1INODE(7)                   Linux Programmer's Manual                  INODE(7)


6       inode - file inode information


9       Each file has an inode containing metadata about the file.  An applica‐
10       tion can retrieve this metadata using stat(2) (or related calls), which
11       returns a stat structure, or statx(2), which returns a statx structure.
13       The following is a list of the information typically found in, or asso‐
14       ciated with, the file inode, with the names of the corresponding struc‐
15       ture fields returned by stat(2) and statx(2):
17       Device where inode resides
18              stat.st_dev; statx.stx_dev_minor and statx.stx_dev_major
20              Each  inode  (as  well  as  the  associated  file)  resides in a
21              filesystem that is hosted on a device.  That device  is  identi‐
22              fied  by  the  combination of its major ID (which identifies the
23              general class of device) and minor ID (which identifies  a  spe‐
24              cific instance in the general class).
26       Inode number
27              stat.st_ino; statx.stx_ino
29              Each file in a filesystem has a unique inode number.  Inode num‐
30              bers are guaranteed to be unique only within a filesystem (i.e.,
31              the  same  inode  numbers  may be used by different filesystems,
32              which is the reason that hard links  may  not  cross  filesystem
33              boundaries).  This field contains the file's inode number.
35       File type and mode
36              stat.st_mode; statx.stx_mode
38              See the discussion of file type and mode, below.
40       Link count
41              stat.st_nlink; statx.stx_nlink
43              This field contains the number of hard links to the file.  Addi‐
44              tional links to an existing file are created using link(2).
46       User ID
47              st_uid stat.st_uid; statx.stx_uid
49              This field records the user ID of the owner of  the  file.   For
50              newly  created  files, the file user ID is the effective user ID
51              of the creating process.  The user ID of a file can  be  changed
52              using chown(2).
54       Group ID
55              stat.st_gid; statx.stx_gid
57              The  inode  records  the ID of the group owner of the file.  For
58              newly created files, the file group ID is either the group ID of
59              the  parent  directory or the effective group ID of the creating
60              process, depending on whether or not the set-group-ID bit is set
61              on the parent directory (see below).  The group ID of a file can
62              be changed using chown(2).
64       Device represented by this inode
65              stat.st_rdev; statx.stx_rdev_minor and statx.stx_rdev_major
67              If this file (inode) represents a device, then the inode records
68              the major and minor ID of that device.
70       File size
71              stat.st_size; statx.stx_size
73              This  field  gives the size of the file (if it is a regular file
74              or a symbolic link) in bytes.  The size of a  symbolic  link  is
75              the  length  of  the pathname it contains, without a terminating
76              null byte.
78       Preferred block size for I/O
79              stat.st_blksize; statx.stx_blksize
81              This  field  gives  the  "preferred"  blocksize  for   efficient
82              filesystem  I/O.  (Writing to a file in smaller chunks may cause
83              an inefficient read-modify-rewrite.)
85       Number of blocks allocated to the file
86              stat.st_blocks; statx.stx_size
88              This field indicates the number of blocks allocated to the file,
89              512-byte  units,  (This may be smaller than st_size/512 when the
90              file has holes.)
92              The POSIX.1 standard notes that the unit for the st_blocks  mem‐
93              ber  of  the  stat structure is not defined by the standard.  On
94              many  implementations it is 512 bytes; on a few systems, a  dif‐
95              ferent  unit  is  used, such as 1024.  Furthermore, the unit may
96              differ on a per-filesystem basis.
98       Last access timestamp (atime)
99              stat.st_atime; statx.stx_atime
101              This is the file's last access timestamp.  It is changed by file
102              accesses,   for   example,   by  execve(2),  mknod(2),  pipe(2),
103              utime(2), and read(2) (of more than zero bytes).   Other  inter‐
104              faces,  such  as  mmap(2), may or may not update the atime time‐
105              stamp
107              Some filesystem types allow mounting in such  a  way  that  file
108              and/or  directory  accesses  do not cause an update of the atime
109              timestamp.  (See noatime, nodiratime, and relatime in  mount(8),
110              and  related  information  in mount(2).)  In addition, the atime
111              timestamp is not updated if a file is opened with the  O_NOATIME
112              flag; see open(2).
114       File creation (birth) timestamp (btime)
115              (not returned in the stat structure); statx.stx_btime
117              The file's creation timestamp.  This is set on file creation and
118              not changed subsequently.
120              The btime timestamp was not historically present on UNIX systems
121              and is not currently supported by most Linux filesystems.
123       Last modification timestamp (mtime)
124              stat.st_atime; statx.stx_mtime
126              This  is  the file's last modification timestamp.  It is changed
127              by file modifications, for example,  by  mknod(2),  truncate(2),
128              utime(2), and write(2) (of more than zero bytes).  Moreover, the
129              mtime timestamp of a directory is changed  by  the  creation  or
130              deletion of files in that directory.  The mtime timestamp is not
131              changed for changes in owner, group, hard link count, or mode.
133       Last status change timestamp (ctime)
134              stat.st_ctime; statx.stx_ctime
136              This is the file's last status change timestamp.  It is  changed
137              by  writing or by setting inode information (i.e., owner, group,
138              link count, mode, etc.).
140       Nanosecond timestamps are supported on XFS, JFS, Btrfs, and ext4 (since
141       Linux  2.6.23).  Nanosecond timestamps are not supported in ext2, ext3,
142       and Reiserfs.  On filesystems that do not support subsecond timestamps,
143       the  nanosecond  fields  in  the stat and statx structures are returned
144       with the value 0.
146   The file type and mode
147       The stat.st_mode field (for statx(2), the  statx.stx_mode  field)  con‐
148       tains the file type and mode.
150       POSIX  refers to the stat.st_mode bits corresponding to the mask S_IFMT
151       (see below) as the file type, the 12 bits  corresponding  to  the  mask
152       07777  as the file mode bits and the least significant 9 bits (0777) as
153       the file permission bits.
155       The following mask values are defined for the file type:
157           S_IFMT     0170000   bit mask for the file type bit field
159           S_IFSOCK   0140000   socket
160           S_IFLNK    0120000   symbolic link
161           S_IFREG    0100000   regular file
162           S_IFBLK    0060000   block device
163           S_IFDIR    0040000   directory
164           S_IFCHR    0020000   character device
165           S_IFIFO    0010000   FIFO
167       Thus, to test for a regular file (for example), one could write:
169           stat(pathname, &sb);
170           if ((sb.st_mode & S_IFMT) == S_IFREG) {
171               /* Handle regular file */
172           }
174       Because tests of the above  form  are  common,  additional  macros  are
175       defined  by  POSIX  to allow the test of the file type in st_mode to be
176       written more concisely:
178           S_ISREG(m)  is it a regular file?
180           S_ISDIR(m)  directory?
182           S_ISCHR(m)  character device?
184           S_ISBLK(m)  block device?
186           S_ISFIFO(m) FIFO (named pipe)?
188           S_ISLNK(m)  symbolic link?  (Not in POSIX.1-1996.)
190           S_ISSOCK(m) socket?  (Not in POSIX.1-1996.)
192       The preceding code snippet could thus be rewritten as:
194           stat(pathname, &sb);
195           if (S_ISREG(sb.st_mode)) {
196               /* Handle regular file */
197           }
199       The definitions of most of the above file type test macros are provided
200       if any of the following feature test macros is defined: _BSD_SOURCE (in
201       glibc 2.19 and earlier), _SVID_SOURCE (in glibc 2.19 and  earlier),  or
202       _DEFAULT_SOURCE (in glibc 2.20 and later).  In addition, definitions of
203       all of the above macros except S_IFSOCK and S_ISSOCK() are provided  if
204       _XOPEN_SOURCE is defined.
206       The  definition  of  S_IFSOCK  can  also  be exposed either by defining
207       _XOPEN_SOURCE with a value of 500 or greater or (since glibc  2.24)  by
208       defining both _XOPEN_SOURCE and _XOPEN_SOURCE_EXTENDED.
210       The definition of S_ISSOCK() is exposed if any of the following feature
211       test macros is  defined:  _BSD_SOURCE  (in  glibc  2.19  and  earlier),
212       _DEFAULT_SOURCE  (in  glibc 2.20 and later), _XOPEN_SOURCE with a value
213       of 500 or greater, _POSIX_C_SOURCE with a value of 200112L or  greater,
214       or   (since   glibc   2.24)   by   defining   both   _XOPEN_SOURCE  and
217       The following mask values are defined for the file  mode  component  of
218       the st_mode field:
220           S_ISUID     04000   set-user-ID bit
221           S_ISGID     02000   set-group-ID bit (see below)
222           S_ISVTX     01000   sticky bit (see below)
224           S_IRWXU     00700   owner has read, write, and execute permission
225           S_IRUSR     00400   owner has read permission
226           S_IWUSR     00200   owner has write permission
227           S_IXUSR     00100   owner has execute permission
229           S_IRWXG     00070   group has read, write, and execute permission
230           S_IRGRP     00040   group has read permission
231           S_IWGRP     00020   group has write permission
232           S_IXGRP     00010   group has execute permission
234           S_IRWXO     00007   others  (not  in group) have read, write, and
235                               execute permission
236           S_IROTH     00004   others have read permission
237           S_IWOTH     00002   others have write permission
238           S_IXOTH     00001   others have execute permission
240       The set-group-ID bit (S_ISGID) has several special uses.  For a  direc‐
241       tory, it indicates that BSD semantics is to be used for that directory:
242       files created there inherit their group ID from the directory, not from
243       the effective group ID of the creating process, and directories created
244       there will also get the S_ISGID bit set.  For a file that does not have
245       the  group  execution bit (S_IXGRP) set, the set-group-ID bit indicates
246       mandatory file/record locking.
248       The sticky bit (S_ISVTX) on a directory  means  that  a  file  in  that
249       directory  can  be renamed or deleted only by the owner of the file, by
250       the owner of the directory, and by a privileged process.


253       If you need to obtain the definition of the blkcnt_t or blksize_t types
254       from  <sys/stat.h>,  then  define  _XOPEN_SOURCE  with the value 500 or
255       greater (before including any header files).
257       POSIX.1-1990 did not describe the S_IFMT, S_IFSOCK,  S_IFLNK,  S_IFREG,
258       S_IFBLK,  S_IFDIR,  S_IFCHR,  S_IFIFO,  S_ISVTX  constants, but instead
259       specified the use of the macros S_ISDIR(), and so on.  The  S_IF*  con‐
260       stants are present in POSIX.1-2001 and later.
262       The  S_ISLNK() and S_ISSOCK() macros were not in POSIX.1-1996, but both
263       are present in POSIX.1-2001; the former is from SVID 4, the latter from
264       SUSv2.
266       UNIX V7 (and later systems) had S_IREAD, S_IWRITE, S_IEXEC, where POSIX
267       prescribes the synonyms S_IRUSR, S_IWUSR, S_IXUSR.


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


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


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289Linux                             2017-09-15                          INODE(7)