| 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):
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 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:
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)
| 2023-10-31 | Linux man-pages 6.7 |