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:
() 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
- chmod() changes the mode of the file specified whose
pathname is given in pathname, which is dereferenced if it is a
- 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
- 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)
- 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
directories, and means that entries within the directory can be
- 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
If the calling process is not privileged (Linux: does not have the
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
bit will be turned off, but this will not cause an error to be
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.
() system call operates in exactly the same way as
(), 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).
is relative and dirfd
is the special value
, then pathname
is interpreted relative to the current
working directory of the calling process (like chmod
is absolute, then dirfd
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
for an explanation of the need for fchmodat
On success, zero is returned. On error, -1 is returned, and errno
Depending on the filesystem, errors other than those listed below can be
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
- An I/O error occurred.
- Too many symbolic links were encountered in resolving
- 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
- The file is marked immutable or append-only. (See
- 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
(). The following additional errors can occur for
- 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
() was added to Linux in kernel 2.6.16; library support was added
to glibc in version 2.4.
(): 4.4BSD, SVr4, POSIX.1-2001i, 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
On older kernels where fchmodat
() is unavailable, the glibc wrapper
function falls back to the use of chmod
(). When pathname
relative pathname, glibc constructs a pathname based on the symbolic link in
that corresponds to the dirfd
This page is part of release 4.16 of the Linux man-pages
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