NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | CONFORMING TO | NOTES | EXAMPLE | SEE ALSO | COLOPHON

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

NAME         top

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

SYNOPSIS         top

       #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         top

       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.  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().

   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         top

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

ERRORS         top

       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.

       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         top

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

CONFORMING TO         top

       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         top

   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 2.6.25, 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.

EXAMPLE         top

       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         top

       chmod(2), flock(2), path_resolution(7), symlink(7)

COLOPHON         top

       This page is part of release 4.07 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                            2016-03-15                         CHOWN(2)