fchown

       #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():
           _BSD_SOURCE || _XOPEN_SOURCE >= 500 ||
           _XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED
           || /* Since glibc 2.12: */ _POSIX_C_SOURCE >= 200809L

       fchownat():
           Since glibc 2.10:
               _XOPEN_SOURCE >= 700 || _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 are changed by an unpriv-
       ileged  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().


       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.

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

       The general errors for fchown() are listed below:
       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 2.6.25, 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.

       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);
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