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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 || _XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED
|| /* Since glibc 2.12: */ _POSIX_C_SOURCE >= 200809L
DESCRIPTION
These system calls change the owner and group of a file. They 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 filesystem, 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 filesystem.
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 be used only by the superuser (that is, ordinary users cannot give
away files).
NOTES
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 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 permission 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.
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.
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);
}
exit(EXIT_SUCCESS);
}
SEE ALSO
chmod(2), fchownat(2), flock(2), path_resolution(7), symlink(7)
COLOPHON
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