NAME
chmod,
lchmod,
fchmod,
fchmodat —
change mode of file
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <sys/stat.h>
int
chmod(
const char
*path,
mode_t mode);
int
lchmod(
const char
*path,
mode_t mode);
int
fchmod(
int
fd,
mode_t mode);
#include <sys/stat.h>
#include <fcntl.h>
int
fchmodat(
int
fd,
const char *path,
mode_t mode,
int flag);
DESCRIPTION
The function
chmod() sets the file permission bits of the file
specified by the pathname
path to
mode.
fchmod() sets the permission
bits of the specified file descriptor
fd.
lchmod() is like
chmod() except in the
case where the named file is a symbolic link, in which case
lchmod() sets the permission bits of the link, while
chmod() sets the bits of the file the link references.
fchmodat() works the same way as
chmod() (or
lchmod() if
AT_SYMLINK_NOFOLLOW
is
set in
flag) except if
path is
relative. In that case, it is looked up from a directory whose file descriptor
was passed as
fd. Search permission is required on this
directory.
fd except if that file descriptor was opened
with the
O_SEARCH
flag.
fd can
be set to
AT_FDCWD
in order to specify the current
directory.
chmod() verifies that the process owner (user) either owns the
file specified by
path (or
fd), or
is the super-user. A mode is created from
or'd permission
bit masks defined in
<sys/stat.h>:
#define S_IRWXU 0000700 /* RWX mask for owner */
#define S_IRUSR 0000400 /* R for owner */
#define S_IWUSR 0000200 /* W for owner */
#define S_IXUSR 0000100 /* X for owner */
#define S_IRWXG 0000070 /* RWX mask for group */
#define S_IRGRP 0000040 /* R for group */
#define S_IWGRP 0000020 /* W for group */
#define S_IXGRP 0000010 /* X for group */
#define S_IRWXO 0000007 /* RWX mask for other */
#define S_IROTH 0000004 /* R for other */
#define S_IWOTH 0000002 /* W for other */
#define S_IXOTH 0000001 /* X for other */
#define S_ISUID 0004000 /* set user id on execution */
#define S_ISGID 0002000 /* set group id on execution */
#define S_ISVTX 0001000 /* sticky bit */
The mode
ISVTX
(the ‘sticky bit’) can be set
on regular files, but has no effect. For historical reasons this can be done
only by the super-user.
If mode
ISVTX
(the ‘sticky bit’) is set on a
directory, an unprivileged user may not delete or rename files of other users
in that directory. The sticky bit may be set by any user on a directory which
the user owns or has appropriate permissions.
For more information about the history and properties of the sticky bit, see
sticky(7).
Changing the owner of a file turns off the set-user-id and set-group-id bits;
writing to a file turns off the set-user-id and set-group-id bits unless the
user is the super-user. This makes the system somewhat more secure by
protecting set-user-id (set-group-id) files from remaining set-user-id
(set-group-id) if they are modified, at the expense of a degree of
compatibility.
RETURN VALUES
The
chmod(),
lchmod(),
fchmod(), and
fchmodat() functions return
the value 0 if successful; otherwise the value -1 is returned and
the global variable
errno is set to indicate the error.
ERRORS
chmod(),
lchmod() and
fchmodat() will fail and the file mode will be unchanged if:
-
-
- [
EACCES
]
- Search permission is denied for a component of the path
prefix.
-
-
- [
EFAULT
]
- path points outside the process's
allocated address space.
-
-
- [
EFTYPE
]
- The effective user ID is not the super-user, the
mode includes the sticky bit
(
S_ISVTX
), and path does not
refer to a directory.
-
-
- [
EIO
]
- An I/O error occurred while reading from or writing to the
file system.
-
-
- [
ELOOP
]
- Too many symbolic links were encountered in translating the
pathname.
-
-
- [
ENAMETOOLONG
]
- A component of a pathname exceeded
{
NAME_MAX
} characters, or an entire path name
exceeded {PATH_MAX
} characters.
-
-
- [
ENOENT
]
- The named file does not exist.
-
-
- [
ENOTDIR
]
- A component of the path prefix is not a directory.
-
-
- [
EPERM
]
- The effective user ID does not match the owner of the file
and the effective user ID is not the super-user; or the mode includes the
setgid bit (
S_ISGID
) but the file's group is
neither the effective group ID nor is it in the group access list.
-
-
- [
EROFS
]
- The named file resides on a read-only file system.
In addition,
fchmodat() will fail if:
-
-
- [
EBADF
]
- path does not specify an absolute
path and fd is neither
AT_FDCWD
nor a valid file descriptor open for
reading or searching.
-
-
- [
ENOTDIR
]
- path is not an absolute path and
fd is a file descriptor associated with a
non-directory file.
fchmod() will fail if:
-
-
- [
EBADF
]
- The descriptor is not valid.
-
-
- [
EFTYPE
]
- The effective user ID is not the super-user, the
mode includes the sticky bit
(
S_ISVTX
), and fd does not
refer to a directory.
-
-
- [
EINVAL
]
- fd refers to a socket, not to a
file.
-
-
- [
EIO
]
- An I/O error occurred while reading from or writing to the
file system.
-
-
- [
EPERM
]
- The effective user ID does not match the owner of the file
and the effective user ID is not the super-user; or the mode includes the
setgid bit (
S_ISGID
) but the file's group is
neither the effective group ID nor is it in the group access list.
-
-
- [
EROFS
]
- The file resides on a read-only file system.
SEE ALSO
chmod(1),
chflags(2),
chown(2),
open(2),
stat(2),
getmode(3),
setmode(3),
sticky(7),
symlink(7)
STANDARDS
The
chmod() function conforms to
IEEE Std
1003.1-1990 (“POSIX.1”).
fchmodat()
function conforms to
IEEE Std 1003.1-2008
(“POSIX.1”).
HISTORY
The
fchmod() function call appeared in
4.2BSD. The
lchmod() function call
appeared in
NetBSD 1.3.