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) ; |
Note | |||||||||||
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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 to
indicate the error.
Depending on the filesystem, errors other than those listed below can be returned.
The more general errors for chmod
() are listed below:
Search permission is denied on a component of the path prefix. (See also path_resolution(7).)
pathname
points outside your accessible address space.
An I/O error occurred.
Too many symbolic links were encountered in
resolving pathname
.
pathname
is
too long.
The file does not exist.
Insufficient kernel memory was available.
A component of the path prefix is not a directory.
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).
The file is marked immutable or append-only. (See ioctl_iflags(2).)
The named file resides on a read-only filesystem.
The general errors for fchmod
() are listed below:
The file descriptor fd
is not valid.
See above.
See above.
See above.
The same errors that occur for chmod
() can also occur for fchmodat
(). The following additional errors
can occur for fchmodat
():
dirfd
is not
a valid file descriptor.
Invalid flag specified in flags
.
pathname
is
relative and dirfd
is a file
descriptor referring to a file other than a
directory.
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.