pivot_root - change the root file system
_syscall2(int,pivot_root,const char *,new_root,const char
int pivot_root(const char *new_root, const char *put_old);
pivot_root moves the root file system of the current pro
cess 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
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 sys
tem as the current root.
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 (/).
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 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
EINVAL put_old is not underneath new_root.
new_root or put_old is not a directory.
EPERM The current process does not have the administrator
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.
pivot_root is Linux-specific and hence is not portable.
pivot_root was introduced in Linux 2.3.41.