These system calls change the permissions of a file.
They differ only in how the file is specified:
*
chmod()
changes the permissions of the file specified whose pathname is given in
path,
which is dereferenced if it is a symbolic link.
*
fchmod()
changes the permissions of the file referred to by the open file descriptor
fd.
The new file permissions are 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 file system,
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 file systems, 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
stat(2).
On NFS file systems, 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.
RETURN VALUE
On success, zero is returned.
On error, -1 is returned, and
errno
is set appropriately.
ERRORS
Depending on the file system, other errors 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
path
points outside your accessible address space.
EIO
An I/O error occurred.
ELOOP
Too many symbolic links were encountered in resolving
path.
ENAMETOOLONG
path
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).
EROFS
The named file resides on a read-only file system.
This page is part of release 3.14 of the Linux
man-pages
project.
A description of the project,
and information about reporting bugs,
can be found at
http://www.kernel.org/doc/man-pages/.
