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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | STANDARDS | HISTORY | SEE ALSO | COLOPHON |
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chmod(2) System Calls Manual chmod(2)
chmod, fchmod, fchmodat - change permissions of a file
Standard C library (libc, -lc)
#include <sys/stat.h>
int chmod(const char *path, 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 *path, 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 glibc 2.23
_POSIX_C_SOURCE
glibc 2.16 to glibc 2.19:
_BSD_SOURCE || _POSIX_C_SOURCE
glibc 2.12 to glibc 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 file's 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 path, 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.
fchmodat()
The fchmodat() system call operates in exactly the same way as
chmod(), except for the differences described here.
If path 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 path is relative and dirfd is the special value AT_FDCWD, then
path is interpreted relative to the current working directory of
the calling process (like chmod()).
If path is absolute, then dirfd is ignored.
flags can either be 0, or include the following flags:
AT_EMPTY_PATH (since Linux 6.6)
If path is an empty string, operate on the file referred to
by dirfd (which may have been obtained using the open(2)
O_PATH flag). In this case, dirfd can refer to any type of
file, not just a directory. If dirfd is AT_FDCWD, the call
operates on the current working directory. This flag is
Linux-specific; define _GNU_SOURCE to obtain its
definition.
AT_SYMLINK_NOFOLLOW
If path is a symbolic link, do not dereference it: instead
operate on the link itself.
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:
EACCES Search permission is denied on a component of the path
prefix. (See also path_resolution(7).)
EBADF (fchmod()) The file descriptor fd is not valid.
EBADF (fchmodat()) path is relative but dirfd is neither AT_FDCWD
nor a valid file descriptor.
EFAULT path points outside your accessible address space.
EINVAL (fchmodat()) Invalid flag specified in flags.
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.
ENOTDIR
(fchmodat()) path is relative and dirfd is a file
descriptor referring to a file other than a directory.
ENOTSUP
(fchmodat()) flags specified AT_SYMLINK_NOFOLLOW, which is
not supported.
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
FS_IOC_SETFLAGS(2const).)
EROFS The named file resides on a read-only filesystem.
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 argument.
glibc notes
On older kernels where fchmodat() is unavailable, the glibc
wrapper function falls back to the use of chmod(). When path is a
relative pathname, glibc constructs a pathname based on the
symbolic link in /proc/self/fd that corresponds to the dirfd
argument.
POSIX.1-2008.
chmod()
fchmod()
4.4BSD, SVr4, POSIX.1-2001.
fchmodat()
POSIX.1-2008. Linux 2.6.16, glibc 2.4.
AT_SYMLINK_NOFOLLOW
glibc 2.32, Linux 6.5.
chmod(1), chown(2), execve(2), open(2), stat(2), inode(7),
path_resolution(7), symlink(7)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
man-pages@man7.org
Linux man-pages 6.15 2025-06-28 chmod(2)
Pages that refer to this page: chmod(1), access(2), chown(2), execve(2), fcntl_locking(2), F_NOTIFY(2const), mkdir(2), mknod(2), open(2), rename(2), rmdir(2), stat(2), statx(2), syscalls(2), umask(2), unlink(2), euidaccess(3), mode_t(3type), shm_open(3), capabilities(7), inotify(7), landlock(7), shm_overview(7), signal-safety(7), spufs(7), symlink(7), unix(7), logrotate(8), xfs_db(8)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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