chown(2)
NAME
chown, fchown, lchown, fchownat - change ownership of a file
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 versions <= 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. 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().
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 appropriately.
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).)
EFAULT pathname points outside your accessible address space.
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.
EPERM The calling process did not have the required permissions (see
above) to change owner and/or group.
EROFS The named file resides on a read-only filesystem.
The general errors for fchown() are listed below:
EBADF fd is not a valid open file descriptor.
EIO A low-level I/O error occurred while modifying the inode.
ENOENT See above.
EPERM See above.
EROFS See above.
The same errors that occur for chown() can also occur for fchownat().
The following additional errors can occur for fchownat():
EBADF dirfd is not a valid file descriptor.
EINVAL Invalid flag specified in flags.
ENOTDIR
pathname is relative and dirfd is a file descriptor referring to
a file other than a directory.
VERSIONS
fchownat() was added to Linux in kernel 2.6.16; library support was
added to glibc in version 2.4.
CONFORMING TO
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 2.6.25, 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.
In versions of Linux prior to 2.1.81 (and distinct from 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.
EXAMPLE
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[])
{
uid_t uid;
struct passwd *pwd;
char *endptr;
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)
COLOPHON
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