man2/chmod.2

   1 .\" Copyright (c) 1992 Drew Eckhardt (drew@cs.colorado.edu), March 28, 1992
   2 .\" and Copyright (C) 2006, 2014 Michael Kerrisk
   3 .\"
   4 .\" %%%LICENSE_START(VERBATIM)
   5 .\" Permission is granted to make and distribute verbatim copies of this
   6 .\" manual provided the copyright notice and this permission notice are
   7 .\" preserved on all copies.
   8 .\"
   9 .\" Permission is granted to copy and distribute modified versions of this
  10 .\" manual under the conditions for verbatim copying, provided that the
  11 .\" entire resulting derived work is distributed under the terms of a
  12 .\" permission notice identical to this one.
  13 .\"
  14 .\" Since the Linux kernel and libraries are constantly changing, this
  15 .\" manual page may be incorrect or out-of-date.  The author(s) assume no
  16 .\" responsibility for errors or omissions, or for damages resulting from
  17 .\" the use of the information contained herein.  The author(s) may not
  18 .\" have taken the same level of care in the production of this manual,
  19 .\" which is licensed free of charge, as they might when working
  20 .\" professionally.
  21 .\"
  22 .\" Formatted or processed versions of this manual, if unaccompanied by
  23 .\" the source, must acknowledge the copyright and authors of this work.
  24 .\" %%%LICENSE_END
  25 .\"
  26 .\" Modified by Michael Haardt <michael@moria.de>
  27 .\" Modified 1993-07-21 by Rik Faith <faith@cs.unc.edu>
  28 .\" Modified 1997-01-12 by Michael Haardt
  29 .\"   <michael@cantor.informatik.rwth-aachen.de>: NFS details
  30 .\" Modified 2004-06-23 by Michael Kerrisk <mtk.manpages@gmail.com>
  31 .\"
  32 .TH CHMOD 2 2017-09-15 "Linux" "Linux Programmer's Manual"
  33 .SH NAME
  34 chmod, fchmod, fchmodat \- change permissions of a file
  35 .SH SYNOPSIS
  36 .nf
  37 .B #include <sys/stat.h>
  38 .PP
  39 .BI "int chmod(const char *" pathname ", mode_t " mode );
  40 .br
  41 .BI "int fchmod(int " fd ", mode_t " mode );
  42
  43 .BR "#include <fcntl.h>" "           /* Definition of AT_* constants */"
  44 .B #include <sys/stat.h>
  45 .PP
  46 .BI "int fchmodat(int " dirfd ", const char *" pathname ", mode_t " \
  47 mode ", int " flags );
  48 .fi
  49 .PP
  50 .in -4n
  51 Feature Test Macro Requirements for glibc (see
  52 .BR feature_test_macros (7)):
  53 .in
  54 .PP
  55 .ad l
  56 .PD 0
  57 .BR fchmod ():
  58 .RS 4
  59 Since glibc 2.24:
  60     _POSIX_C_SOURCE\ >=\ 199309L
  61 .\"        || (_XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED)
  62 .PP
  63 Glibc 2.19 to 2.23
  64     _POSIX_C_SOURCE
  65 .PP
  66 Glibc 2.16 to 2.19:
  67     _BSD_SOURCE || _POSIX_C_SOURCE
  68 .PP
  69 Glibc 2.12 to 2.16:
  70     _BSD_SOURCE || _XOPEN_SOURCE\ >=\ 500 ||
  71         _POSIX_C_SOURCE >= 200809L
  72 .PP
  73 Glibc 2.11 and earlier:
  74     _BSD_SOURCE || _XOPEN_SOURCE\ >=\ 500
  75 .\"        || (_XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED)
  76 .RE
  77 .PD
  78 .PP
  79 .BR fchmodat ():
  80 .PD 0
  81 .ad l
  82 .RS 4
  83 .TP 4
  84 Since glibc 2.10:
  85 _POSIX_C_SOURCE\ >=\ 200809L
  86 .TP
  87 Before glibc 2.10:
  88 _ATFILE_SOURCE
  89 .RE
  90 .ad
  91 .PD
  92 .ad
  93 .SH DESCRIPTION
  94 The
  95 .BR chmod ()
  96 and
  97 .BR fchmod ()
  98 system calls change a files mode bits.
  99 (The file mode consists of the file permission bits plus the set-user-ID,
 100 set-group-ID, and sticky bits.)
 101 These system calls differ only in how the file is specified:
 102 .IP * 2
 103 .BR chmod ()
 104 changes the mode of the file specified whose pathname is given in
 105 .IR pathname ,
 106 which is dereferenced if it is a symbolic link.
 107 .IP *
 108 .BR fchmod ()
 109 changes the mode of the file referred to by the open file descriptor
 110 .IR fd .
 111 .PP
 112 The new file mode is specified in
 113 .IR mode ,
 114 which is a bit mask created by ORing together zero or
 115 more of the following:
 116 .TP 18
 117 .BR S_ISUID "  (04000)"
 118 set-user-ID (set process effective user ID on
 119 .BR execve (2))
 120 .TP
 121 .BR S_ISGID "  (02000)"
 122 set-group-ID (set process effective group ID on
 123 .BR execve (2);
 124 mandatory locking, as described in
 125 .BR fcntl (2);
 126 take a new file's group from parent directory, as described in
 127 .BR chown (2)
 128 and
 129 .BR mkdir (2))
 130 .TP
 131 .BR S_ISVTX "  (01000)"
 132 sticky bit (restricted deletion flag, as described in
 133 .BR unlink (2))
 134 .TP
 135 .BR S_IRUSR "  (00400)"
 136 read by owner
 137 .TP
 138 .BR S_IWUSR "  (00200)"
 139 write by owner
 140 .TP
 141 .BR S_IXUSR "  (00100)"
 142 execute/search by owner ("search" applies for directories,
 143 and means that entries within the directory can be accessed)
 144 .TP
 145 .BR S_IRGRP "  (00040)"
 146 read by group
 147 .TP
 148 .BR S_IWGRP "  (00020)"
 149 write by group
 150 .TP
 151 .BR S_IXGRP "  (00010)"
 152 execute/search by group
 153 .TP
 154 .BR S_IROTH "  (00004)"
 155 read by others
 156 .TP
 157 .BR S_IWOTH "  (00002)"
 158 write by others
 159 .TP
 160 .BR S_IXOTH "  (00001)"
 161 execute/search by others
 162 .PP
 163 The effective UID of the calling process must match the owner of the file,
 164 or the process must be privileged (Linux: it must have the
 165 .B CAP_FOWNER
 166 capability).
 167 .PP
 168 If the calling process is not privileged (Linux: does not have the
 169 .B CAP_FSETID
 170 capability), and the group of the file does not match
 171 the effective group ID of the process or one of its
 172 supplementary group IDs, the
 173 .B S_ISGID
 174 bit will be turned off,
 175 but this will not cause an error to be returned.
 176 .PP
 177 As a security measure, depending on the filesystem,
 178 the set-user-ID and set-group-ID execution bits
 179 may be turned off if a file is written.
 180 (On Linux, this occurs if the writing process does not have the
 181 .B CAP_FSETID
 182 capability.)
 183 On some filesystems, only the superuser can set the sticky bit,
 184 which may have a special meaning.
 185 For the sticky bit, and for set-user-ID and set-group-ID bits on
 186 directories, see
 187 .BR inode (7).
 188 .PP
 189 On NFS filesystems, restricting the permissions will immediately influence
 190 already open files, because the access control is done on the server, but
 191 open files are maintained by the client.
 192 Widening the permissions may be
 193 delayed for other clients if attribute caching is enabled on them.
 194 .\"
 195 .\"
 196 .SS fchmodat()
 197 The
 198 .BR fchmodat ()
 199 system call operates in exactly the same way as
 200 .BR chmod (),
 201 except for the differences described here.
 202 .PP
 203 If the pathname given in
 204 .I pathname
 205 is relative, then it is interpreted relative to the directory
 206 referred to by the file descriptor
 207 .I dirfd
 208 (rather than relative to the current working directory of
 209 the calling process, as is done by
 210 .BR chmod ()
 211 for a relative pathname).
 212 .PP
 213 If
 214 .I pathname
 215 is relative and
 216 .I dirfd
 217 is the special value
 218 .BR AT_FDCWD ,
 219 then
 220 .I pathname
 221 is interpreted relative to the current working
 222 directory of the calling process (like
 223 .BR chmod ()).
 224 .PP
 225 If
 226 .I pathname
 227 is absolute, then
 228 .I dirfd
 229 is ignored.
 230 .PP
 231 .I flags
 232 can either be 0, or include the following flag:
 233 .TP
 234 .B AT_SYMLINK_NOFOLLOW
 235 If
 236 .I pathname
 237 is a symbolic link, do not dereference it:
 238 instead operate on the link itself.
 239 This flag is not currently implemented.
 240 .PP
 241 See
 242 .BR openat (2)
 243 for an explanation of the need for
 244 .BR fchmodat ().
 245 .SH RETURN VALUE
 246 On success, zero is returned.
 247 On error, \-1 is returned, and
 248 .I errno
 249 is set appropriately.
 250 .SH ERRORS
 251 Depending on the filesystem,
 252 errors other than those listed below can be returned.
 253 .PP
 254 The more general errors for
 255 .BR chmod ()
 256 are listed below:
 257 .TP
 258 .B EACCES
 259 Search permission is denied on a component of the path prefix.
 260 (See also
 261 .BR path_resolution (7).)
 262 .TP
 263 .B EFAULT
 264 .I pathname
 265 points outside your accessible address space.
 266 .TP
 267 .B EIO
 268 An I/O error occurred.
 269 .TP
 270 .B ELOOP
 271 Too many symbolic links were encountered in resolving
 272 .IR pathname .
 273 .TP
 274 .B ENAMETOOLONG
 275 .I pathname
 276 is too long.
 277 .TP
 278 .B ENOENT
 279 The file does not exist.
 280 .TP
 281 .B ENOMEM
 282 Insufficient kernel memory was available.
 283 .TP
 284 .B ENOTDIR
 285 A component of the path prefix is not a directory.
 286 .TP
 287 .B EPERM
 288 The effective UID does not match the owner of the file,
 289 and the process is not privileged (Linux: it does not have the
 290 .B CAP_FOWNER
 291 capability).
 292 .TP
 293 .B EPERM
 294 The file is marked immutable or append-only.
 295 (See
 296 .BR ioctl_iflags (2).)
 297 .TP
 298 .B EROFS
 299 The named file resides on a read-only filesystem.
 300 .PP
 301 The general errors for
 302 .BR fchmod ()
 303 are listed below:
 304 .TP
 305 .B EBADF
 306 The file descriptor
 307 .I fd
 308 is not valid.
 309 .TP
 310 .B EIO
 311 See above.
 312 .TP
 313 .B EPERM
 314 See above.
 315 .TP
 316 .B EROFS
 317 See above.
 318 .PP
 319 The same errors that occur for
 320 .BR chmod ()
 321 can also occur for
 322 .BR fchmodat ().
 323 The following additional errors can occur for
 324 .BR fchmodat ():
 325 .TP
 326 .B EBADF
 327 .I dirfd
 328 is not a valid file descriptor.
 329 .TP
 330 .B EINVAL
 331 Invalid flag specified in
 332 .IR flags .
 333 .TP
 334 .B ENOTDIR
 335 .I pathname
 336 is relative and
 337 .I dirfd
 338 is a file descriptor referring to a file other than a directory.
 339 .TP
 340 .B ENOTSUP
 341 .I flags
 342 specified
 343 .BR AT_SYMLINK_NOFOLLOW ,
 344 which is not supported.
 345 .SH VERSIONS
 346 .BR fchmodat ()
 347 was added to Linux in kernel 2.6.16;
 348 library support was added to glibc in version 2.4.
 349 .SH CONFORMING TO
 350 .BR chmod (),
 351 .BR fchmod ():
 352 4.4BSD, SVr4, POSIX.1-2001i, POSIX.1-2008.
 353 .PP
 354 .BR fchmodat ():
 355 POSIX.1-2008.
 356 .SH NOTES
 357 .SS C library/kernel differences
 358 The GNU C library
 359 .BR fchmodat ()
 360 wrapper function implements the POSIX-specified
 361 interface described in this page.
 362 This interface differs from the underlying Linux system call, which does
 363 .I not
 364 have a
 365 .I flags
 366 argument.
 367 .SS Glibc notes
 368 On older kernels where
 369 .BR fchmodat ()
 370 is unavailable, the glibc wrapper function falls back to the use of
 371 .BR chmod ().
 372 When
 373 .I pathname
 374 is a relative pathname,
 375 glibc constructs a pathname based on the symbolic link in
 376 .IR /proc/self/fd
 377 that corresponds to the
 378 .IR dirfd
 379 argument.
 380 .SH SEE ALSO
 381 .BR chmod (1),
 382 .BR chown (2),
 383 .BR execve (2),
 384 .BR open (2),
 385 .BR stat (2),
 386 .BR inode (7),
 387 .BR path_resolution (7),
 388 .BR symlink (7)