man2/chown.2

   1 .\" Copyright (c) 1992 Drew Eckhardt (drew@cs.colorado.edu), March 28, 1992
   2 .\" and Copyright (c) 1998 Andries Brouwer (aeb@cwi.nl)
   3 .\" and Copyright (c) 2006, 2007, 2008, 2014 Michael Kerrisk <mtk.manpages@gmail.com>
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  26 .\"
  27 .\" Modified by Michael Haardt <michael@moria.de>
  28 .\" Modified 1993-07-21 by Rik Faith <faith@cs.unc.edu>
  29 .\" Modified 1996-07-09 by Andries Brouwer <aeb@cwi.nl>
  30 .\" Modified 1996-11-06 by Eric S. Raymond <esr@thyrsus.com>
  31 .\" Modified 1997-05-18 by Michael Haardt <michael@cantor.informatik.rwth-aachen.de>
  32 .\" Modified 2004-06-23 by Michael Kerrisk <mtk.manpages@gmail.com>
  33 .\" 2007-07-08, mtk, added an example program; updated SYNOPSIS
  34 .\" 2008-05-08, mtk, Describe rules governing ownership of new files
  35 .\"     (bsdgroups versus sysvgroups, and the effect of the parent
  36 .\"     directory's set-group-ID mode bit).
  37 .\"
  38 .TH CHOWN 2 2016-03-15 "Linux" "Linux Programmer's Manual"
  39 .SH NAME
  40 chown, fchown, lchown, fchownat \- change ownership of a file
  41 .SH SYNOPSIS
  42 .nf
  43 .B #include <unistd.h>
  44 .sp
  45 .BI "int chown(const char *" pathname ", uid_t " owner ", gid_t " group );
  46 .br
  47 .BI "int fchown(int " fd ", uid_t " owner ", gid_t " group );
  48 .br
  49 .BI "int lchown(const char *" pathname ", uid_t " owner ", gid_t " group );
  50 .sp
  51 .BR "#include <fcntl.h>           " "/* Definition of AT_* constants */"
  52 .B #include <unistd.h>
  53 .sp
  54 .BI "int fchownat(int " dirfd ", const char *" pathname ,
  55 .BI "             uid_t " owner ", gid_t " group ", int " flags );
  56 .fi
  57 .sp
  58 .in -4n
  59 Feature Test Macro Requirements for glibc (see
  60 .BR feature_test_macros (7)):
  61 .in
  62 .sp
  63 .BR fchown (),
  64 .BR lchown ():
  65 .PD 0
  66 .ad l
  67 .RS 4
  68 /* Since glibc 2.12: */ _POSIX_C_SOURCE\ >=\ 200809L
  69     || _XOPEN_SOURCE\ >=\ 500
  70 .\"    || _XOPEN_SOURCE\ &&\ _XOPEN_SOURCE_EXTENDED
  71     || /* Glibc versions <= 2.19: */ _BSD_SOURCE
  72 .RE
  73 .sp
  74 .BR fchownat ():
  75 .PD 0
  76 .ad l
  77 .RS 4
  78 .TP 4
  79 Since glibc 2.10:
  80 _POSIX_C_SOURCE\ >=\ 200809L
  81 .TP
  82 Before glibc 2.10:
  83 _ATFILE_SOURCE
  84 .RE
  85 .ad
  86 .PD
  87 .SH DESCRIPTION
  88 These system calls change the owner and group of a file.
  89 The
  90 .BR chown (),
  91 .BR fchown (),
  92 and
  93 .BR lchown ()
  94 system calls differ only in how the file is specified:
  95 .IP * 2
  96 .BR chown ()
  97 changes the ownership of the file specified by
  98 .IR pathname ,
  99 which is dereferenced if it is a symbolic link.
 100 .IP *
 101 .BR fchown ()
 102 changes the ownership of the file referred to by the open file descriptor
 103 .IR fd .
 104 .IP *
 105 .BR lchown ()
 106 is like
 107 .BR chown (),
 108 but does not dereference symbolic links.
 109 .PP
 110 Only a privileged process (Linux: one with the
 111 .B CAP_CHOWN
 112 capability) may change the owner of a file.
 113 The owner of a file may change the group of the file
 114 to any group of which that owner is a member.
 115 A privileged process (Linux: with
 116 .BR CAP_CHOWN )
 117 may change the group arbitrarily.
 118
 119 If the
 120 .I owner
 121 or
 122 .I group
 123 is specified as \-1, then that ID is not changed.
 124
 125 When the owner or group of an executable file is
 126 changed by an unprivileged user, the
 127 .B S_ISUID
 128 and
 129 .B S_ISGID
 130 mode bits are cleared.
 131 POSIX does not specify whether
 132 this also should happen when root does the
 133 .BR chown ();
 134 the Linux behavior depends on the kernel version.
 135 .\" In Linux 2.0 kernels, superuser was like everyone else
 136 .\" In 2.2, up to 2.2.12, these bits were not cleared for superuser.
 137 .\" Since 2.2.13, superuser is once more like everyone else.
 138 In case of a non-group-executable file (i.e., one for which the
 139 .B S_IXGRP
 140 bit is not set) the
 141 .B S_ISGID
 142 bit indicates mandatory locking, and is not cleared by a
 143 .BR chown ().
 144 .SS fchownat()
 145 The
 146 .BR fchownat ()
 147 system call operates in exactly the same way as
 148 .BR chown (),
 149 except for the differences described here.
 150
 151 If the pathname given in
 152 .I pathname
 153 is relative, then it is interpreted relative to the directory
 154 referred to by the file descriptor
 155 .I dirfd
 156 (rather than relative to the current working directory of
 157 the calling process, as is done by
 158 .BR chown ()
 159 for a relative pathname).
 160
 161 If
 162 .I pathname
 163 is relative and
 164 .I dirfd
 165 is the special value
 166 .BR AT_FDCWD ,
 167 then
 168 .I pathname
 169 is interpreted relative to the current working
 170 directory of the calling process (like
 171 .BR chown ()).
 172
 173 If
 174 .I pathname
 175 is absolute, then
 176 .I dirfd
 177 is ignored.
 178
 179 The
 180 .I flags
 181 argument is a bit mask created by ORing together
 182 0 or more of the following values;
 183 .TP
 184 .BR AT_EMPTY_PATH " (since Linux 2.6.39)"
 185 .\" commit 65cfc6722361570bfe255698d9cd4dccaf47570d
 186 If
 187 .I pathname
 188 is an empty string, operate on the file referred to by
 189 .IR dirfd
 190 (which may have been obtained using the
 191 .BR open (2)
 192 .B O_PATH
 193 flag).
 194 In this case,
 195 .I dirfd
 196 can refer to any type of file, not just a directory.
 197 If
 198 .I dirfd
 199 is
 200 .BR AT_FDCWD ,
 201 the call operates on the current working directory.
 202 This flag is Linux-specific; define
 203 .B _GNU_SOURCE
 204 .\" Before glibc 2.16, defining _ATFILE_SOURCE sufficed
 205 to obtain its definition.
 206 .TP
 207 .B AT_SYMLINK_NOFOLLOW
 208 If
 209 .I pathname
 210 is a symbolic link, do not dereference it:
 211 instead operate on the link itself, like
 212 .BR lchown ().
 213 (By default,
 214 .BR fchownat ()
 215 dereferences symbolic links, like
 216 .BR chown ().)
 217 .PP
 218 See
 219 .BR openat (2)
 220 for an explanation of the need for
 221 .BR fchownat ().
 222 .SH RETURN VALUE
 223 On success, zero is returned.
 224 On error, \-1 is returned, and
 225 .I errno
 226 is set appropriately.
 227 .SH ERRORS
 228 Depending on the filesystem,
 229 errors other than those listed below can be returned.
 230
 231 The more general errors for
 232 .BR chown ()
 233 are listed below.
 234 .TP
 235 .B EACCES
 236 Search permission is denied on a component of the path prefix.
 237 (See also
 238 .BR path_resolution (7).)
 239 .TP
 240 .B EFAULT
 241 .I pathname
 242 points outside your accessible address space.
 243 .TP
 244 .B ELOOP
 245 Too many symbolic links were encountered in resolving
 246 .IR pathname .
 247 .TP
 248 .B ENAMETOOLONG
 249 .I pathname
 250 is too long.
 251 .TP
 252 .B ENOENT
 253 The file does not exist.
 254 .TP
 255 .B ENOMEM
 256 Insufficient kernel memory was available.
 257 .TP
 258 .B ENOTDIR
 259 A component of the path prefix is not a directory.
 260 .TP
 261 .B EPERM
 262 The calling process did not have the required permissions
 263 (see above) to change owner and/or group.
 264 .TP
 265 .B EROFS
 266 The named file resides on a read-only filesystem.
 267 .PP
 268 The general errors for
 269 .BR fchown ()
 270 are listed below:
 271 .TP
 272 .B EBADF
 273 .I fd
 274 is not a valid open file descriptor.
 275 .TP
 276 .B EIO
 277 A low-level I/O error occurred while modifying the inode.
 278 .TP
 279 .B ENOENT
 280 See above.
 281 .TP
 282 .B EPERM
 283 See above.
 284 .TP
 285 .B EROFS
 286 See above.
 287 .PP
 288 The same errors that occur for
 289 .BR chown ()
 290 can also occur for
 291 .BR fchownat ().
 292 The following additional errors can occur for
 293 .BR fchownat ():
 294 .TP
 295 .B EBADF
 296 .I dirfd
 297 is not a valid file descriptor.
 298 .TP
 299 .B EINVAL
 300 Invalid flag specified in
 301 .IR flags .
 302 .TP
 303 .B ENOTDIR
 304 .I pathname
 305 is relative and
 306 .I dirfd
 307 is a file descriptor referring to a file other than a directory.
 308 .SH VERSIONS
 309 .BR fchownat ()
 310 was added to Linux in kernel 2.6.16;
 311 library support was added to glibc in version 2.4.
 312 .SH CONFORMING TO
 313 .BR chown (),
 314 .BR fchown (),
 315 .BR lchown ():
 316 4.4BSD, SVr4, POSIX.1-2001, POSIX.1-2008.
 317
 318 The 4.4BSD version can be
 319 used only by the superuser (that is, ordinary users cannot give away files).
 320 .\" chown():
 321 .\" SVr4 documents EINVAL, EINTR, ENOLINK and EMULTIHOP returns, but no
 322 .\" ENOMEM.  POSIX.1 does not document ENOMEM or ELOOP error conditions.
 323 .\" fchown():
 324 .\" SVr4 documents additional EINVAL, EIO, EINTR, and ENOLINK
 325 .\" error conditions.
 326
 327 .BR fchownat ():
 328 POSIX.1-2008.
 329 .SH NOTES
 330 .SS Ownership of new files
 331 When a new file is created (by, for example,
 332 .BR open (2)
 333 or
 334 .BR mkdir (2)),
 335 its owner is made the same as the filesystem user ID of the
 336 creating process.
 337 The group of the file depends on a range of factors,
 338 including the type of filesystem,
 339 the options used to mount the filesystem,
 340 and whether or not the set-group-ID mode bit is enabled
 341 on the parent directory.
 342 If the filesystem supports the
 343 .I "\-o\ grpid"
 344 (or, synonymously
 345 .IR "\-o\ bsdgroups" )
 346 and
 347 .I "\-o\ nogrpid"
 348 (or, synonymously
 349 .IR "\-o\ sysvgroups" )
 350 .BR mount (8)
 351 options, then the rules are as follows:
 352 .IP * 2
 353 If the filesystem is mounted with
 354 .IR "\-o\ grpid" ,
 355 then the group of a new file is made
 356 the same as that of the parent directory.
 357 .IP *
 358 If the filesystem is mounted with
 359 .IR "\-o\ nogrpid"
 360 and the set-group-ID bit is disabled on the parent directory,
 361 then the group of a new file is made the same as the
 362 process's filesystem GID.
 363 .IP *
 364 If the filesystem is mounted with
 365 .IR "\-o\ nogrpid"
 366 and the set-group-ID bit is enabled on the parent directory,
 367 then the group of a new file is made
 368 the same as that of the parent directory.
 369 .PP
 370 As at Linux 2.6.25,
 371 the
 372 .IR "\-o\ grpid"
 373 and
 374 .IR "\-o\ nogrpid"
 375 mount options are supported by ext2, ext3, ext4, and XFS.
 376 Filesystems that don't support these mount options follow the
 377 .IR "\-o\ nogrpid"
 378 rules.
 379 .SS Glibc notes
 380 On older kernels where
 381 .BR fchownat ()
 382 is unavailable, the glibc wrapper function falls back to the use of
 383 .BR chown ()
 384 and
 385 .BR lchown ().
 386 When
 387 .I pathname
 388 is a relative pathname,
 389 glibc constructs a pathname based on the symbolic link in
 390 .IR /proc/self/fd
 391 that corresponds to the
 392 .IR dirfd
 393 argument.
 394 .SS NFS
 395 The
 396 .BR chown ()
 397 semantics are deliberately violated on NFS filesystems
 398 which have UID mapping enabled.
 399 Additionally, the semantics of all system
 400 calls which access the file contents are violated, because
 401 .BR chown ()
 402 may cause immediate access revocation on already open files.
 403 Client side
 404 caching may lead to a delay between the time where ownership have
 405 been changed to allow access for a user and the time where the file can
 406 actually be accessed by the user on other clients.
 407 .SS Historical details
 408 The original Linux
 409 .BR chown (),
 410 .BR fchown (),
 411 and
 412 .BR lchown ()
 413 system calls supported only 16-bit user and group IDs.
 414 Subsequently, Linux 2.4 added
 415 .BR chown32 (),
 416 .BR fchown32 (),
 417 and
 418 .BR lchown32 (),
 419 supporting 32-bit IDs.
 420 The glibc
 421 .BR chown (),
 422 .BR fchown (),
 423 and
 424 .BR lchown ()
 425 wrapper functions transparently deal with the variations across kernel versions.
 426
 427 In versions of Linux prior to 2.1.81 (and distinct from 2.1.46),
 428 .BR chown ()
 429 did not follow symbolic links.
 430 Since Linux 2.1.81,
 431 .BR chown ()
 432 does follow symbolic links, and there is a new system call
 433 .BR lchown ()
 434 that does not follow symbolic links.
 435 Since Linux 2.1.86, this new call (that has the same semantics
 436 as the old
 437 .BR chown ())
 438 has got the same syscall number, and
 439 .BR chown ()
 440 got the newly introduced number.
 441 .SH EXAMPLE
 442 .PP
 443 The following program changes the ownership of the file named in
 444 its second command-line argument to the value specified in its
 445 first command-line argument.
 446 The new owner can be specified either as a numeric user ID,
 447 or as a username (which is converted to a user ID by using
 448 .BR getpwnam (3)
 449 to perform a lookup in the system password file).
 450 .SS Program source
 451 .nf
 452 #include <pwd.h>
 453 #include <stdio.h>
 454 #include <stdlib.h>
 455 #include <unistd.h>
 456
 457 int
 458 main(int argc, char *argv[])
 459 {
 460     uid_t uid;
 461     struct passwd *pwd;
 462     char *endptr;
 463
 464     if (argc != 3 || argv[1][0] == \(aq\\0\(aq) {
 465         fprintf(stderr, "%s <owner> <file>\\n", argv[0]);
 466         exit(EXIT_FAILURE);
 467     }
 468
 469     uid = strtol(argv[1], &endptr, 10);  /* Allow a numeric string */
 470
 471     if (*endptr != \(aq\\0\(aq) {         /* Was not pure numeric string */
 472         pwd = getpwnam(argv[1]);   /* Try getting UID for username */
 473         if (pwd == NULL) {
 474             perror("getpwnam");
 475             exit(EXIT_FAILURE);
 476         }
 477
 478         uid = pwd\->pw_uid;
 479     }
 480
 481     if (chown(argv[2], uid, \-1) == \-1) {
 482         perror("chown");
 483         exit(EXIT_FAILURE);
 484     }
 485
 486     exit(EXIT_SUCCESS);
 487 }
 488 .fi
 489 .SH SEE ALSO
 490 .BR chgrp (1),
 491 .BR chown (1),
 492 .BR chmod (2),
 493 .BR flock (2),
 494 .BR path_resolution (7),
 495 .BR symlink (7)