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CHOWN(2) Linux Programmer's Manual CHOWN(2)
chown, fchown, lchown - change ownership of a file
#include <unistd.h>
int chown(const char *path, uid_t owner, gid_t group);
int fchown(int fd, uid_t owner, gid_t group);
int lchown(const char *path, uid_t owner, gid_t group);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
fchown(), lchown():
_BSD_SOURCE || _XOPEN_SOURCE >= 500 ||
_XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED
|| /* Since glibc 2.12: */ _POSIX_C_SOURCE >= 200809L
These system calls change the owner and group of a file. The differ only in
how the file is specified:
* chown() changes the ownership of the file specified by path, 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 are 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().
On success, zero is returned. On error, -1 is returned, and errno is set
appropriately.
Depending on the file system, other errors 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 path points outside your accessible address space.
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.
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 file system.
The general errors for fchown() are listed below:
EBADF The descriptor is not valid.
EIO A low-level I/O error occurred while modifying the inode.
ENOENT See above.
EPERM See above.
EROFS See above.
4.4BSD, SVr4, POSIX.1-2001.
The 4.4BSD version can only be used by the superuser (that is, ordinary users
cannot give away files).
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.
When a new file is created (by, for example, open(2) or mkdir(2)), its owner
is made the same as the file system user ID of the creating process. The
group of the file depends on a range of factors, including the type of file
system, the options used to mount the file system, and whether or not the set-
group-ID permission bit is enabled on the parent directory. If the file
system 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 file system is mounted with -o grpid, then the group of a new file is
made the same as that of the parent directory.
* If the file system 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 file system GID.
* If the file system 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. File systems that don't support these mount
options follow the -o nogrpid rules.
The chown() semantics are deliberately violated on NFS file systems 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.
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.
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).
#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);
}
chmod(2), fchownat(2), flock(2), path_resolution(7), symlink(7)
This page is part of release 3.32 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can be found
at http://www.kernel.org/doc/man-pages/.
Linux 2010-11-22 CHOWN(2)
HTML rendering created 2010-12-03 by Michael Kerrisk, author of The Linux Programming Interface