pivot_root() - Unix, Linux System Call
pivot_root - change the root file system
int pivot_root(const char *new_root, const char *put_old);
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
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
The paragraph above is intentionally vague because the implementation
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.
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
Note that this also applies to the current process:
or may not affect its cwd.
It is therefore recommended to call
chdir("/") immediately after
The following restrictions apply to new_root and put_old:
for additional usage examples.
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 directory as new_root.
No other file system may be mounted on put_old.
If the current root is not a mount point (e.g. after
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 (/).
On success, zero is returned.
On error, -1 is returned, and
errno is set appropriately.
pivot_root() may return (in errno) any of the errors returned by
Additionally, it may return:
new_root or put_old are on the current root file system,
or a file system is already mounted on put_old.
put_old is not underneath new_root.
new_root or put_old is not a directory.
The current process does not have the
pivot_root() was introduced in Linux 2.3.41.
pivot_root() is Linux specific and hence is not portable.
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
Glibc does not provide a wrapper for this system call; call it using