INODE

Section: Linux Programmer's Manual (7)
Updated: 2020-08-13
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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_IFMT0170000bit mask for the file type bit field

S_IFSOCK0140000socket
S_IFLNK0120000symbolic link
S_IFREG0100000regular file
S_IFBLK0060000block device
S_IFDIR0040000directory
S_IFCHR0020000character device
S_IFIFO0010000FIFO

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

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

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

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

NAME
DESCRIPTION
The file type and mode
CONFORMING TO
NOTES
SEE ALSO
COLOPHON