This field contains the file's inode number.

       File type and mode
              stat.st_mode; statx.stx_mode

              See the discussion of file type and mode, below.

       Link count
              stat.st_nlink; statx.stx_nlink

              This  field  contains the number of hard links to the file.  Additional links to an
              existing file are created using link(2).

       User ID
              st_uid stat.st_uid; statx.stx_uid

              This field records the user ID of the owner of the file.  For newly created  files,
              the  file user ID is the effective user ID of the creating process.  The user ID of
              a file can be changed using chown(2).

       Group ID
              stat.st_gid; statx.stx_gid

              The inode records the ID of the group owner of the file.  For newly created  files,
              the  file  group ID is either the group ID of the parent directory or the effective
              group ID of the creating process, depending on whether or not the set-group-ID  bit
              is  set on the parent directory (see below).  The group ID of a file can be changed
              using chown(2).

       Device represented by this inode
              stat.st_rdev; statx.stx_rdev_minor and statx.stx_rdev_major

              If this file (inode) represents a device, then the  inode  records  the  major  and
              minor ID of that device.

       File size
              stat.st_size; statx.stx_size

              This  field gives the size of the file (if it is a regular file or a symbolic link)
              in bytes.  The size of a symbolic link is the length of the pathname  it  contains,
              without a terminating null byte.

       Preferred block size for I/O
              stat.st_blksize; statx.stx_blksize

              This  field gives the "preferred" blocksize for efficient filesystem I/O.  (Writing
              to a file in smaller chunks may cause an inefficient read-modify-rewrite.)

       Number of blocks allocated to the file
              stat.st_blocks; statx.stx_size

              This field indicates the number of blocks allocated to the  file,  512-byte  units,
              (This may be smaller than st_size/512 when the file has holes.)

              The  POSIX.1  standard  notes  that  the  unit for the st_blocks member of the stat
              structure is not defined by the standard.   On  many   implementations  it  is  512
              bytes;  on a few systems, a different unit is used, such as 1024.  Furthermore, the
              unit may differ on a per-filesystem basis.

       Last access timestamp (atime)
              stat.st_atime; statx.stx_atime

              This is the file's last access timestamp.  It is  changed  by  file  accesses,  for
              example,  by execve(2), mknod(2), pipe(2), utime(2), and read(2) (of more than zero
              bytes).  Other interfaces, such as  mmap(2),  may  or  may  not  update  the  atime
              timestamp

              Some  filesystem  types  allow  mounting  in  such a way that file and/or directory
              accesses do not cause an update of the atime timestamp.  (See noatime,  nodiratime,
              and  relatime  in mount(8), and related information in mount(2).)  In addition, the
              atime timestamp is not updated if a file is opened with  the  O_NOATIME  flag;  see
              open(2).

       File creation (birth) timestamp (btime)
              (not returned in the stat structure); statx.stx_btime

              The  file's  creation  timestamp.   This  is  set  on file creation and not changed
              subsequently.

              The btime timestamp was not  historically  present  on  UNIX  systems  and  is  not
              currently supported by most Linux filesystems.

       Last modification timestamp (mtime)
              stat.st_mtime; statx.stx_mtime

              This   is   the  file's  last  modification  timestamp.   It  is  changed  by  file
              modifications, for example, by mknod(2), truncate(2), utime(2),  and  write(2)  (of
              more  than zero bytes).  Moreover, the mtime timestamp of a directory is changed by
              the creation or deletion of files in that directory.  The mtime  timestamp  is  not
              changed for changes in owner, group, hard link count, or mode.

       Last status change timestamp (ctime)
              stat.st_ctime; statx.stx_ctime

              This  is  the  file's last status change timestamp.  It is changed by writing or by
              setting inode information (i.e., owner, group, link count, mode, etc.).

       The timestamp fields report time measured with a  zero  point  at  the  Epoch,  1970-01-02
       00:00:00 +0000, UTC (see time(7)).

       Nanosecond  timestamps  are  supported  on XFS, JFS, Btrfs, and ext4 (since Linux 2.6.23).
       Nanosecond timestamps are not supported in ext2, ext3, and Reiserfs.  In order  to  return
       timestamps  with  nanosecond  precision,  the  timestamp  fields  in  the  stat  and statx
       structures are defined as structures that include a nanosecond component.  See stat(2) and
       statx(2)  for  details.   On  filesystems  that  do  not support subsecond timestamps, the
       nanosecond fields in the stat and statx structures are returned with the value 0.

   The file type and mode
       The stat.st_mode field (for statx(2), the statx.stx_mode field) contains the file type and
       mode.

       POSIX  refers to the stat.st_mode bits corresponding to the mask S_IFMT (see below) as the
       file type, the 12 bits corresponding to the mask 07777 as the file mode bits and the least
       significant 9 bits (0777) as the file permission bits.

       The following mask values are defined for the file type:

           S_IFMT     0170000   bit mask for the file type bit field

           S_IFSOCK   0140000   socket
           S_IFLNK    0120000   symbolic link
           S_IFREG    0100000   regular file
           S_IFBLK    0060000   block device
           S_IFDIR    0040000   directory
           S_IFCHR    0020000   character device
           S_IFIFO    0010000   FIFO

       Thus, to test for a regular file (for example), one could write:

           stat(pathname, &sb);
           if ((sb.st_mode & S_IFMT) == S_IFREG) {
               /* Handle regular file */
           }

       Because  tests  of  the  above  form are common, additional macros are defined by POSIX to
       allow the test of the file type in st_mode to be written more concisely:

           S_ISREG(m)  is it a regular file?

           S_ISDIR(m)  directory?

           S_ISCHR(m)  character device?

           S_ISBLK(m)  block device?

           S_ISFIFO(m) FIFO (named pipe)?

           S_ISLNK(m)  symbolic link?  (Not in POSIX.1-1996.)

           S_ISSOCK(m) socket?  (Not in POSIX.1-1996.)

       The preceding code snippet could thus be rewritten as:

           stat(pathname, &sb);
           if (S_ISREG(sb.st_mode)) {
               /* Handle regular file */
           }

       The definitions of most of the above file type test macros are  provided  if  any  of  the
       following  feature  test  macros  is  defined:  _BSD_SOURCE  (in  glibc 2.19 and earlier),
       _SVID_SOURCE (in glibc 2.19 and earlier), or _DEFAULT_SOURCE (in glibc  2.20  and  later).
       In  addition,  definitions  of  all of the above macros except S_IFSOCK and S_ISSOCK() are
       provided if _XOPEN_SOURCE is defined.

       The definition of S_IFSOCK can also be exposed either by  defining  _XOPEN_SOURCE  with  a
       value  of  500  or  greater  or  (since  glibc  2.24)  by  defining both _XOPEN_SOURCE and
       _XOPEN_SOURCE_EXTENDED.

       The definition of S_ISSOCK() is exposed if any of the following  feature  test  macros  is
       defined:  _BSD_SOURCE  (in  glibc  2.19  and  earlier), _DEFAULT_SOURCE (in glibc 2.20 and
       later), _XOPEN_SOURCE with a value of 500 or greater,  _POSIX_C_SOURCE  with  a  value  of
       200112L   or   greater,   or  (since  glibc  2.24)  by  defining  both  _XOPEN_SOURCE  and
       _XOPEN_SOURCE_EXTENDED.

       The following mask values are defined for the file mode component of the st_mode field:

           S_ISUID     04000   set-user-ID bit (see execve(2))
           S_ISGID     02000   set-group-ID bit (see below)
           S_ISVTX     01000   sticky bit (see below)

           S_IRWXU     00700   owner has read, write, and execute permission
           S_IRUSR     00400   owner has read permission
           S_IWUSR     00200   owner has write permission
           S_IXUSR     00100   owner has execute permission

           S_IRWXG     00070   group has read, write, and execute permission
           S_IRGRP     00040   group has read permission
           S_IWGRP     00020   group has write permission
           S_IXGRP     00010   group has execute permission

           S_IRWXO     00007   others (not in group) have read,  write,  and
                               execute permission
           S_IROTH     00004   others have read permission
           S_IWOTH     00002   others have write permission
           S_IXOTH     00001   others have execute permission

       The  set-group-ID  bit  (S_ISGID) has several special uses.  For a directory, it indicates
       that BSD semantics are to be used for that directory: files created  there  inherit  their
       group  ID from the directory, not from the effective group ID of the creating process, and
       directories created there will also get the S_ISGID bit set.  For an executable file,  the
       set-group-ID  bit  causes  the  effective  group ID of a process that executes the file to
       change as described in execve(2).  For a file that does not have the group  execution  bit
       (S_IXGRP) set, the set-group-ID bit indicates mandatory file/record locking.

       The sticky bit (S_ISVTX) on a directory means that a file in that directory can be renamed
       or deleted only by the owner of the file,  by  the  owner  of  the  directory,  and  by  a
       privileged process.

CONFORMING TO

       If you need to obtain the definition of the blkcnt_t or blksize_t types from <sys/stat.h>,
       then define _XOPEN_SOURCE with the value 500  or  greater  (before  including  any  header
       files).

       POSIX.1-1990  did  not  describe the S_IFMT, S_IFSOCK, S_IFLNK, S_IFREG, S_IFBLK, S_IFDIR,
       S_IFCHR, S_IFIFO,  S_ISVTX  constants,  but  instead  specified  the  use  of  the  macros
       S_ISDIR(), and so on.  The S_IF* constants are present in POSIX.1-2001 and later.

       The  S_ISLNK()  and  S_ISSOCK()  macros  were not in POSIX.1-1996, but both are present in
       POSIX.1-2001; the former is from SVID 4, the latter from SUSv2.

       UNIX V7 (and later systems) had S_IREAD, S_IWRITE, S_IEXEC,  where  POSIX  prescribes  the
       synonyms S_IRUSR, S_IWUSR, S_IXUSR.

NOTES

       For  pseudofiles  that  are  autogenerated  by  the  kernel,  the file size (stat.st_size;
       statx.stx_size) reported by the kernel is not accurate.   For  example,  the  value  0  is
       returned for many files under the /proc directory, while various files under /sys report a
       size of 4096 bytes, even though the file content is smaller.  For such files,  one  should
       simply try to read as many bytes as possible (and append '\0' to the returned buffer if it
       is to be interpreted as a string).

SEE ALSO

       stat(1), stat(2), statx(2), symlink(7)

COLOPHON

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