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Linux 2.6 - man page for fchown (linux section 2)

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

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

SYNOPSIS
       #include <unistd.h>

       int chown(const char *path, uid_t owner, gid_t group);
       int fchown(int fd, uid_t owner, gid_t group);
       int lchown(const char *path, uid_t owner, gid_t group);

   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

DESCRIPTION
       These system calls change the owner and group of a file.  They differ only in how the file
       is specified:

       * chown() changes the ownership of the file specified by path, 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 are 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  ver-
       sion.   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().

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

ERRORS
       Depending on the filesystem, other errors 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_res-
	      olution(7).)

       EFAULT path points outside your accessible address space.

       ELOOP  Too many symbolic links were encountered in resolving path.

       ENAMETOOLONG
	      path 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  The descriptor is not valid.

       EIO    A low-level I/O error occurred while modifying the inode.

       ENOENT See above.

       EPERM  See above.

       EROFS  See above.

CONFORMING TO
       4.4BSD, SVr4, POSIX.1-2001.

       The  4.4BSD version can be used only by the superuser (that is, ordinary users cannot give
       away files).

NOTES
       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(), sup-
       porting 32-bit IDs.  The glibc chown(), fchown(), and lchown() wrapper functions transpar-
       ently deal with the variations across kernel versions.

       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 permission 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  fol-
       lows:

       * 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.

       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.

       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).

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

COLOPHON
       This page is part of release 3.55 of the Linux man-pages project.  A  description  of  the
       project,     and    information	  about    reporting	bugs,	 can	be    found    at
       http://www.kernel.org/doc/man-pages/.

Linux					    2010-11-22					 CHOWN(2)
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