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 <= 2.19: */ _BSD_SOURCE
27
28       fchownat():
29           Since glibc 2.10:
30               _POSIX_C_SOURCE >= 200809L
31           Before glibc 2.10:
32               _ATFILE_SOURCE
33

DESCRIPTION

35       These  system calls change the owner and group of a file.  The chown(),
36       fchown(), and lchown() system calls differ only  in  how  the  file  is
37       specified:
38
39       * chown()  changes  the  ownership  of  the file specified by pathname,
40         which is dereferenced if it is a symbolic link.
41
42       * fchown() changes the ownership of the file referred to  by  the  open
43         file descriptor fd.
44
45       * lchown() is like chown(), but does not dereference symbolic links.
46
47       Only  a  privileged  process (Linux: one with the CAP_CHOWN capability)
48       may change the owner of a file.  The owner of a  file  may  change  the
49       group  of  the  file  to  any group of which that owner is a member.  A
50       privileged process (Linux: with CAP_CHOWN) may change the  group  arbi‐
51       trarily.
52
53       If the owner or group is specified as -1, then that ID is not changed.
54
55       When the owner or group of an executable file is changed by an unprivi‐
56       leged user, the S_ISUID and S_ISGID mode bits are cleared.  POSIX  does
57       not specify whether this also should happen when root does the chown();
58       the Linux behavior depends on  the  kernel  version,  and  since  Linux
59       2.2.13,  root is treated like other users.  In case of a non-group-exe‐
60       cutable file (i.e., one for which the S_IXGRP bit is not set) the S_IS‐
61       GID bit indicates mandatory locking, and is not cleared by a 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; de‐
90              fine _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 to indicate the error.
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

EXAMPLES

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 5.12 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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279
280Linux                             2021-03-22                          CHOWN(2)
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