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man:chown

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

     This page is part of release 4.16 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 2017-09-15 CHOWN(2)

/data/webs/external/dokuwiki/data/pages/man/chown.txt · Last modified: 2019/05/17 09:47 by 127.0.0.1

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