NAME
chown, fchown, lchown - change ownership of a file
SYNOPSIS
#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
DESCRIPTION
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().
RETURN VALUE
On success, zero is returned. On error, -1 is returned, and errno is
set appropriately.
ERRORS
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.
CONFORMING TO
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).
NOTES
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.
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).
#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);
} /* if */
exit(EXIT_SUCCESS);
} /* main */
SEE ALSO
chmod(2), fchownat(2), flock(2), path_resolution(7), symlink(7)
COLOPHON
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