1UMOUNT(2)                  Linux Programmer's Manual                 UMOUNT(2)
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NAME

6       umount, umount2 - unmount filesystem
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SYNOPSIS

9       #include <sys/mount.h>
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11       int umount(const char *target);
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13       int umount2(const char *target, int flags);
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DESCRIPTION

16       umount()  and umount2() remove the attachment of the (topmost) filesys‐
17       tem mounted on target.
18
19       Appropriate privilege (Linux: the CAP_SYS_ADMIN capability) is required
20       to unmount filesystems.
21
22       Linux  2.1.116  added  the umount2() system call, which, like umount(),
23       unmounts a target, but allows additional flags controlling the behavior
24       of the operation:
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26       MNT_FORCE (since Linux 2.1.116)
27              Ask  the  filesystem to abort pending requests before attempting
28              the unmount.  This may allow the  unmount  to  complete  without
29              waiting  for  an inaccessible server, but could cause data loss.
30              If, after aborting requests, some processes  still  have  active
31              references  to  the filesystem, the unmount will still fail.  As
32              at Linux 4.12, MNT_FORCE is  supported  only  on  the  following
33              filesystems: 9p (since Linux 2.6.16), ceph (since Linux 2.6.34),
34              cifs (since Linux 2.6.12), fuse  (since  Linux  2.6.16),  lustre
35              (since Linux 3.11), and NFS (since Linux 2.1.116).
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37       MNT_DETACH (since Linux 2.4.11)
38              Perform a lazy unmount: make the mount point unavailable for new
39              accesses, immediately disconnect the filesystem and all filesys‐
40              tems  mounted below it from each other and from the mount table,
41              and actually perform the unmount when the mount point ceases  to
42              be busy.
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44       MNT_EXPIRE (since Linux 2.6.8)
45              Mark  the  mount point as expired.  If a mount point is not cur‐
46              rently in use, then an initial call to umount2() with this  flag
47              fails  with  the  error  EAGAIN,  but  marks  the mount point as
48              expired.  The mount point remains expired as long  as  it  isn't
49              accessed  by  any  process.   A second umount2() call specifying
50              MNT_EXPIRE unmounts an expired mount point.  This flag cannot be
51              specified with either MNT_FORCE or MNT_DETACH.
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53       UMOUNT_NOFOLLOW (since Linux 2.6.34)
54              Don't  dereference  target  if it is a symbolic link.  This flag
55              allows security problems to be avoided in set-user-ID-root  pro‐
56              grams that allow unprivileged users to unmount filesystems.
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RETURN VALUE

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

63       The error values given below result from  filesystem  type  independent
64       errors.   Each  filesystem type may have its own special errors and its
65       own special behavior.  See the Linux kernel source code for details.
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67       EAGAIN A call to umount2() specifying MNT_EXPIRE successfully marked an
68              unbusy filesystem as expired.
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70       EBUSY  target could not be unmounted because it is busy.
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72       EFAULT target points outside the user address space.
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74       EINVAL target is not a mount point.
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76       EINVAL umount2()  was  called  with MNT_EXPIRE and either MNT_DETACH or
77              MNT_FORCE.
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79       EINVAL (since Linux 2.6.34)
80              umount2() was called with an invalid flag value in flags.
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82       ENAMETOOLONG
83              A pathname was longer than MAXPATHLEN.
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85       ENOENT A pathname was empty or had a nonexistent component.
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87       ENOMEM The kernel could not allocate a free page to copy  filenames  or
88              data into.
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90       EPERM  The caller does not have the required privileges.
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VERSIONS

93       MNT_DETACH and MNT_EXPIRE are available in glibc since version 2.11.
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CONFORMING TO

96       These  functions  are Linux-specific and should not be used in programs
97       intended to be portable.
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NOTES

100   umount() and shared mount points
101       Shared mount points cause any mount activity on a mount point,  includ‐
102       ing umount() operations, to be forwarded to every shared mount point in
103       the peer group and every slave mount of that peer  group.   This  means
104       that  umount()  of any peer in a set of shared mounts will cause all of
105       its peers to be unmounted and all of their slaves to  be  unmounted  as
106       well.
107
108       This  propagation of unmount activity can be particularly surprising on
109       systems where every mount point is shared by default.  On such systems,
110       recursively  bind  mounting the root directory of the filesystem onto a
111       subdirectory  and  then  later  unmounting   that   subdirectory   with
112       MNT_DETACH  will  cause every mount in the mount namespace to be lazily
113       unmounted.
114
115       To ensure umount() does not propagate in this fashion, the mount  point
116       may  be remounted using a mount() call with a mount_flags argument that
117       includes both MS_REC and MS_PRIVATE prior to umount() being called.
118
119   Historical details
120       The original umount() function was called as umount(device)  and  would
121       return  ENOTBLK  when  called with something other than a block device.
122       In Linux 0.98p4, a call umount(dir) was  added,  in  order  to  support
123       anonymous  devices.   In Linux 2.3.99-pre7, the call umount(device) was
124       removed, leaving only umount(dir) (since now devices can be mounted  in
125       more than one place, so specifying the device does not suffice).
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SEE ALSO

128       mount(2), mount_namespaces(7), path_resolution(7), mount(8), umount(8)
129

COLOPHON

131       This  page  is  part of release 4.16 of the Linux man-pages project.  A
132       description of the project, information about reporting bugs,  and  the
133       latest     version     of     this    page,    can    be    found    at
134       https://www.kernel.org/doc/man-pages/.
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138Linux                             2017-09-15                         UMOUNT(2)
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