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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 directory 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 relin-
       quish 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 non-zero 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 directory, 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 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 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 3.22 of the Linux man-pages project.  A description of the project, and informa-
       tion about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



Linux                             2007-06-01                     PIVOT_ROOT(2)