stat, fstat, lstat — get file status
#include <sys/types.h> #include <sys/stat.h> #include <unistd.h>
int
stat( |
const char * | path, |
struct stat * | buf) ; |
int
fstat( |
int | filedes, |
struct stat * | buf) ; |
int
lstat( |
const char * | path, |
struct stat * | buf) ; |
These functions return information about a file. No
permissions are required on the file itself, but — in
the case of stat
() and
lstat
() — execute
(search) permission is required on all of the directories in
path
that lead to the
file.
stat
() stats the file
pointed to by path
and fills in buf
.
lstat
() is identical to
stat
(), except that if
path
is a symbolic
link, then the link itself is stat-ed, not the file that it
refers to.
fstat
() is identical to
stat
(), except that the file to
be stat-ed is specified by the file descriptor filedes
.
All of these system calls return a stat
structure, which contains the
following fields:
struct stat { }; dev_t st_dev
;/* ID of device containing file */ ino_t st_ino
;/* inode number */ mode_t st_mode
;/* protection */ nlink_t st_nlink
;/* number of hard links */ uid_t st_uid
;/* user ID of owner */ gid_t st_gid
;/* group ID of owner */ dev_t st_rdev
;/* device ID (if special file) */ off_t st_size
;/* total size, in bytes */ blksize_t st_blksize
;/* blocksize for filesystem I/O */ blkcnt_t st_blocks
;/* number of blocks allocated */ time_t st_atime
;/* time of last access */ time_t st_mtime
;/* time of last modification */ time_t st_ctime
;/* time of last status change */
The st_dev
field
describes the device on which this file resides.
The st_rdev
field describes the device that this file (inode)
represents.
The st_size
field gives the size of the file (if it is a regular file or
a symbolic link) in bytes. The size of a symlink is the
length of the pathname it contains, without a trailing null
byte.
The st_blocks
field indicates the number of blocks allocated to the file,
512-byte units. (This may be smaller than st_size
/512, for example,
when the file has holes.)
The st_blksize
field gives the "preferred" blocksize for efficient file
system I/O. (Writing to a file in smaller chunks may cause an
inefficient read-modify-rewrite.)
Not all of the Linux filesystems implement all of the time
fields. Some file system types allow mounting in such a way
that file accesses do not cause an update of the st_atime
field. (See
`noatime' in mount(8).)
The field st_atime
is changed by file
accesses, e.g. by execve(2), mknod(2), pipe(2), utime(2) and read(2) (of more than zero
bytes). Other routines, like mmap(2), may or may not
update st_atime
.
The field st_mtime
is changed by file
modifications, e.g. by mknod(2), truncate(2), utime(2) and write(2) (of more than zero
bytes). Moreover, st_mtime
of a directory is
changed by the creation or deletion of files in that
directory. The st_mtime
field is not
changed for changes in
owner, group, hard link count, or mode.
The field st_ctime
is changed by
writing or by setting inode information (i.e., owner, group,
link count, mode, etc.).
The following POSIX macros are defined to check the file
type using the st_mode
field:
- 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 following flags are defined for the st_mode
field:
S_IFMT | 0170000 | bitmask for the file type bitfields |
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 |
S_ISUID | 0004000 | set UID bit |
S_ISGID | 0002000 | set-group-ID bit (see below) |
S_ISVTX | 0001000 | sticky bit (see below) |
S_IRWXU | 00700 | mask for file owner permissions |
S_IRUSR | 00400 | owner has read permission |
S_IWUSR | 00200 | owner has write permission |
S_IXUSR | 00100 | owner has execute permission |
S_IRWXG | 00070 | mask for group permissions |
S_IRGRP | 00040 | group has read permission |
S_IWGRP | 00020 | group has write permission |
S_IXGRP | 00010 | group has execute permission |
S_IRWXO | 00007 | mask for permissions for others (not in group) |
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 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 S_ISGID bit set. 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.
Since kernel 2.5.48, the stat
structure supports nanosecond
resolution for the three file timestamp fields. Glibc exposes
the nanosecond component of each field using names either of
the form st_atim.tv_nsec
, if the
_BSD_SOURCE or _SVID_SOURCE feature test macro is defined, or
of the form st_atimensec
, if neither of
these macros is defined. On file systems that do not support
sub-second timestamps, these nanosecond fields are returned
with the value 0.
For most files under the /proc
directory, stat
() does not return the file size in the
st_size
field;
instead the field is returned with the value 0.
On success, zero is returned. On error, −1 is
returned, and errno
is set
appropriately.
Search permission is denied for one of the
directories in the path prefix of path
. (See also path_resolution(2).)
filedes
is
bad.
Bad address.
Too many symbolic links encountered while traversing the path.
File name too long.
A component of the path path
does not exist, or
the path is an empty string.
Out of memory (i.e. kernel memory).
A component of the path is not a directory.
These system calls conform to SVr4, 4.3BSD, POSIX.1-2001.
Use of the st_blocks
and st_blksize
fields may be less
portable. (They were introduced in BSD. The interpretation
differs between systems, and possibly on a single system when
NFS mounts are involved.)
POSIX does not describe the S_IFMT, S_IFSOCK, S_IFLNK, S_IFREG, S_IFBLK, S_IFDIR, S_IFCHR, S_IFIFO, S_ISVTX bits, but instead demands the use of the macros S_ISDIR(), etc. The S_ISLNK and S_ISSOCK macros are 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.
Values that have been (or are) in use on various systems:
hex | name | ls | octal | description |
f000 | S_IFMT | 170000 | mask for file type | |
0000 | 000000 | SCO out-of-service inode, BSD unknown type SVID-v2 and XPG2 have both 0 and 0100000 for ordinary file | ||
1000 | S_IFIFO | p| | 010000 | FIFO (named pipe) |
2000 | S_IFCHR | c | 020000 | character special (V7) |
3000 | S_IFMPC | 030000 | multiplexed character special (V7) | |
4000 | S_IFDIR | d/ | 040000 | directory (V7) |
5000 | S_IFNAM | 050000 | XENIX named special file with two subtypes, distinguished by st_rdev values 1, 2: | |
0001 | S_INSEM | s | 000001 | XENIX semaphore subtype of IFNAM |
0002 | S_INSHD | m | 000002 | XENIX shared data subtype of IFNAM |
6000 | S_IFBLK | b | 060000 | block special (V7) |
7000 | S_IFMPB | 070000 | multiplexed block special (V7) | |
8000 | S_IFREG | - | 100000 | regular (V7) |
9000 | S_IFCMP | 110000 | VxFS compressed | |
9000 | S_IFNWK | n | 110000 | network special (HP-UX) |
a000 | S_IFLNK | l@ | 120000 | symbolic link (BSD) |
b000 | S_IFSHAD | 130000 | Solaris shadow inode for ACL (not seen by userspace) | |
c000 | S_IFSOCK | s= | 140000 | socket (BSD; also "S_IFSOC" on VxFS) |
d000 | S_IFDOOR | D> | 150000 | Solaris door |
e000 | S_IFWHT | w% | 160000 | BSD whiteout (not used for inode) |
0200 | S_ISVTX | 001000 | `sticky bit': save swapped text even after use (V7) reserved (SVID-v2) On non-directories: don't cache this file (SunOS) On directories: restricted deletion flag (SVID-v4.2) | |
0400 | S_ISGID | 002000 | set-group-ID on execution (V7) for directories: use BSD semantics for propagation of GID | |
0400 | S_ENFMT | 002000 | SysV file locking enforcement (shared with S_ISGID) | |
0800 | S_ISUID | 004000 | set-user-ID on execution (V7) | |
0800 | S_CDF | 004000 | directory is a context dependent file (HP-UX) |
A sticky command appeared in Version 32V AT&T UNIX.
access(2), chmod(2), chown(2), fstatat(2), readlink(2), utime(2), capabilities(7)
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