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NAME | DESCRIPTION | STANDARDS | HISTORY | NOTES | SEE ALSO | COLOPHON |
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inode(7) Miscellaneous Information Manual inode(7)
inode - file inode information
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
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_blocks
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, 1970-01-01 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.
POSIX.1-2008.
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.
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).
stat(1), stat(2), statx(2), symlink(7)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
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for this manual page, see
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2024-06-26. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
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Linux man-pages 6.9.1 2024-06-15 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)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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