chown32

CHOWN(2)                   Linux Programmer's Manual                  CHOWN(2)

NAME
       chown, fchown, lchown, fchownat - change ownership of a file

SYNOPSIS
       #include <unistd.h>

       int chown(const char *pathname, uid_t owner, gid_t group);
       int fchown(int fd, uid_t owner, gid_t group);
       int lchown(const char *pathname, uid_t owner, gid_t group);

       #include <fcntl.h>           /* Definition of AT_* constants */
       #include <unistd.h>

       int fchownat(int dirfd, const char *pathname,
                    uid_t owner, gid_t group, int flags);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       fchown(), lchown():
           /* Since glibc 2.12: */ _POSIX_C_SOURCE >= 200809L
               || _XOPEN_SOURCE >= 500
               || /* Glibc versions <= 2.19: */ _BSD_SOURCE
       fchownat():
           Since glibc 2.10:
               _POSIX_C_SOURCE >= 200809L
           Before glibc 2.10:
               _ATFILE_SOURCE

DESCRIPTION
       These  system calls change the owner and group of a file.  The chown(),
       fchown(), and lchown() system calls differ only  in  how  the  file  is
       specified:

       * chown()  changes  the  ownership  of  the file specified by pathname,
         which is dereferenced if it is a symbolic link.

       * fchown() changes the ownership of the file referred to  by  the  open
         file descriptor fd.

       * lchown() is like chown(), but does not dereference symbolic links.

       Only  a  privileged  process (Linux: one with the 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 CAP_CHOWN) may change the  group  arbi-
       trarily.

       If the owner or group is specified as -1, then that ID is not changed.

       When the owner or group of an executable file is changed by an unprivi-
       leged user, the S_ISUID and S_ISGID mode bits are cleared.  POSIX  does
       not specify whether this also should happen when root does the chown();
       the Linux behavior depends on  the  kernel  version,  and  since  Linux
       2.2.13,  root is treated like other users.  In case of a non-group-exe-
       cutable file (i.e., one for which the  S_IXGRP  bit  is  not  set)  the
       S_ISGID  bit  indicates  mandatory  locking,  and  is  not cleared by a
       chown().

       When the owner or group of an executable file is changed (by any user),
       all capability sets for the file are cleared.

   fchownat()
       The fchownat() system call operates in exactly the same way as chown(),
       except for the differences described here.

       If the pathname given in pathname is relative, then it  is  interpreted
       relative  to  the  directory  referred  to by the file descriptor dirfd
       (rather than relative to the current working directory of  the  calling
       process, as is done by chown() for a relative pathname).

       If  pathname  is relative and dirfd is the special value AT_FDCWD, then
       pathname is interpreted relative to the current  working  directory  of
       the calling process (like chown()).

       If pathname is absolute, then dirfd is ignored.

       The flags argument is a bit mask created by ORing together 0 or more of
       the following values;

       AT_EMPTY_PATH (since Linux 2.6.39)
              If pathname is an empty string, operate on the file referred  to
              by  dirfd (which may have been obtained using the open(2) O_PATH
              flag).  In this case, dirfd can refer to any type of  file,  not
              just  a  directory.   If dirfd is AT_FDCWD, the call operates on
              the current working directory.   This  flag  is  Linux-specific;
              define _GNU_SOURCE to obtain its definition.

       AT_SYMLINK_NOFOLLOW
              If  pathname  is a symbolic link, do not dereference it: instead
              operate on the link itself, like lchown().  (By default,  fchow-
              nat() dereferences symbolic links, like chown().)

       See openat(2) for an explanation of the need for fchownat().

RETURN VALUE
       On  success,  zero is returned.  On error, -1 is returned, and errno is
       set appropriately.

ERRORS
       Depending on the filesystem, errors other than those listed  below  can
       be returned.

       The more general errors for chown() are listed below.

       EACCES Search  permission  is denied on a component of the path prefix.
              (See also path_resolution(7).)

       EFAULT pathname points outside your accessible address space.

       ELOOP  Too many symbolic links were encountered in resolving pathname.

       ENAMETOOLONG
              pathname is too long.

       ENOENT The file does not exist.

       ENOMEM Insufficient kernel memory was available.

       ENOTDIR
              A component of the path prefix is not a directory.

       EPERM  The calling process did not have the required  permissions  (see
              above) to change owner and/or group.

       EPERM  The   file   is   marked   immutable   or   append-only.    (See
              ioctl_iflags(2).)

       EROFS  The named file resides on a read-only filesystem.

       The general errors for fchown() are listed below:

       EBADF  fd is not a valid open file descriptor.

       EIO    A low-level I/O error occurred while modifying the inode.

       ENOENT See above.

       EPERM  See above.

       EROFS  See above.

       The same errors that occur for chown() can also occur  for  fchownat().
       The following additional errors can occur for fchownat():

       EBADF  dirfd is not a valid file descriptor.

       EINVAL Invalid flag specified in flags.

       ENOTDIR
              pathname is relative and dirfd is a file descriptor referring to
              a file other than a directory.

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

CONFORMING TO
       chown(), fchown(), lchown(): 4.4BSD, SVr4, POSIX.1-2001, POSIX.1-2008.

       The 4.4BSD version can be used only by the superuser (that is, ordinary
       users cannot give away files).

       fchownat(): POSIX.1-2008.

NOTES
   Ownership of new files
       When a new file is created (by, for example, open(2) or 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,  includ-
       ing  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 -o grpid (or, synonymously
       -o bsdgroups) and -o nogrpid (or, synonymously -o sysvgroups)  mount(8)
       options, then the rules are as follows:

       * If  the  filesystem is mounted with -o grpid, then the group of a new
         file is made the same as that of the parent directory.

       * If the filesystem is mounted with -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.

       * If the filesystem is mounted with -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.

       As at Linux 4.12, the -o grpid and -o nogrpid mount  options  are  sup-
       ported  by  ext2,  ext3, ext4, and XFS.  Filesystems that don't support
       these mount options follow the -o nogrpid rules.

   Glibc notes
       On older kernels where fchownat() is  unavailable,  the  glibc  wrapper
       function  falls back to the use of chown() and lchown().  When pathname
       is a relative pathname, glibc constructs a pathname based on  the  sym-
       bolic link in /proc/self/fd that corresponds to the dirfd argument.

   NFS
       The  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
       chown() may cause immediate access revocation on  already  open  files.
       Client  side  caching may lead to a delay between the time where owner-
       ship 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.

   Historical details
       The  original  Linux  chown(), fchown(), and lchown() system calls sup-
       ported only 16-bit user and group IDs.  Subsequently, Linux  2.4  added
       chown32(),  fchown32(),  and  lchown32(),  supporting  32-bit IDs.  The
       glibc chown(), fchown(), and lchown() wrapper  functions  transparently
       deal with the variations across kernel versions.

       In  versions  of  Linux  prior  to  2.1.81  (and distinct from 2.1.46),
       chown() did not follow symbolic links.   Since  Linux  2.1.81,  chown()
       does  follow  symbolic  links,  and there is a new system call lchown()
       that does not follow symbolic links.  Since Linux 2.1.86, this new call
       (that  has  the  same  semantics  as  the old chown()) has got the same
       syscall number, and chown() got the newly introduced number.

EXAMPLE
       The following program changes the ownership of the file  named  in  its
       second  command-line  argument to the value specified in its first com-
       mand-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
       getpwnam(3) to perform a lookup in the system password file).

   Program source
       #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] == '\0') {
               fprintf(stderr, "%s <owner> <file>\n", argv[0]);
               exit(EXIT_FAILURE);
           }

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

           if (*endptr != '\0') {         /* 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);

       }

SEE ALSO
       chgrp(1), chown(1), chmod(2), flock(2), path_resolution(7), symlink(7)

COLOPHON
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Linux                             2017-09-15                          CHOWN(2)
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