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.
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.
