inode(7) - Linux manual page (original) (raw)
inode(7) Miscellaneous Information Manual inode(7)
NAME top
inode - file inode informationDESCRIPTION top
Each file has an inode containing metadata about the file. An
application can retrieve this metadata using [stat(2)](../man2/stat.2.html) (or related
calls), which returns a _stat_ structure, or [statx(2)](../man2/statx.2.html), 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)](../man2/stat.2.html) and [statx(2)](../man2/statx.2.html):
Device where inode resides
_stat.stdev_; _statx.stxdevminor_ and _statx.stxdevmajor_
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.stino_; _statx.stxino_
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.stmode_; _statx.stxmode_
See the discussion of file type and mode, below.
Link count
_stat.stnlink_; _statx.stxnlink_
This field contains the number of hard links to the file.
Additional links to an existing file are created using
[link(2)](../man2/link.2.html).
User ID
_stat.stuid_; _statx.stxuid_
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)](../man2/chown.2.html).
Group ID
_stat.stgid_; _statx.stxgid_
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)](../man2/chown.2.html).
Device represented by this inode
_stat.strdev_; _statx.stxrdevminor_ and _statx.stxrdevmajor_
If this file (inode) represents a device, then the inode
records the major and minor ID of that device.
File size
_stat.stsize_; _statx.stxsize_
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.stblksize_; _statx.stxblksize_
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.stblocks_; _statx.stxblocks_
This field indicates the number of blocks allocated to the
file, 512-byte units, (This may be smaller than _stsize_/512
when the file has holes.)
The POSIX.1 standard notes that the unit for the _stblocks_
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.statime_; _statx.stxatime_
This is the file's last access timestamp. It is changed by
file accesses, for example, by [execve(2)](../man2/execve.2.html), [mknod(2)](../man2/mknod.2.html),
[pipe(2)](../man2/pipe.2.html), [utime(2)](../man2/utime.2.html), and [read(2)](../man2/read.2.html) (of more than zero bytes).
Other interfaces, such as [mmap(2)](../man2/mmap.2.html), 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)](../man8/mount.8.html), and related information in [mount(2)](../man2/mount.2.html).)
In addition, the atime timestamp is not updated if a file
is opened with the **O_NOATIME** flag; see [open(2)](../man2/open.2.html).
File creation (birth) timestamp (btime)
(not returned in the _stat_ structure); _statx.stxbtime_
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.stmtime_; _statx.stxmtime_
This is the file's last modification timestamp. It is
changed by file modifications, for example, by [mknod(2)](../man2/mknod.2.html),
[truncate(2)](../man2/truncate.2.html), [utime(2)](../man2/utime.2.html), and [write(2)](../man2/write.2.html) (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.stctime_; _statx.stxctime_
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_, 1970-01-01 00:00:00 +0000, UTC (see [time(7)](../man7/time.7.html)).
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)](../man2/stat.2.html) and [statx(2)](../man2/statx.2.html) 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.stmode field (for statx(2), the statx.stxmode field) contains the file type and mode.
POSIX refers to the _stat.stmode_ 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 _stmode_ 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 _stmode_ field:
**S_ISUID** 04000 set-user-ID bit (see [execve(2)](../man2/execve.2.html))
**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)](../man2/execve.2.html). 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.STANDARDS top
POSIX.1-2008.HISTORY top
POSIX.1-2001.
POSIX.1-1990 did not describe the **S_IFMT**, **S_IFSOCK**, **S_IFLNK**,
**S_IFREG**, **S_IFBLK**, **S_IFDIR**, **S_IFCHR**, **S_IFIFO**, and **S_ISVTX**
constants, but instead specified the use of the macros **S_ISDIR**()
and so on.
The **S_ISLNK**() and **S_ISSOCK**() macros were not in POSIX.1-1996; the
former is from SVID 4, the latter from SUSv2.
UNIX V7 (and later systems) had **S_IREAD**, **S_IWRITE**, **S_IEXEC**, and
where POSIX prescribes the synonyms **S_IRUSR**, **S_IWUSR**, and **S_IXUSR**.NOTES top
For pseudofiles that are autogenerated by the kernel, the file
size (_stat.stsize_; _statx.stxsize_) 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 top
[stat(1)](../man1/stat.1.html), [stat(2)](../man2/stat.2.html), [statx(2)](../man2/statx.2.html), [symlink(7)](../man7/symlink.7.html)COLOPHON top
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man-pages@man7.orgLinux man-pages 6.10 2024-09-01 inode(7)
Pages that refer to this page:systemd-vpick(1), chmod(2), fsync(2), getdents(2), mkdir(2), mknod(2), open(2), stat(2), statx(2), truncate(2), umask(2), utime(2), utimensat(2), stat(3type), systemd.exec(5), symlink(7)