table of contents
| CHOWN(2) | Linux Programmer's Manual | CHOWN(2) |
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 a non-superuser, 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:
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¶
This page is part of release 3.22 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/.
| 2008-06-16 | Linux |