CHOWN(3P) POSIX Programmer's Manual CHOWN(3P)PROLOG
This manual page is part of the POSIX Programmer's Manual. The Linux
implementation of this interface may differ (consult the corresponding
Linux manual page for details of Linux behavior), or the interface may
not be implemented on Linux.
NAME
chown, fchownat — change owner and group of a file relative to direc‐
tory file descriptor
SYNOPSIS
#include <unistd.h>
int chown(const char *path, uid_t owner, gid_t group);
int fchownat(int fd, const char *path, uid_t owner, gid_t group,
int flag);
DESCRIPTION
The chown() function shall change the user and group ownership of a
file.
The path argument points to a pathname naming a file. The user ID and
group ID of the named file shall be set to the numeric values contained
in owner and group, respectively.
Only processes with an effective user ID equal to the user ID of the
file or with appropriate privileges may change the ownership of a file.
If _POSIX_CHOWN_RESTRICTED is in effect for path:
* Changing the user ID is restricted to processes with appropriate
privileges.
* Changing the group ID is permitted to a process with an effective
user ID equal to the user ID of the file, but without appropriate
privileges, if and only if owner is equal to the file's user ID or
(uid_t)−1 and group is equal either to the calling process' effec‐
tive group ID or to one of its supplementary group IDs.
If the specified file is a regular file, one or more of the S_IXUSR,
S_IXGRP, or S_IXOTH bits of the file mode are set, and the process does
not have appropriate privileges, the set-user-ID (S_ISUID) and set-
group-ID (S_ISGID) bits of the file mode shall be cleared upon success‐
ful return from chown(). If the specified file is a regular file, one
or more of the S_IXUSR, S_IXGRP, or S_IXOTH bits of the file mode are
set, and the process has appropriate privileges, it is implementation-
defined whether the set-user-ID and set-group-ID bits are altered. If
the chown() function is successfully invoked on a file that is not a
regular file and one or more of the S_IXUSR, S_IXGRP, or S_IXOTH bits
of the file mode are set, the set-user-ID and set-group-ID bits may be
cleared.
If owner or group is specified as (uid_t)−1 or (gid_t)−1, respectively,
the corresponding ID of the file shall not be changed. If both owner
and group are −1, the times need not be updated.
Upon successful completion, chown() shall mark for update the last file
status change timestamp of the file.
The fchownat() function shall be equivalent to the chown() and lchown()
functions except in the case where path specifies a relative path. In
this case the file to be changed is determined relative to the direc‐
tory associated with the file descriptor fd instead of the current
working directory. If the file descriptor was opened without O_SEARCH,
the function shall check whether directory searches are permitted using
the current permissions of the directory underlying the file descrip‐
tor. If the file descriptor was opened with O_SEARCH, the function
shall not perform the check.
Values for flag are constructed by a bitwise-inclusive OR of flags from
the following list, defined in <fcntl.h>:
AT_SYMLINK_NOFOLLOW
If path names a symbolic link, ownership of the symbolic link is
changed.
If fchownat() is passed the special value AT_FDCWD in the fd parameter,
the current working directory shall be used and the behavior shall be
identical to a call to chown() or lchown() respectively, depending on
whether or not the AT_SYMLINK_NOFOLLOW bit is set in the flag argument.
RETURN VALUE
Upon successful completion, these functions shall return 0. Otherwise,
these functions shall return −1 and set errno to indicate the error. If
−1 is returned, no changes are made in the user ID and group ID of the
file.
ERRORS
These functions shall fail if:
EACCES Search permission is denied on a component of the path prefix.
ELOOP A loop exists in symbolic links encountered during resolution of
the path argument.
ENAMETOOLONG
The length of a component of a pathname is longer than
{NAME_MAX}.
ENOENT A component of path does not name an existing file or path is an
empty string.
ENOTDIR
A component of the path prefix names an existing file that is
neither a directory nor a symbolic link to a directory, or the
path argument contains at least one non-<slash> character and
ends with one or more trailing <slash> characters and the last
pathname component names an existing file that is neither a
directory nor a symbolic link to a directory.
EPERM The effective user ID does not match the owner of the file, or
the calling process does not have appropriate privileges and
_POSIX_CHOWN_RESTRICTED indicates that such privilege is
required.
EROFS The named file resides on a read-only file system.
The fchownat() function shall fail if:
EACCES fd was not opened with O_SEARCH and the permissions of the
directory underlying fd do not permit directory searches.
EBADF The path argument does not specify an absolute path and the fd
argument is neither AT_FDCWD nor a valid file descriptor open
for reading or searching.
ENOTDIR
The path argument is not an absolute path and fd is a file
descriptor associated with a non-directory file.
These functions may fail if:
EIO An I/O error occurred while reading or writing to the file sys‐
tem.
EINTR The chown() function was interrupted by a signal which was
caught.
EINVAL The owner or group ID supplied is not a value supported by the
implementation.
ELOOP More than {SYMLOOP_MAX} symbolic links were encountered during
resolution of the path argument.
ENAMETOOLONG
The length of a pathname exceeds {PATH_MAX}, or pathname resolu‐
tion of a symbolic link produced an intermediate result with a
length that exceeds {PATH_MAX}.
The fchownat() function may fail if:
EINVAL The value of the flag argument is not valid.
The following sections are informative.
EXAMPLES
None.
APPLICATION USAGE
Although chown() can be used on some implementations by the file owner
to change the owner and group to any desired values, the only portable
use of this function is to change the group of a file to the effective
GID of the calling process or to a member of its group set.
RATIONALE
System III and System V allow a user to give away files; that is, the
owner of a file may change its user ID to anything. This is a serious
problem for implementations that are intended to meet government secu‐
rity regulations. Version 7 and 4.3 BSD permit only the superuser to
change the user ID of a file. Some government agencies (usually not
ones concerned directly with security) find this limitation too confin‐
ing. This volume of POSIX.1‐2008 uses may to permit secure implementa‐
tions while not disallowing System V.
System III and System V allow the owner of a file to change the group
ID to anything. Version 7 permits only the superuser to change the
group ID of a file. 4.3 BSD permits the owner to change the group ID
of a file to its effective group ID or to any of the groups in the list
of supplementary group IDs, but to no others.
The POSIX.1‐1990 standard requires that the chown() function invoked by
a non-appropriate privileged process clear the S_ISGID and the S_ISUID
bits for regular files, and permits them to be cleared for other types
of files. This is so that changes in accessibility do not accidentally
cause files to become security holes. Unfortunately, requiring these
bits to be cleared on non-executable data files also clears the manda‐
tory file locking bit (shared with S_ISGID), which is an extension on
many implementations (it first appeared in System V). These bits should
only be required to be cleared on regular files that have one or more
of their execute bits set.
The purpose of the fchownat() function is to enable changing ownership
of files in directories other than the current working directory with‐
out exposure to race conditions. Any part of the path of a file could
be changed in parallel to a call to chown() or lchown(), resulting in
unspecified behavior. By opening a file descriptor for the target
directory and using the fchownat() function it can be guaranteed that
the changed file is located relative to the desired directory.
FUTURE DIRECTIONS
None.
SEE ALSOchmod(), fpathconf(), lchown()
The Base Definitions volume of POSIX.1‐2008, <fcntl.h>, <sys_types.h>,
<unistd.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri‐
cal and Electronics Engineers, Inc and The Open Group. (This is
POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) In the
event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online
at http://www.unix.org/online.html .
Any typographical or formatting errors that appear in this page are
most likely to have been introduced during the conversion of the source
files to man page format. To report such errors, see https://www.ker‐
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IEEE/The Open Group 2013 CHOWN(3P)
