CHMOD
Section: Linux Programmer's Manual (2)
Updated: 2017-09-15
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NAME
chmod, fchmod, fchmodat - change permissions of a file
SYNOPSIS
#include <sys/stat.h>
int chmod(const char *pathname, mode_t mode);
int fchmod(int fd, mode_t mode);
#include <fcntl.h> /* Definition of AT_* constants */
#include <sys/stat.h>
int fchmodat(int dirfd, const char *pathname, mode_t mode, int flags);
Feature Test Macro Requirements for glibc (see
feature_test_macros(7)):
fchmod():
-
Since glibc 2.24:
_POSIX_C_SOURCE >= 199309L
Glibc 2.19 to 2.23
_POSIX_C_SOURCE
Glibc 2.16 to 2.19:
_BSD_SOURCE || _POSIX_C_SOURCE
Glibc 2.12 to 2.16:
_BSD_SOURCE || _XOPEN_SOURCE >= 500 ||
_POSIX_C_SOURCE >= 200809L
Glibc 2.11 and earlier:
_BSD_SOURCE || _XOPEN_SOURCE >= 500
fchmodat():
-
- Since glibc 2.10:
-
_POSIX_C_SOURCE >= 200809L
- Before glibc 2.10:
-
_ATFILE_SOURCE
DESCRIPTION
The
chmod()
and
fchmod()
system calls change a files mode bits.
(The file mode consists of the file permission bits plus the set-user-ID,
set-group-ID, and sticky bits.)
These system calls differ only in how the file is specified:
- *
-
chmod()
changes the mode of the file specified whose pathname is given in
pathname,
which is dereferenced if it is a symbolic link.
- *
-
fchmod()
changes the mode of the file referred to by the open file descriptor
fd.
The new file mode is specified in
mode,
which is a bit mask created by ORing together zero or
more of the following:
- S_ISUID (04000)
-
set-user-ID (set process effective user ID on
execve(2))
- S_ISGID (02000)
-
set-group-ID (set process effective group ID on
execve(2);
mandatory locking, as described in
fcntl(2);
take a new file's group from parent directory, as described in
chown(2)
and
mkdir(2))
- S_ISVTX (01000)
-
sticky bit (restricted deletion flag, as described in
unlink(2))
- S_IRUSR (00400)
-
read by owner
- S_IWUSR (00200)
-
write by owner
- S_IXUSR (00100)
-
execute/search by owner ("search" applies for directories,
and means that entries within the directory can be accessed)
- S_IRGRP (00040)
-
read by group
- S_IWGRP (00020)
-
write by group
- S_IXGRP (00010)
-
execute/search by group
- S_IROTH (00004)
-
read by others
- S_IWOTH (00002)
-
write by others
- S_IXOTH (00001)
-
execute/search by others
The effective UID of the calling process must match the owner of the file,
or the process must be privileged (Linux: it must have the
CAP_FOWNER
capability).
If the calling process is not privileged (Linux: does not have the
CAP_FSETID
capability), and the group of the file does not match
the effective group ID of the process or one of its
supplementary group IDs, the
S_ISGID
bit will be turned off,
but this will not cause an error to be returned.
As a security measure, depending on the filesystem,
the set-user-ID and set-group-ID execution bits
may be turned off if a file is written.
(On Linux, this occurs if the writing process does not have the
CAP_FSETID
capability.)
On some filesystems, only the superuser can set the sticky bit,
which may have a special meaning.
For the sticky bit, and for set-user-ID and set-group-ID bits on
directories, see
inode(7).
On NFS filesystems, restricting the permissions will immediately influence
already open files, because the access control is done on the server, but
open files are maintained by the client.
Widening the permissions may be
delayed for other clients if attribute caching is enabled on them.
fchmodat()
The
fchmodat()
system call operates in exactly the same way as
chmod(),
except for the differences described here.
If the pathname given in
pathname
is relative, then it is interpreted relative to the directory
referred to by the file descriptor
dirfd
(rather than relative to the current working directory of
the calling process, as is done by
chmod()
for a relative pathname).
If
pathname
is relative and
dirfd
is the special value
AT_FDCWD,
then
pathname
is interpreted relative to the current working
directory of the calling process (like
chmod()).
If
pathname
is absolute, then
dirfd
is ignored.
flags
can either be 0, or include the following flag:
- AT_SYMLINK_NOFOLLOW
-
If
pathname
is a symbolic link, do not dereference it:
instead operate on the link itself.
This flag is not currently implemented.
See
openat(2)
for an explanation of the need for
fchmodat().
RETURN VALUE
On success, zero is returned.
On error, -1 is returned, and
errno
is set appropriately.
ERRORS
Depending on the filesystem,
errors other than those listed below can be returned.
The more general errors for
chmod()
are listed below:
- EACCES
-
Search permission is denied on a component of the path prefix.
(See also
path_resolution(7).)
- EFAULT
-
pathname
points outside your accessible address space.
- EIO
-
An I/O error occurred.
- ELOOP
-
Too many symbolic links were encountered in resolving
pathname.
- ENAMETOOLONG
-
pathname
is too long.
- ENOENT
-
The file does not exist.
- ENOMEM
-
Insufficient kernel memory was available.
- ENOTDIR
-
A component of the path prefix is not a directory.
- EPERM
-
The effective UID does not match the owner of the file,
and the process is not privileged (Linux: it does not have the
CAP_FOWNER
capability).
- EPERM
-
The file is marked immutable or append-only.
(See
ioctl_iflags(2).)
- EROFS
-
The named file resides on a read-only filesystem.
The general errors for
fchmod()
are listed below:
- EBADF
-
The file descriptor
fd
is not valid.
- EIO
-
See above.
- EPERM
-
See above.
- EROFS
-
See above.
The same errors that occur for
chmod()
can also occur for
fchmodat().
The following additional errors can occur for
fchmodat():
- EBADF
-
dirfd
is not a valid file descriptor.
- EINVAL
-
Invalid flag specified in
flags.
- ENOTDIR
-
pathname
is relative and
dirfd
is a file descriptor referring to a file other than a directory.
- ENOTSUP
-
flags
specified
AT_SYMLINK_NOFOLLOW,
which is not supported.
VERSIONS
fchmodat()
was added to Linux in kernel 2.6.16;
library support was added to glibc in version 2.4.
CONFORMING TO
chmod(),
fchmod():
4.4BSD, SVr4, POSIX.1-2001i, POSIX.1-2008.
fchmodat():
POSIX.1-2008.
NOTES
C library/kernel differences
The GNU C library
fchmodat()
wrapper function implements the POSIX-specified
interface described in this page.
This interface differs from the underlying Linux system call, which does
not
have a
flags
argument.
Glibc notes
On older kernels where
fchmodat()
is unavailable, the glibc wrapper function falls back to the use of
chmod().
When
pathname
is a relative pathname,
glibc constructs a pathname based on the symbolic link in
/proc/self/fd
that corresponds to the
dirfd
argument.
SEE ALSO
chmod(1),
chown(2),
execve(2),
open(2),
stat(2),
inode(7),
path_resolution(7),
symlink(7)
COLOPHON
This page is part of release 4.13 of the Linux
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/.
Index
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- fchmodat()
-
- RETURN VALUE
-
- ERRORS
-
- VERSIONS
-
- CONFORMING TO
-
- NOTES
-
- C library/kernel differences
-
- Glibc notes
-
- SEE ALSO
-
- COLOPHON
-
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