PIVOT_ROOT(2)							   System Calls 						     PIVOT_ROOT(2)

NAME

       pivot_root - change the root file system

SYNOPSIS

       #include <linux/unistd.h>

       _syscall2(int,pivot_root,const char *,new_root,const char *,put_old)

       int pivot_root(const char *new_root, const char *put_old);

DESCRIPTION

       pivot_root  moves  the  root file system of the current process to the directory put_old and makes new_root the new root file system of the
       current process.

       The typical use of pivot_root is during system startup, when the system mounts a temporary root file system (e.g. an initrd),  then  mounts
       the real root file system, and eventually turns the latter into the current root of all relevant processes or threads.

       pivot_root  may	or  may  not change the current root and the current working directory (cwd) of any processes or threads which use the old
       root directory. The caller of pivot_root must ensure that processes with root or cwd at the old root operate correctly in either  case.	An
       easy way to ensure this is to change their root and cwd to new_root before invoking pivot_root.

       The  paragraph  above  is  intentionally  vague	because the implementation of pivot_root may change in the future. At the time of writing,
       pivot_root changes root and cwd of each process or thread to new_root if they point to the old root directory. This is necessary  in  order
       to  prevent  kernel  threads from keeping the old root directory busy with their root and cwd, even if they never access the file system in
       any way. In the future, there may be a mechanism for kernel threads to explicitly relinquish any access to the file system, such that  this
       fairly intrusive mechanism can be removed from pivot_root.

       Note  that  this  also  applies	to  the  current  process:  pivot_root	may or may not affect its cwd. It is therefore recommended to call
       chdir("/") immediately after pivot_root.

       The following restrictions apply to new_root and put_old:

       -  They must be directories.

       -  new_root and put_old must not be on the same file system as the current root.

       -  put_old must be underneath new_root, i.e. adding a non-zero number of /.. to the string pointed to by put_old must yield the same direc-
	  tory as new_root.

       -  No other file system may be mounted on put_old.

       See also pivot_root(8) for additional usage examples.

       If  the	current  root is not a mount point (e.g. after chroot(2) or pivot_root, see also below), not the old root directory, but the mount
       point of that file system is mounted on put_old.

NOTES

       new_root does not have to be a mount point. In this case, /proc/mounts will show the mount point of the file system containing new_root	as
       root (/).

RETURN VALUE

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

ERRORS

       pivot_root may return (in errno) any of the errors returned by stat(2). Additionally, it may return:

       EBUSY  new_root or put_old are on the current root file system, or a file system is already mounted on put_old.

       EINVAL put_old is not underneath new_root.

       ENOTDIR
	      new_root or put_old is not a directory.

       EPERM  The current process does not have the administrator capability.

BUGS

       pivot_root should not have to change root and cwd of all other processes in the system.

       Some of the more obscure uses of pivot_root may quickly lead to insanity.

CONFORMING TO

       pivot_root is Linux-specific and hence is not portable.

HISTORY

       pivot_root was introduced in Linux 2.3.41.

SEE ALSO

       chdir(2), chroot(2), initrd(4), pivot_root(8), stat(2)

Linux								    2000-02-23							     PIVOT_ROOT(2)