1CHOWN(2)                   Linux Programmer's Manual                  CHOWN(2)
2
3
4

NAME

6       chown, fchown, lchown, fchownat - change ownership of a file
7

SYNOPSIS

9       #include <unistd.h>
10
11       int chown(const char *pathname, uid_t owner, gid_t group);
12       int fchown(int fd, uid_t owner, gid_t group);
13       int lchown(const char *pathname, uid_t owner, gid_t group);
14
15       #include <fcntl.h>           /* Definition of AT_* constants */
16       #include <unistd.h>
17
18       int fchownat(int dirfd, const char *pathname,
19                    uid_t owner, gid_t group, int flags);
20
21   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
22
23       fchown(), lchown():
24           /* Since glibc 2.12: */ _POSIX_C_SOURCE >= 200809L
25               || _XOPEN_SOURCE >= 500
26               || /* Glibc versions <= 2.19: */ _BSD_SOURCE
27       fchownat():
28           Since glibc 2.10:
29               _POSIX_C_SOURCE >= 200809L
30           Before glibc 2.10:
31               _ATFILE_SOURCE
32

DESCRIPTION

34       These  system calls change the owner and group of a file.  The chown(),
35       fchown(), and lchown() system calls differ only  in  how  the  file  is
36       specified:
37
38       * chown()  changes  the  ownership  of  the file specified by pathname,
39         which is dereferenced if it is a symbolic link.
40
41       * fchown() changes the ownership of the file referred to  by  the  open
42         file descriptor fd.
43
44       * lchown() is like chown(), but does not dereference symbolic links.
45
46       Only  a  privileged  process (Linux: one with the CAP_CHOWN capability)
47       may change the owner of a file.  The owner of a  file  may  change  the
48       group  of  the  file  to  any group of which that owner is a member.  A
49       privileged process (Linux: with CAP_CHOWN) may change the  group  arbi‐
50       trarily.
51
52       If the owner or group is specified as -1, then that ID is not changed.
53
54       When the owner or group of an executable file is changed by an unprivi‐
55       leged user, the S_ISUID and S_ISGID mode bits are cleared.  POSIX  does
56       not specify whether this also should happen when root does the chown();
57       the Linux behavior depends on  the  kernel  version,  and  since  Linux
58       2.2.13,  root is treated like other users.  In case of a non-group-exe‐
59       cutable file (i.e., one for which the  S_IXGRP  bit  is  not  set)  the
60       S_ISGID  bit  indicates  mandatory  locking,  and  is  not cleared by a
61       chown().
62
63       When the owner or group of an executable file is changed (by any user),
64       all capability sets for the file are cleared.
65
66   fchownat()
67       The fchownat() system call operates in exactly the same way as chown(),
68       except for the differences described here.
69
70       If the pathname given in pathname is relative, then it  is  interpreted
71       relative  to  the  directory  referred  to by the file descriptor dirfd
72       (rather than relative to the current working directory of  the  calling
73       process, as is done by chown() for a relative pathname).
74
75       If  pathname  is relative and dirfd is the special value AT_FDCWD, then
76       pathname is interpreted relative to the current  working  directory  of
77       the calling process (like chown()).
78
79       If pathname is absolute, then dirfd is ignored.
80
81       The flags argument is a bit mask created by ORing together 0 or more of
82       the following values;
83
84       AT_EMPTY_PATH (since Linux 2.6.39)
85              If pathname is an empty string, operate on the file referred  to
86              by  dirfd (which may have been obtained using the open(2) O_PATH
87              flag).  In this case, dirfd can refer to any type of  file,  not
88              just  a  directory.   If dirfd is AT_FDCWD, the call operates on
89              the current working directory.   This  flag  is  Linux-specific;
90              define _GNU_SOURCE to obtain its definition.
91
92       AT_SYMLINK_NOFOLLOW
93              If  pathname  is a symbolic link, do not dereference it: instead
94              operate on the link itself, like lchown().  (By default,  fchow‐
95              nat() dereferences symbolic links, like chown().)
96
97       See openat(2) for an explanation of the need for fchownat().
98

RETURN VALUE

100       On  success,  zero is returned.  On error, -1 is returned, and errno is
101       set appropriately.
102

ERRORS

104       Depending on the filesystem, errors other than those listed  below  can
105       be returned.
106
107       The more general errors for chown() are listed below.
108
109       EACCES Search  permission  is denied on a component of the path prefix.
110              (See also path_resolution(7).)
111
112       EFAULT pathname points outside your accessible address space.
113
114       ELOOP  Too many symbolic links were encountered in resolving pathname.
115
116       ENAMETOOLONG
117              pathname is too long.
118
119       ENOENT The file does not exist.
120
121       ENOMEM Insufficient kernel memory was available.
122
123       ENOTDIR
124              A component of the path prefix is not a directory.
125
126       EPERM  The calling process did not have the required  permissions  (see
127              above) to change owner and/or group.
128
129       EPERM  The   file   is   marked   immutable   or   append-only.    (See
130              ioctl_iflags(2).)
131
132       EROFS  The named file resides on a read-only filesystem.
133
134       The general errors for fchown() are listed below:
135
136       EBADF  fd is not a valid open file descriptor.
137
138       EIO    A low-level I/O error occurred while modifying the inode.
139
140       ENOENT See above.
141
142       EPERM  See above.
143
144       EROFS  See above.
145
146       The same errors that occur for chown() can also occur  for  fchownat().
147       The following additional errors can occur for fchownat():
148
149       EBADF  dirfd is not a valid file descriptor.
150
151       EINVAL Invalid flag specified in flags.
152
153       ENOTDIR
154              pathname is relative and dirfd is a file descriptor referring to
155              a file other than a directory.
156

VERSIONS

158       fchownat() was added to Linux in kernel  2.6.16;  library  support  was
159       added to glibc in version 2.4.
160

CONFORMING TO

162       chown(), fchown(), lchown(): 4.4BSD, SVr4, POSIX.1-2001, POSIX.1-2008.
163
164       The 4.4BSD version can be used only by the superuser (that is, ordinary
165       users cannot give away files).
166
167       fchownat(): POSIX.1-2008.
168

NOTES

170   Ownership of new files
171       When a new file is created (by, for example, open(2) or mkdir(2)),  its
172       owner  is  made  the  same  as  the  filesystem user ID of the creating
173       process.  The group of the file depends on a range of factors,  includ‐
174       ing  the  type of filesystem, the options used to mount the filesystem,
175       and whether or not the set-group-ID mode bit is enabled on  the  parent
176       directory.   If  the filesystem supports the -o grpid (or, synonymously
177       -o bsdgroups) and -o nogrpid (or, synonymously -o sysvgroups)  mount(8)
178       options, then the rules are as follows:
179
180       * If  the  filesystem is mounted with -o grpid, then the group of a new
181         file is made the same as that of the parent directory.
182
183       * If the filesystem is mounted with -o nogrpid and the set-group-ID bit
184         is  disabled on the parent directory, then the group of a new file is
185         made the same as the process's filesystem GID.
186
187       * If the filesystem is mounted with -o nogrpid and the set-group-ID bit
188         is  enabled  on the parent directory, then the group of a new file is
189         made the same as that of the parent directory.
190
191       As at Linux 4.12, the -o grpid and -o nogrpid mount  options  are  sup‐
192       ported  by  ext2,  ext3, ext4, and XFS.  Filesystems that don't support
193       these mount options follow the -o nogrpid rules.
194
195   Glibc notes
196       On older kernels where fchownat() is  unavailable,  the  glibc  wrapper
197       function  falls back to the use of chown() and lchown().  When pathname
198       is a relative pathname, glibc constructs a pathname based on  the  sym‐
199       bolic link in /proc/self/fd that corresponds to the dirfd argument.
200
201   NFS
202       The  chown()  semantics  are  deliberately  violated on NFS filesystems
203       which have UID mapping enabled.  Additionally,  the  semantics  of  all
204       system  calls  which  access  the  file  contents are violated, because
205       chown() may cause immediate access revocation on  already  open  files.
206       Client  side  caching may lead to a delay between the time where owner‐
207       ship have been changed to allow access for a user and  the  time  where
208       the file can actually be accessed by the user on other clients.
209
210   Historical details
211       The  original  Linux  chown(), fchown(), and lchown() system calls sup‐
212       ported only 16-bit user and group IDs.  Subsequently, Linux  2.4  added
213       chown32(),  fchown32(),  and  lchown32(),  supporting  32-bit IDs.  The
214       glibc chown(), fchown(), and lchown() wrapper  functions  transparently
215       deal with the variations across kernel versions.
216
217       In  versions  of  Linux  prior  to  2.1.81  (and distinct from 2.1.46),
218       chown() did not follow symbolic links.   Since  Linux  2.1.81,  chown()
219       does  follow  symbolic  links,  and there is a new system call lchown()
220       that does not follow symbolic links.  Since Linux 2.1.86, this new call
221       (that  has  the  same  semantics  as  the old chown()) has got the same
222       syscall number, and chown() got the newly introduced number.
223

EXAMPLE

225       The following program changes the ownership of the file  named  in  its
226       second  command-line  argument to the value specified in its first com‐
227       mand-line argument.  The new owner can be specified either as a numeric
228       user  ID,  or  as  a username (which is converted to a user ID by using
229       getpwnam(3) to perform a lookup in the system password file).
230
231   Program source
232       #include <pwd.h>
233       #include <stdio.h>
234       #include <stdlib.h>
235       #include <unistd.h>
236
237       int
238       main(int argc, char *argv[])
239       {
240           uid_t uid;
241           struct passwd *pwd;
242           char *endptr;
243
244           if (argc != 3 || argv[1][0] == '\0') {
245               fprintf(stderr, "%s <owner> <file>\n", argv[0]);
246               exit(EXIT_FAILURE);
247           }
248
249           uid = strtol(argv[1], &endptr, 10);  /* Allow a numeric string */
250
251           if (*endptr != '\0') {         /* Was not pure numeric string */
252               pwd = getpwnam(argv[1]);   /* Try getting UID for username */
253               if (pwd == NULL) {
254                   perror("getpwnam");
255                   exit(EXIT_FAILURE);
256               }
257
258               uid = pwd->pw_uid;
259           }
260
261           if (chown(argv[2], uid, -1) == -1) {
262               perror("chown");
263               exit(EXIT_FAILURE);
264           }
265
266           exit(EXIT_SUCCESS);
267       }
268

SEE ALSO

270       chgrp(1), chown(1), chmod(2), flock(2), path_resolution(7), symlink(7)
271

COLOPHON

273       This page is part of release 4.16 of the Linux  man-pages  project.   A
274       description  of  the project, information about reporting bugs, and the
275       latest    version    of    this    page,    can     be     found     at
276       https://www.kernel.org/doc/man-pages/.
277
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280Linux                             2017-09-15                          CHOWN(2)
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