fchown(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.  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

   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.

   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  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

   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 4.09 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/.

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