chmod, fchmod, fchmodat - change permissions of a file
#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
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.
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().
On success, zero is returned. On error, -1 is returned, and
errno is set
appropriately.
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.
fchmodat() was added to Linux in kernel 2.6.16; library support was added
to glibc in version 2.4.
chmod(),
fchmod(): 4.4BSD, SVr4, POSIX.1-2001i, POSIX.1-2008.
fchmodat(): POSIX.1-2008.
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.
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.
chmod(1),
chown(2),
execve(2),
open(2),
stat(2),
inode(7),
path_resolution(7),
symlink(7)