'\" t .\" Copyright (c) 2017 Michael Kerrisk .\" .\" %%%LICENSE_START(VERBATIM) .\" Permission is granted to make and distribute verbatim copies of this .\" manual provided the copyright notice and this permission notice are .\" preserved on all copies. .\" .\" Permission is granted to copy and distribute modified versions of this .\" manual under the conditions for verbatim copying, provided that the .\" entire resulting derived work is distributed under the terms of a .\" permission notice identical to this one. .\" .\" Since the Linux kernel and libraries are constantly changing, this .\" manual page may be incorrect or out-of-date. The author(s) assume no .\" responsibility for errors or omissions, or for damages resulting from .\" the use of the information contained herein. The author(s) may not .\" have taken the same level of care in the production of this manual, .\" which is licensed free of charge, as they might when working .\" professionally. .\" .\" Formatted or processed versions of this manual, if unaccompanied by .\" the source, must acknowledge the copyright and authors of this work. .\" %%%LICENSE_END .\" .TH INODE 7 2017-09-15 "Linux" "Linux Programmer's Manual" .SH NAME inode \- file inode information .SH DESCRIPTION Each file has an inode containing metadata about the file. An application can retrieve this metadata using .BR stat (2) (or related calls), which returns a .I stat structure, or .BR statx (2), which returns a .I statx structure. .PP 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 .BR stat (2) and .BR statx (2): .TP Device where inode resides \fIstat.st_dev\fP; \fIstatx.stx_dev_minor\fP and \fIstatx.stx_dev_major\fP .IP 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). .TP Inode number \fIstat.st_ino\fP; \fIstatx.stx_ino\fP .IP 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. .TP File type and mode \fIstat.st_mode\fP; \fIstatx.stx_mode\fP .IP See the discussion of file type and mode, below. .TP Link count \fIstat.st_nlink\fP; \fIstatx.stx_nlink\fP .IP This field contains the number of hard links to the file. Additional links to an existing file are created using .BR link (2). .TP User ID .I st_uid \fIstat.st_uid\fP; \fIstatx.stx_uid\fP .IP 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 .BR chown (2). .TP Group ID \fIstat.st_gid\fP; \fIstatx.stx_gid\fP .IP 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 .BR chown (2). .TP Device represented by this inode \fIstat.st_rdev\fP; \fIstatx.stx_rdev_minor\fP and \fIstatx.stx_rdev_major\fP .IP If this file (inode) represents a device, then the inode records the major and minor ID of that device. .TP File size \fIstat.st_size\fP; \fIstatx.stx_size\fP .IP 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. .TP Preferred block size for I/O \fIstat.st_blksize\fP; \fIstatx.stx_blksize\fP .IP 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.) .TP Number of blocks allocated to the file \fIstat.st_blocks\fP; \fIstatx.stx_size\fP .IP This field indicates the number of blocks allocated to the file, 512-byte units, (This may be smaller than .IR st_size /512 when the file has holes.) .IP The POSIX.1 standard notes .\" Rationale for sys/stat.h in POSIX.1-2008 that the unit for the .I st_blocks member of the .I 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. .TP Last access timestamp (atime) \fIstat.st_atime\fP; \fIstatx.stx_atime\fP .IP This is the file's last access timestamp. It is changed by file accesses, for example, by .BR execve (2), .BR mknod (2), .BR pipe (2), .BR utime (2), and .BR read (2) (of more than zero bytes). Other interfaces, such as .BR mmap (2), may or may not update the atime timestamp .IP 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 .IR noatime , .IR nodiratime , and .I relatime in .BR mount (8), and related information in .BR mount (2).) In addition, the atime timestamp is not updated if a file is opened with the .BR O_NOATIME flag; see .BR open (2). .TP File creation (birth) timestamp (btime) (not returned in the \fIstat\fP structure); \fIstatx.stx_btime\fP .IP The file's creation timestamp. This is set on file creation and not changed subsequently. .IP The btime timestamp was not historically present on UNIX systems and is not currently supported by most Linux filesystems. .\" FIXME Is it supported on ext4 and XFS? .TP Last modification timestamp (mtime) \fIstat.st_atime\fP; \fIstatx.stx_mtime\fP .IP This is the file's last modification timestamp. It is changed by file modifications, for example, by .BR mknod (2), .BR truncate (2), .BR utime (2), and .BR 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 .I not changed for changes in owner, group, hard link count, or mode. .TP Last status change timestamp (ctime) \fIstat.st_ctime\fP; \fIstatx.stx_ctime\fP .IP 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.). .PP Nanosecond timestamps are supported on XFS, JFS, Btrfs, and ext4 (since Linux 2.6.23). .\" commit ef7f38359ea8b3e9c7f2cae9a4d4935f55ca9e80 Nanosecond timestamps are not supported in ext2, ext3, and Reiserfs. On filesystems that do not support subsecond timestamps, the nanosecond fields in the .I stat and .I statx structures are returned with the value 0. .\" .SS The file type and mode The .I stat.st_mode field (for .BR statx (2), the .I statx.stx_mode field) contains the file type and mode. .PP POSIX refers to the .I stat.st_mode bits corresponding to the mask .B S_IFMT (see below) as the .IR "file type" , the 12 bits corresponding to the mask 07777 as the .IR "file mode bits" and the least significant 9 bits (0777) as the .IR "file permission bits" . .PP The following mask values are defined for the file type: .in +4n .TS lB l l. 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 .TE .in .PP Thus, to test for a regular file (for example), one could write: .PP .in +4n .EX stat(pathname, &sb); if ((sb.st_mode & S_IFMT) == S_IFREG) { /* Handle regular file */ } .EE .in .PP Because tests of the above form are common, additional macros are defined by POSIX to allow the test of the file type in .I st_mode to be written more concisely: .RS 4 .TP 1.2i .BR S_ISREG (m) is it a regular file? .TP .BR S_ISDIR (m) directory? .TP .BR S_ISCHR (m) character device? .TP .BR S_ISBLK (m) block device? .TP .BR S_ISFIFO (m) FIFO (named pipe)? .TP .BR S_ISLNK (m) symbolic link? (Not in POSIX.1-1996.) .TP .BR S_ISSOCK (m) socket? (Not in POSIX.1-1996.) .RE .PP The preceding code snippet could thus be rewritten as: .PP .in +4n .EX stat(pathname, &sb); if (S_ISREG(sb.st_mode)) { /* Handle regular file */ } .EE .in .PP The definitions of most of the above file type test macros are provided if any of the following feature test macros is defined: .BR _BSD_SOURCE (in glibc 2.19 and earlier), .BR _SVID_SOURCE (in glibc 2.19 and earlier), or .BR _DEFAULT_SOURCE (in glibc 2.20 and later). In addition, definitions of all of the above macros except .BR S_IFSOCK and .BR S_ISSOCK () are provided if .BR _XOPEN_SOURCE is defined. .PP The definition of .BR S_IFSOCK can also be exposed either by defining .BR _XOPEN_SOURCE with a value of 500 or greater or (since glibc 2.24) by defining both .BR _XOPEN_SOURCE and .BR _XOPEN_SOURCE_EXTENDED . .PP The definition of .BR S_ISSOCK () is exposed if any of the following feature test macros is defined: .BR _BSD_SOURCE (in glibc 2.19 and earlier), .BR _DEFAULT_SOURCE (in glibc 2.20 and later), .BR _XOPEN_SOURCE with a value of 500 or greater, .BR _POSIX_C_SOURCE with a value of 200112L or greater, or (since glibc 2.24) by defining both .BR _XOPEN_SOURCE and .BR _XOPEN_SOURCE_EXTENDED . .PP The following mask values are defined for the file mode component of the .I st_mode field: .in +4n .TS lB l l. S_ISUID 04000 set-user-ID bit 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 T{ others (not in group) have read, write, and execute permission T} S_IROTH 00004 others have read permission S_IWOTH 00002 others have write permission S_IXOTH 00001 others have execute permission .TE .in .PP The set-group-ID bit .RB ( S_ISGID ) has several special uses. For a directory, it indicates that BSD semantics is 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 .B S_ISGID bit set. For a file that does not have the group execution bit .RB ( S_IXGRP ) set, the set-group-ID bit indicates mandatory file/record locking. .PP The sticky bit .RB ( 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. .SH CONFORMING TO If you need to obtain the definition of the .IR blkcnt_t or .IR blksize_t types from .IR , then define .BR _XOPEN_SOURCE with the value 500 or greater (before including .I any header files). .PP POSIX.1-1990 did not describe the .BR S_IFMT , .BR S_IFSOCK , .BR S_IFLNK , .BR S_IFREG , .BR S_IFBLK , .BR S_IFDIR , .BR S_IFCHR , .BR S_IFIFO , .B S_ISVTX constants, but instead specified the use of the macros .BR S_ISDIR (), and so on. The .BR S_IF* constants are present in POSIX.1-2001 and later. .PP The .BR S_ISLNK () and .BR 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. .PP UNIX\ V7 (and later systems) had .BR S_IREAD , .BR S_IWRITE , .BR S_IEXEC , where POSIX prescribes the synonyms .BR S_IRUSR , .BR S_IWUSR , .BR S_IXUSR . .SH NOTES For pseudofiles that are autogenerated by the kernel, the file size (\fIstat.st_size\fP; \fIstatx.stx_size\fP) reported by the kernel is not accurate. For example, the value 0 is returned for many files under the .I /proc directory, while various files under .IR /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 \(aq\e0\(aq to the returned buffer if it is to be interpreted as a string). .IR st_atimensec . .SH SEE ALSO .BR stat (1), .BR stat (2), .BR statx (2), .BR symlink (7) .SH COLOPHON This page is part of release 4.16 of the Linux .I man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at \%https://www.kernel.org/doc/man\-pages/.