fchown32 (2)
Leading comments
Copyright (c) 1992 Drew Eckhardt (drew@cs.colorado.edu), March 28, 1992 and Copyright (c) 1998 Andries Brouwer (aeb@cwi.nl) and Copyright (c) 2006, 2007, 2008, 2014 Michael Kerrisk <mtk.manpages@gmail.com> %%%LICENSE_START(VERBATIM) Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions f...
NAME
chown, fchown, lchown, fchownat - change ownership of a fileSYNOPSIS
#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():
-
_BSD_SOURCE || _XOPEN_SOURCE >= 500 ||
_XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED
|| /* Since glibc 2.12: */ _POSIX_C_SOURCE >= 200809L
fchownat():
-
- Since glibc 2.10:
- _XOPEN_SOURCE >= 700 || _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 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().
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
- 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.
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] == aq\0aq) { fprintf(stderr, "%s <owner> <file>\n", argv[0]); exit(EXIT_FAILURE); } uid = strtol(argv[1], &endptr, 10); /* Allow a numeric string */ if (*endptr != aq\0aq) { /* 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); }