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NAME
chown, fchown, lchown, fchownat - change ownership of a file
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#include <unistd.h>
int chown(const char *pathname, uid_t owner, gid_t group);
int fchown(int fd, uid_t owner, gid_t group);
int lchown(const char *pathname, uid_t owner, gid_t group);
#include <fcntl.h> /* Definition of AT_* constants */
#include <unistd.h>
int fchownat(int dirfd, const char *pathname,
uid_t owner, gid_t group, int flags);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
fchown(), lchown():
/* Since glibc 2.12: */ _POSIX_C_SOURCE >= 200809L
|| _XOPEN_SOURCE >= 500
|| /* glibc <= 2.19: */ _BSD_SOURCE
fchownat():
Since glibc 2.10:
_POSIX_C_SOURCE >= 200809L
Before glibc 2.10:
_ATFILE_SOURCE
DESCRIPTION
These system calls change the owner and group of a file. The chown(), fchown(), and
lchown() system calls differ only in how the file is specified:
• chown() changes the ownership of the file specified by pathname, which is dereferenced
if it is a symbolic link.
• fchown() changes the ownership of the file referred to by the open file descriptor fd.
• lchown() is like chown(), but does not dereference symbolic links.
Only a privileged process (Linux: one with the CAP_CHOWN capability) may change the owner
of a file. The owner of a file may change the group of the file to any group of which
that owner is a member. A privileged process (Linux: with CAP_CHOWN) may change the group
arbitrarily.
If the owner or group is specified as -1, then that ID is not changed.
When the owner or group of an executable file is changed by an unprivileged user, the
S_ISUID and S_ISGID mode bits are cleared. POSIX does not specify whether this also
should happen when root does the chown(); the Linux behavior depends on the kernel
version, and since Linux 2.2.13, root is treated like other users. In case of a non-
group-executable file (i.e., one for which the S_IXGRP bit is not set) the S_ISGID bit
indicates mandatory locking, and is not cleared by a chown().
When the owner or group of an executable file is changed (by any user), all capability
sets for the file are cleared.
fchownat()
The fchownat() system call operates in exactly the same way as chown(), 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 chown() 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
chown()).
If pathname is absolute, then dirfd is ignored.
The flags argument is a bit mask created by ORing together 0 or more of the following
values;
AT_EMPTY_PATH (since Linux 2.6.39)
If pathname 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 pathname is a symbolic link, do not dereference it: instead operate on the link
itself, like lchown(). (By default, fchownat() dereferences symbolic links, like
chown().)
See openat(2) for an explanation of the need for fchownat().
RETURN VALUE
On success, zero is returned. On error, -1 is returned, and errno is set to indicate the
error.
ERRORS
Depending on the filesystem, errors other than those listed below can be returned.
The more general errors for chown() are listed below.
EACCES Search permission is denied on a component of the path prefix. (See also
path_resolution(7).)
EBADF (fchown()) fd is not a valid open file descriptor.
EBADF (fchownat()) pathname is relative but dirfd is neither AT_FDCWD nor a valid file
descriptor.
EFAULT pathname points outside your accessible address space.
EINVAL (fchownat()) Invalid flag specified in flags.
EIO (fchown()) A low-level I/O error occurred while modifying the inode.
ELOOP Too many symbolic links were encountered in resolving pathname.
ENAMETOOLONG
pathname 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
(fchownat()) pathname is relative and dirfd is a file descriptor referring to a
file other than a directory.
EPERM The calling process did not have the required permissions (see above) to change
owner and/or group.
EPERM The file is marked immutable or append-only. (See ioctl_iflags(2).)
EROFS The named file resides on a read-only filesystem.
VERSIONS
fchownat() was added in Linux 2.6.16; library support was added in glibc 2.4.
STANDARDS
chown(), fchown(), lchown(): 4.4BSD, SVr4, POSIX.1-2001, POSIX.1-2008.
The 4.4BSD version can be used only by the superuser (that is, ordinary users cannot give
away files).
fchownat(): POSIX.1-2008.
NOTES
Ownership of new files
When a new file is created (by, for example, open(2) or mkdir(2)), its owner is made the
same as the filesystem user ID of the creating process. The group of the file depends on
a range of factors, including the type of filesystem, the options used to mount the
filesystem, and whether or not the set-group-ID mode bit is enabled on the parent
directory. If the filesystem supports the -o grpid (or, synonymously -o bsdgroups) and
-o nogrpid (or, synonymously -o sysvgroups) mount(8) options, then the rules are as
follows:
• If the filesystem is mounted with -o grpid, then the group of a new file is made the
same as that of the parent directory.
• If the filesystem is mounted with -o nogrpid and the set-group-ID bit is disabled on
the parent directory, then the group of a new file is made the same as the process's
filesystem GID.
• If the filesystem is mounted with -o nogrpid and the set-group-ID bit is enabled on the
parent directory, then the group of a new file is made the same as that of the parent
directory.
As at Linux 4.12, the -o grpid and -o nogrpid mount options are supported by ext2, ext3,
ext4, and XFS. Filesystems that don't support these mount options follow the -o nogrpid
rules.
glibc notes
On older kernels where fchownat() is unavailable, the glibc wrapper function falls back to
the use of chown() and lchown(). 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.
NFS
The chown() semantics are deliberately violated on NFS filesystems which have UID mapping
enabled. Additionally, the semantics of all system calls which access the file contents
are violated, because chown() may cause immediate access revocation on already open files.
Client side caching may lead to a delay between the time where ownership have been changed
to allow access for a user and the time where the file can actually be accessed by the
user on other clients.
Historical details
The original Linux chown(), fchown(), and lchown() system calls supported only 16-bit user
and group IDs. Subsequently, Linux 2.4 added chown32(), fchown32(), and lchown32(),
supporting 32-bit IDs. The glibc chown(), fchown(), and lchown() wrapper functions
transparently deal with the variations across kernel versions.
Before Linux 2.1.81 (except 2.1.46), chown() did not follow symbolic links. Since Linux
2.1.81, chown() does follow symbolic links, and there is a new system call lchown() that
does not follow symbolic links. Since Linux 2.1.86, this new call (that has the same
semantics as the old chown()) has got the same syscall number, and chown() got the newly
introduced number.
EXAMPLES
The following program changes the ownership of the file named in its second command-line
argument to the value specified in its first command-line argument. The new owner can be
specified either as a numeric user ID, or as a username (which is converted to a user ID
by using getpwnam(3) to perform a lookup in the system password file).
Program source
#include <pwd.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
int
main(int argc, char *argv[])
{
char *endptr;
uid_t uid;
struct passwd *pwd;
if (argc != 3 || argv[1][0] == '\0') {
fprintf(stderr, "%s <owner> <file>\n", argv[0]);
exit(EXIT_FAILURE);
}
uid = strtol(argv[1], &endptr, 10); /* Allow a numeric string */
if (*endptr != '\0') { /* Was not pure numeric string */
pwd = getpwnam(argv[1]); /* Try getting UID for username */
if (pwd == NULL) {
perror("getpwnam");
exit(EXIT_FAILURE);
}
uid = pwd->pw_uid;
}
if (chown(argv[2], uid, -1) == -1) {
perror("chown");
exit(EXIT_FAILURE);
}
exit(EXIT_SUCCESS);
}
SEE ALSO
chgrp(1), chown(1), chmod(2), flock(2), path_resolution(7), symlink(7)