CHMOD(2) Linux Programmer's Manual CHMOD(2)
chmod, fchmod, fchmodat - change permissions of a file
int chmod(const char *pathname, mode_t mode);
int fchmod(int fd, mode_t mode);
#include <fcntl.h> /* Definition of AT_* constants */
int fchmodat(int dirfd, const char *pathname, mode_t mode, int flags);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
Since glibc 2.24:
_POSIX_C_SOURCE >= 199309L
Glibc 2.19 to 2.23
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
Since glibc 2.10:
_POSIX_C_SOURCE >= 200809L
Before glibc 2.10:
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
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 ex-
S_ISGID (02000) set-group-ID (set process effective group ID on ex-
ecve(2); mandatory locking, as described in fcntl(2);
take a new file's group from parent directory, as de-
scribed in chown(2) and mkdir(2))
S_ISVTX (01000) sticky bit (restricted deletion flag, as described in
S_IRUSR (00400) read by owner
S_IWUSR (00200) write by owner
S_IXUSR (00100) execute/search by owner ("search" applies for direc-
tories, 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
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 er-
ror 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 in-
On NFS filesystems, restricting the permissions will immediately influ-
ence already open files, because the access control is done on the
server, but open files are maintained by the client. Widening the per-
missions may be delayed for other clients if attribute caching is en-
abled 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:
If pathname is a symbolic link, do not dereference it: instead
operate on the link itself. This flag is not currently imple-
See openat(2) for an explanation of the need for fchmodat().
On success, zero is returned. On error, -1 is returned, and errno is
Depending on the filesystem, errors other than those listed below can
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.
pathname is too long.
ENOENT The file does not exist.
ENOMEM Insufficient kernel memory was available.
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
EPERM The file is marked immutable or append-only. (See
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.
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.
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 ar-
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_resolu-
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Linux 2017-09-15 CHMOD(2)
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