NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | CONFORMING TO | NOTES | BUGS | SEE ALSO | COLOPHON

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

NAME         top

       pivot_root - change the root filesystem

SYNOPSIS         top

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

       Note: There is no glibc wrapper for this system call; see NOTES.

DESCRIPTION         top

       pivot_root() moves the root filesystem of the calling process to the
       directory put_old and makes new_root the new root filesystem of the
       calling process.

       The typical use of pivot_root() is during system startup, when the
       system mounts a temporary root filesystem (e.g., an initrd), then
       mounts the real root filesystem, 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 filesystem in any
       way.  In the future, there may be a mechanism for kernel threads to
       explicitly relinquish any access to the filesystem, 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 filesystem 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 filesystem 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 filesystem 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 filesystem containing
       new_root as root (/).

RETURN VALUE         top

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

ERRORS         top

       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 filesystem, or a
              filesystem 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         top

       pivot_root() was introduced in Linux 2.3.41.

CONFORMING TO         top

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

NOTES         top

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

BUGS         top

       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         top

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

COLOPHON         top

       This page is part of release 4.08 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                            2012-07-13                    PIVOT_ROOT(2)