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



NAME
       pivot_root - change the root file system

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

DESCRIPTION
       pivot_root()  moves  the root file system of the calling process to the directory put_old and makes new_root the new root
       file system of the calling 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 of any processes or  threads  which
       use the old root directory.  The caller of pivot_root() must ensure that processes with root or current working directory
       at the old root operate correctly in either case.  An easy way to ensure this is to change their root and current working
       directory 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 current working directory 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 direc-
       tory busy with their root and current working directory, 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 calling process: pivot_root() may or may not affect its current working directory.  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, that is, adding a nonzero number of /.. to the string pointed to by put_old must
          yield the same directory 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 direc-
       tory, but the mount point of that file system is mounted on put_old.

       new_root does not have to be a mount point.  In this case, /proc/mounts will show the mount point of the file system con-
       taining 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 calling process does not have the CAP_SYS_ADMIN capability.

VERSIONS
       pivot_root() was introduced in Linux 2.3.41.

CONFORMING TO
       pivot_root() is Linux-specific and hence is not portable.

NOTES
       Glibc does not provide a wrapper for this system call; call it using syscall(2).

BUGS
       pivot_root() should not have to change root and current working directory of all other processes in the system.

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

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

COLOPHON
       This page is part of release 3.25 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                                                      2007-06-01                                              PIVOT_ROOT(2)