lchown

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 arbitrarily.

       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
       unprivileged 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-
       executable 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,
              fchownat() 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, including 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 supported
       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 symbolic
       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 ownership 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 supported
       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.

EXAMPLES
       The following program changes the ownership of the file named in its
       second command-line argument to the value specified in its first command-
       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
       This page is part of release 5.10 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at
       https://www.kernel.org/doc/man-pages/.




Linux                              2020-06-09                           CHOWN(2)