proc.5: Note kernel version for /proc/PID/smaps VmFlags "wf" flag

[thirdparty/man-pages.git] / man2 / chown.2

.\" 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>
.\"
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.\" manual under the conditions for verbatim copying, provided that the
.\" entire resulting derived work is distributed under the terms of a
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.\" responsibility for errors or omissions, or for damages resulting from
.\" the use of the information contained herein.  The author(s) may not
.\" have taken the same level of care in the production of this manual,
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.\" Modified by Michael Haardt <michael@moria.de>
.\" Modified 1993-07-21 by Rik Faith <faith@cs.unc.edu>
.\" Modified 1996-07-09 by Andries Brouwer <aeb@cwi.nl>
.\" Modified 1996-11-06 by Eric S. Raymond <esr@thyrsus.com>
.\" Modified 1997-05-18 by Michael Haardt <michael@cantor.informatik.rwth-aachen.de>
.\" Modified 2004-06-23 by Michael Kerrisk <mtk.manpages@gmail.com>
.\" 2007-07-08, mtk, added an example program; updated SYNOPSIS
.\" 2008-05-08, mtk, Describe rules governing ownership of new files
.\"     (bsdgroups versus sysvgroups, and the effect of the parent
.\"     directory's set-group-ID mode bit).
.\"
.TH CHOWN 2 2019-03-06 "Linux" "Linux Programmer's Manual"
.SH NAME
chown, fchown, lchown, fchownat \- change ownership of a file
.SH SYNOPSIS
.nf
.B #include <unistd.h>
.PP
.BI "int chown(const char *" pathname ", uid_t " owner ", gid_t " group );
.BI "int fchown(int " fd ", uid_t " owner ", gid_t " group );
.BI "int lchown(const char *" pathname ", uid_t " owner ", gid_t " group );

.BR "#include <fcntl.h>           " "/* Definition of AT_* constants */"
.B #include <unistd.h>
.PP
.BI "int fchownat(int " dirfd ", const char *" pathname ,
.BI "             uid_t " owner ", gid_t " group ", int " flags );
.fi
.PP
.in -4n
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.in
.PP
.BR fchown (),
.BR lchown ():
.PD 0
.ad l
.RS 4
/* Since glibc 2.12: */ _POSIX_C_SOURCE\ >=\ 200809L
    || _XOPEN_SOURCE\ >=\ 500
.\"    || _XOPEN_SOURCE\ &&\ _XOPEN_SOURCE_EXTENDED
    || /* Glibc versions <= 2.19: */ _BSD_SOURCE
.RE
.PP
.BR fchownat ():
.PD 0
.ad l
.RS 4
.TP 4
Since glibc 2.10:
_POSIX_C_SOURCE\ >=\ 200809L
.TP
Before glibc 2.10:
_ATFILE_SOURCE
.RE
.ad
.PD
.SH DESCRIPTION
These system calls change the owner and group of a file.
The
.BR chown (),
.BR fchown (),
and
.BR lchown ()
system calls differ only in how the file is specified:
.IP * 2
.BR chown ()
changes the ownership of the file specified by
.IR pathname ,
which is dereferenced if it is a symbolic link.
.IP *
.BR fchown ()
changes the ownership of the file referred to by the open file descriptor
.IR fd .
.IP *
.BR lchown ()
is like
.BR chown (),
but does not dereference symbolic links.
.PP
Only a privileged process (Linux: one with the
.B 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
.BR CAP_CHOWN )
may change the group arbitrarily.
.PP
If the
.I owner
or
.I group
is specified as \-1, then that ID is not changed.
.PP
When the owner or group of an executable file is
changed by an unprivileged user, the
.B S_ISUID
and
.B S_ISGID
mode bits are cleared.
POSIX does not specify whether
this also should happen when root does the
.BR chown ();
the Linux behavior depends on the kernel version,
and since Linux 2.2.13, root is treated like other users.
.\" In Linux 2.0 kernels, superuser was like everyone else
.\" In 2.2, up to 2.2.12, these bits were not cleared for superuser.
.\" Since 2.2.13, superuser is once more like everyone else.
In case of a non-group-executable file (i.e., one for which the
.B S_IXGRP
bit is not set) the
.B S_ISGID
bit indicates mandatory locking, and is not cleared by a
.BR chown ().
.PP
When the owner or group of an executable file is changed (by any user),
all capability sets for the file are cleared.
.\"
.SS fchownat()
The
.BR fchownat ()
system call operates in exactly the same way as
.BR chown (),
except for the differences described here.
.PP
If the pathname given in
.I pathname
is relative, then it is interpreted relative to the directory
referred to by the file descriptor
.I dirfd
(rather than relative to the current working directory of
the calling process, as is done by
.BR chown ()
for a relative pathname).
.PP
If
.I pathname
is relative and
.I dirfd
is the special value
.BR AT_FDCWD ,
then
.I pathname
is interpreted relative to the current working
directory of the calling process (like
.BR chown ()).
.PP
If
.I pathname
is absolute, then
.I dirfd
is ignored.
.PP
The
.I flags
argument is a bit mask created by ORing together
0 or more of the following values;
.TP
.BR AT_EMPTY_PATH " (since Linux 2.6.39)"
.\" commit 65cfc6722361570bfe255698d9cd4dccaf47570d
If
.I pathname
is an empty string, operate on the file referred to by
.IR dirfd
(which may have been obtained using the
.BR open (2)
.B O_PATH
flag).
In this case,
.I dirfd
can refer to any type of file, not just a directory.
If
.I dirfd
is
.BR AT_FDCWD ,
the call operates on the current working directory.
This flag is Linux-specific; define
.B _GNU_SOURCE
.\" Before glibc 2.16, defining _ATFILE_SOURCE sufficed
to obtain its definition.
.TP
.B AT_SYMLINK_NOFOLLOW
If
.I pathname
is a symbolic link, do not dereference it:
instead operate on the link itself, like
.BR lchown ().
(By default,
.BR fchownat ()
dereferences symbolic links, like
.BR chown ().)
.PP
See
.BR openat (2)
for an explanation of the need for
.BR fchownat ().
.SH RETURN VALUE
On success, zero is returned.
On error, \-1 is returned, and
.I errno
is set appropriately.
.SH ERRORS
Depending on the filesystem,
errors other than those listed below can be returned.
.PP
The more general errors for
.BR chown ()
are listed below.
.TP
.B EACCES
Search permission is denied on a component of the path prefix.
(See also
.BR path_resolution (7).)
.TP
.B EFAULT
.I pathname
points outside your accessible address space.
.TP
.B ELOOP
Too many symbolic links were encountered in resolving
.IR pathname .
.TP
.B ENAMETOOLONG
.I pathname
is too long.
.TP
.B ENOENT
The file does not exist.
.TP
.B ENOMEM
Insufficient kernel memory was available.
.TP
.B ENOTDIR
A component of the path prefix is not a directory.
.TP
.B EPERM
The calling process did not have the required permissions
(see above) to change owner and/or group.
.TP
.B EPERM
The file is marked immutable or append-only.
(See
.BR ioctl_iflags (2).)
.TP
.B EROFS
The named file resides on a read-only filesystem.
.PP
The general errors for
.BR fchown ()
are listed below:
.TP
.B EBADF
.I fd
is not a valid open file descriptor.
.TP
.B EIO
A low-level I/O error occurred while modifying the inode.
.TP
.B ENOENT
See above.
.TP
.B EPERM
See above.
.TP
.B EROFS
See above.
.PP
The same errors that occur for
.BR chown ()
can also occur for
.BR fchownat ().
The following additional errors can occur for
.BR fchownat ():
.TP
.B EBADF
.I dirfd
is not a valid file descriptor.
.TP
.B EINVAL
Invalid flag specified in
.IR flags .
.TP
.B ENOTDIR
.I pathname
is relative and
.I dirfd
is a file descriptor referring to a file other than a directory.
.SH VERSIONS
.BR fchownat ()
was added to Linux in kernel 2.6.16;
library support was added to glibc in version 2.4.
.SH CONFORMING TO
.BR chown (),
.BR fchown (),
.BR lchown ():
4.4BSD, SVr4, POSIX.1-2001, POSIX.1-2008.
.PP
The 4.4BSD version can be
used only by the superuser (that is, ordinary users cannot give away files).
.\" chown():
.\" SVr4 documents EINVAL, EINTR, ENOLINK and EMULTIHOP returns, but no
.\" ENOMEM.  POSIX.1 does not document ENOMEM or ELOOP error conditions.
.\" fchown():
.\" SVr4 documents additional EINVAL, EIO, EINTR, and ENOLINK
.\" error conditions.
.PP
.BR fchownat ():
POSIX.1-2008.
.SH NOTES
.SS Ownership of new files
When a new file is created (by, for example,
.BR open (2)
or
.BR 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
.B "\-o\ grpid"
(or, synonymously
.BR "\-o\ bsdgroups" )
and
.B "\-o\ nogrpid"
(or, synonymously
.BR "\-o\ sysvgroups" )
.BR mount (8)
options, then the rules are as follows:
.IP * 2
If the filesystem is mounted with
.BR "\-o\ grpid" ,
then the group of a new file is made
the same as that of the parent directory.
.IP *
If the filesystem is mounted with
.BR "\-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.
.IP *
If the filesystem is mounted with
.BR "\-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.
.PP
As at Linux 4.12,
the
.BR "\-o\ grpid"
and
.BR "\-o\ nogrpid"
mount options are supported by ext2, ext3, ext4, and XFS.
Filesystems that don't support these mount options follow the
.BR "\-o\ nogrpid"
rules.
.SS Glibc notes
On older kernels where
.BR fchownat ()
is unavailable, the glibc wrapper function falls back to the use of
.BR chown ()
and
.BR lchown ().
When
.I pathname
is a relative pathname,
glibc constructs a pathname based on the symbolic link in
.IR /proc/self/fd
that corresponds to the
.IR dirfd
argument.
.SS NFS
The
.BR 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
.BR 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.
.SS Historical details
The original Linux
.BR chown (),
.BR fchown (),
and
.BR lchown ()
system calls supported only 16-bit user and group IDs.
Subsequently, Linux 2.4 added
.BR chown32 (),
.BR fchown32 (),
and
.BR lchown32 (),
supporting 32-bit IDs.
The glibc
.BR chown (),
.BR fchown (),
and
.BR lchown ()
wrapper functions transparently deal with the variations across kernel versions.
.PP
In versions of Linux prior to 2.1.81 (and distinct from 2.1.46),
.BR chown ()
did not follow symbolic links.
Since Linux 2.1.81,
.BR chown ()
does follow symbolic links, and there is a new system call
.BR lchown ()
that does not follow symbolic links.
Since Linux 2.1.86, this new call (that has the same semantics
as the old
.BR chown ())
has got the same syscall number, and
.BR chown ()
got the newly introduced number.
.SH EXAMPLE
.PP
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
.BR getpwnam (3)
to perform a lookup in the system password file).
.SS Program source
.EX
#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\e0\(aq) {
        fprintf(stderr, "%s <owner> <file>\en", argv[0]);
        exit(EXIT_FAILURE);
    }

    uid = strtol(argv[1], &endptr, 10);  /* Allow a numeric string */

    if (*endptr != \(aq\e0\(aq) {         /* 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);
}
.EE
.SH SEE ALSO
.BR chgrp (1),
.BR chown (1),
.BR chmod (2),
.BR flock (2),
.BR path_resolution (7),
.BR symlink (7)