1UMOUNT(2) Linux Programmer's Manual UMOUNT(2)
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6 umount, umount2 - unmount filesystem
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9 #include <sys/mount.h>
10 int umount(const char *target);
12 int umount2(const char *target, int flags);
13
15 umount() and umount2() remove the attachment of the (topmost) filesys‐
16 tem mounted on target.
17 Appropriate privilege (Linux: the CAP_SYS_ADMIN capability) is required
19 to unmount filesystems.
20 Linux 2.1.116 added the umount2() system call, which, like umount(),
22 unmounts a target, but allows additional flags controlling the behavior
23 of the operation:
24 MNT_FORCE (since Linux 2.1.116)
26 Ask the filesystem to abort pending requests before attempting
27 the unmount. This may allow the unmount to complete without
28 waiting for an inaccessible server, but could cause data loss.
29 If, after aborting requests, some processes still have active
30 references to the filesystem, the unmount will still fail. As
31 at Linux 4.12, MNT_FORCE is supported only on the following
32 filesystems: 9p (since Linux 2.6.16), ceph (since Linux 2.6.34),
33 cifs (since Linux 2.6.12), fuse (since Linux 2.6.16), lustre
34 (since Linux 3.11), and NFS (since Linux 2.1.116).
35 MNT_DETACH (since Linux 2.4.11)
37 Perform a lazy unmount: make the mount unavailable for new ac‐
38 cesses, immediately disconnect the filesystem and all filesys‐
39 tems mounted below it from each other and from the mount table,
40 and actually perform the unmount when the mount ceases to be
41 busy.
42 MNT_EXPIRE (since Linux 2.6.8)
44 Mark the mount as expired. If a mount is not currently in use,
45 then an initial call to umount2() with this flag fails with the
46 error EAGAIN, but marks the mount as expired. The mount remains
47 expired as long as it isn't accessed by any process. A second
48 umount2() call specifying MNT_EXPIRE unmounts an expired mount.
49 This flag cannot be specified with either MNT_FORCE or MNT_DE‐
50 TACH.
51 UMOUNT_NOFOLLOW (since Linux 2.6.34)
53 Don't dereference target if it is a symbolic link. This flag
54 allows security problems to be avoided in set-user-ID-root pro‐
55 grams that allow unprivileged users to unmount filesystems.
56
58 On success, zero is returned. On error, -1 is returned, and errno is
59 set to indicate the error.
60
62 The error values given below result from filesystem type independent
63 errors. Each filesystem type may have its own special errors and its
64 own special behavior. See the Linux kernel source code for details.
65 EAGAIN A call to umount2() specifying MNT_EXPIRE successfully marked an
67 unbusy filesystem as expired.
68 EBUSY target could not be unmounted because it is busy.
70 EFAULT target points outside the user address space.
72 EINVAL target is not a mount point.
74 EINVAL target is locked; see mount_namespaces(7).
76 EINVAL umount2() was called with MNT_EXPIRE and either MNT_DETACH or
78 MNT_FORCE.
79 EINVAL (since Linux 2.6.34)
81 umount2() was called with an invalid flag value in flags.
82 ENAMETOOLONG
84 A pathname was longer than MAXPATHLEN.
85 ENOENT A pathname was empty or had a nonexistent component.
87 ENOMEM The kernel could not allocate a free page to copy filenames or
89 data into.
90 EPERM The caller does not have the required privileges.
92
94 MNT_DETACH and MNT_EXPIRE are available in glibc since version 2.11.
95
97 These functions are Linux-specific and should not be used in programs
98 intended to be portable.
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101 umount() and shared mounts
102 Shared mounts cause any mount activity on a mount, including umount()
103 operations, to be forwarded to every shared mount in the peer group and
104 every slave mount of that peer group. This means that umount() of any
105 peer in a set of shared mounts will cause all of its peers to be un‐
106 mounted and all of their slaves to be unmounted as well.
107 This propagation of unmount activity can be particularly surprising on
109 systems where every mount is shared by default. On such systems, re‐
110 cursively bind mounting the root directory of the filesystem onto a
111 subdirectory and then later unmounting that subdirectory with MNT_DE‐
112 TACH will cause every mount in the mount namespace to be lazily un‐
113 mounted.
114 To ensure umount() does not propagate in this fashion, the mount may be
116 remounted using a mount(2) call with a mount_flags argument that in‐
117 cludes both MS_REC and MS_PRIVATE prior to umount() being called.
118 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).
126
128 mount(2), mount_namespaces(7), path_resolution(7), mount(8), umount(8)
129
131 This page is part of release 5.13 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/.
135Linux 2021-08-27 UMOUNT(2)