1PIVOT_ROOT(2) Linux Programmer's Manual PIVOT_ROOT(2)
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6 pivot_root - change the root file system
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9 int pivot_root(const char *new_root, const char *put_old);
10 Note: There is no glibc wrapper for this system call; see NOTES.
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14 pivot_root() moves the root file system of the calling process to the
15 directory put_old and makes new_root the new root file system of the
16 calling process.
17 The typical use of pivot_root() is during system startup, when the sys‐
19 tem mounts a temporary root file system (e.g., an initrd), then mounts
20 the real root file system, and eventually turns the latter into the
21 current root of all relevant processes or threads.
22 pivot_root() may or may not change the current root and the current
24 working directory of any processes or threads which use the old root
25 directory. The caller of pivot_root() must ensure that processes with
26 root or current working directory at the old root operate correctly in
27 either case. An easy way to ensure this is to change their root and
28 current working directory to new_root before invoking pivot_root().
29 The paragraph above is intentionally vague because the implementation
31 of pivot_root() may change in the future. At the time of writing,
32 pivot_root() changes root and current working directory of each process
33 or thread to new_root if they point to the old root directory. This is
34 necessary in order to prevent kernel threads from keeping the old root
35 directory busy with their root and current working directory, even if
36 they never access the file system in any way. In the future, there may
37 be a mechanism for kernel threads to explicitly relinquish any access
38 to the file system, such that this fairly intrusive mechanism can be
39 removed from pivot_root().
40 Note that this also applies to the calling process: pivot_root() may or
42 may not affect its current working directory. It is therefore recom‐
43 mended to call chdir("/") immediately after pivot_root().
44 The following restrictions apply to new_root and put_old:
46 - They must be directories.
48 - new_root and put_old must not be on the same file system as the cur‐
50 rent root.
51 - put_old must be underneath new_root, that is, adding a nonzero num‐
53 ber of /.. to the string pointed to by put_old must yield the same
54 directory as new_root.
55 - No other file system may be mounted on put_old.
57 See also pivot_root(8) for additional usage examples.
59 If the current root is not a mount point (e.g., after chroot(2) or
61 pivot_root(), see also below), not the old root directory, but the
62 mount point of that file system is mounted on put_old.
63 new_root does not have to be a mount point. In this case, /proc/mounts
65 will show the mount point of the file system containing new_root as
66 root (/).
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69 On success, zero is returned. On error, -1 is returned, and errno is
70 set appropriately.
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73 pivot_root() may return (in errno) any of the errors returned by
74 stat(2). Additionally, it may return:
75 EBUSY new_root or put_old are on the current root file system, or a
77 file system is already mounted on put_old.
78 EINVAL put_old is not underneath new_root.
80 ENOTDIR
82 new_root or put_old is not a directory.
83 EPERM The calling process does not have the CAP_SYS_ADMIN capability.
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87 pivot_root() was introduced in Linux 2.3.41.
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90 pivot_root() is Linux-specific and hence is not portable.
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93 Glibc does not provide a wrapper for this system call; call it using
94 syscall(2).
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97 pivot_root() should not have to change root and current working direc‐
98 tory of all other processes in the system.
99 Some of the more obscure uses of pivot_root() may quickly lead to
101 insanity.
102
104 chdir(2), chroot(2), stat(2), initrd(4), pivot_root(8)
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107 This page is part of release 3.53 of the Linux man-pages project. A
108 description of the project, and information about reporting bugs, can
109 be found at http://www.kernel.org/doc/man-pages/.
110Linux 2012-07-13 PIVOT_ROOT(2)