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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | STANDARDS | HISTORY | NOTES | EXAMPLES | SEE ALSO | COLOPHON |
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chown(2) System Calls Manual chown(2)
chown, fchown, lchown, fchownat - change ownership of a file
Standard C library (libc, -lc)
#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
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().
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 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
FS_IOC_SETFLAGS(2const).)
EROFS The named file resides on a read-only filesystem.
The 4.4BSD version can be used only by the superuser (that is,
ordinary users cannot give away files).
POSIX.1-2008.
chown()
fchown()
lchown()
4.4BSD, SVr4, POSIX.1-2001.
fchownat()
POSIX.1-2008. Linux 2.6.16, glibc 2.4.
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.
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 <sys/types.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);
}
chgrp(1), chown(1), chmod(2), flock(2), path_resolution(7),
symlink(7)
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2024-06-26. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
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Linux man-pages 6.9.1 2024-06-15 chown(2)
Pages that refer to this page: chgrp(1), chown(1), access(2), chmod(2), fcntl(2), mkdir(2), mknod(2), mount_setattr(2), open(2), open_by_handle_at(2), stat(2), statx(2), symlink(2), syscalls(2), euidaccess(3), fpathconf(3), id_t(3type), shm_open(3), systemd.exec(5), capabilities(7), inode(7), inotify(7), landlock(7), shm_overview(7), signal-safety(7), spufs(7), symlink(7), unix(7)
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HTML rendering created 2024-06-26 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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