man2/umount.2

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   2 .\" and Copyright (C) 1994 Andries E. Brouwer <aeb@cwi.nl>
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  27 .\" 2008-10-06, mtk: Created this as a new page by splitting
  28 .\"     umount/umount2 material out of mount.2
  29 .\"
  30 .TH UMOUNT 2 2017-09-15 "Linux" "Linux Programmer's Manual"
  31 .SH NAME
  32 umount, umount2 \- unmount filesystem
  33 .SH SYNOPSIS
  34 .nf
  35 .B "#include <sys/mount.h>"
  36 .PP
  37 .BI "int umount(const char *" target );
  38 .PP
  39 .BI "int umount2(const char *" target ", int " flags );
  40 .fi
  41 .SH DESCRIPTION
  42 .BR umount ()
  43 and
  44 .BR umount2 ()
  45 remove the attachment of the (topmost) filesystem mounted on
  46 .IR target .
  47 .\" Note: the kernel naming differs from the glibc naming
  48 .\" umount2 is the glibc name for what the kernel now calls umount
  49 .\" and umount is the glibc name for oldumount
  50 .PP
  51 Appropriate privilege (Linux: the
  52 .B CAP_SYS_ADMIN
  53 capability) is required to unmount filesystems.
  54 .PP
  55 Linux 2.1.116 added the
  56 .BR umount2 ()
  57 system call, which, like
  58 .BR umount (),
  59 unmounts a target, but allows additional
  60 .I flags
  61 controlling the behavior of the operation:
  62 .TP
  63 .BR MNT_FORCE " (since Linux 2.1.116)"
  64 Ask the filesystem to abort pending requests before attempting the
  65 unmount.
  66 This may allow the unmount to complete without waiting
  67 for an inaccessible server, but could cause data loss.
  68 If, after aborting requests,
  69 some processes still have active references to the filesystem,
  70 the unmount will still fail.
  71 As at Linux 4.12,
  72 .BR MNT_FORCE
  73 is supported only on the following filesystems:
  74 9p (since Linux 2.6.16),
  75 ceph (since Linux 2.6.34),
  76 cifs (since Linux 2.6.12),
  77 fuse (since Linux 2.6.16),
  78 lustre (since Linux 3.11),
  79 and NFS (since Linux 2.1.116).
  80 .TP
  81 .BR MNT_DETACH " (since Linux 2.4.11)"
  82 Perform a lazy unmount: make the mount point unavailable for new
  83 accesses, immediately disconnect the filesystem and all filesystems
  84 mounted below it from each other and from the mount table, and
  85 actually perform the unmount when the mount point ceases to be busy.
  86 .TP
  87 .BR MNT_EXPIRE " (since Linux 2.6.8)"
  88 Mark the mount point as expired.
  89 If a mount point is not currently in use, then an initial call to
  90 .BR umount2 ()
  91 with this flag fails with the error
  92 .BR EAGAIN ,
  93 but marks the mount point as expired.
  94 The mount point remains expired as long as it isn't accessed
  95 by any process.
  96 A second
  97 .BR umount2 ()
  98 call specifying
  99 .B MNT_EXPIRE
 100 unmounts an expired mount point.
 101 This flag cannot be specified with either
 102 .B MNT_FORCE
 103 or
 104 .BR MNT_DETACH .
 105 .TP
 106 .BR UMOUNT_NOFOLLOW " (since Linux 2.6.34)"
 107 .\" Later added to 2.6.33-stable
 108 Don't dereference
 109 .I target
 110 if it is a symbolic link.
 111 This flag allows security problems to be avoided in set-user-ID-\fIroot\fP
 112 programs that allow unprivileged users to unmount filesystems.
 113 .SH RETURN VALUE
 114 On success, zero is returned.
 115 On error, \-1 is returned, and
 116 .I errno
 117 is set appropriately.
 118 .SH ERRORS
 119 The error values given below result from filesystem type independent
 120 errors.
 121 Each filesystem type may have its own special errors and its
 122 own special behavior.
 123 See the Linux kernel source code for details.
 124 .TP
 125 .B EAGAIN
 126 A call to
 127 .BR umount2 ()
 128 specifying
 129 .B MNT_EXPIRE
 130 successfully marked an unbusy filesystem as expired.
 131 .TP
 132 .B EBUSY
 133 .I target
 134 could not be unmounted because it is busy.
 135 .TP
 136 .B EFAULT
 137 .I target
 138 points outside the user address space.
 139 .TP
 140 .B EINVAL
 141 .I target
 142 is not a mount point.
 143 .TP
 144 .B EINVAL
 145 .BR umount2 ()
 146 was called with
 147 .B MNT_EXPIRE
 148 and either
 149 .B MNT_DETACH
 150 or
 151 .BR MNT_FORCE .
 152 .TP
 153 .BR EINVAL " (since Linux 2.6.34)"
 154 .BR umount2 ()
 155 was called with an invalid flag value in
 156 .IR flags .
 157 .TP
 158 .B ENAMETOOLONG
 159 A pathname was longer than
 160 .BR MAXPATHLEN .
 161 .TP
 162 .B ENOENT
 163 A pathname was empty or had a nonexistent component.
 164 .TP
 165 .B ENOMEM
 166 The kernel could not allocate a free page to copy filenames or data into.
 167 .TP
 168 .B EPERM
 169 The caller does not have the required privileges.
 170 .SH VERSIONS
 171 .BR MNT_DETACH
 172 and
 173 .BR MNT_EXPIRE
 174 .\" http://sourceware.org/bugzilla/show_bug.cgi?id=10092
 175 are available in glibc since version 2.11.
 176 .SH CONFORMING TO
 177 These functions are Linux-specific and should not be used in
 178 programs intended to be portable.
 179 .SH NOTES
 180 .SS umount() and shared mount points
 181 Shared mount points cause any mount activity on a mount point, including
 182 .BR umount ()
 183 operations, to be forwarded to every shared mount point in the
 184 peer group and every slave mount of that peer group.
 185 This means that
 186 .BR umount ()
 187 of any peer in a set of shared mounts will cause all of its
 188 peers to be unmounted and all of their slaves to be unmounted as well.
 189 .PP
 190 This propagation of unmount activity can be particularly surprising
 191 on systems where every mount point is shared by default.
 192 On such systems,
 193 recursively bind mounting the root directory of the filesystem
 194 onto a subdirectory and then later unmounting that subdirectory with
 195 .BR MNT_DETACH
 196 will cause every mount in the mount namespace to be lazily unmounted.
 197 .PP
 198 To ensure
 199 .BR umount ()
 200 does not propagate in this fashion,
 201 the mount point may be remounted using a
 202 .BR mount (2)
 203 call with a
 204 .I mount_flags
 205 argument that includes both
 206 .BR MS_REC
 207 and
 208 .BR MS_PRIVATE
 209 prior to
 210 .BR umount ()
 211 being called.
 212 .SS Historical details
 213 The original
 214 .BR umount ()
 215 function was called as \fIumount(device)\fP and would return
 216 .B ENOTBLK
 217 when called with something other than a block device.
 218 In Linux 0.98p4, a call \fIumount(dir)\fP was added, in order to
 219 support anonymous devices.
 220 In Linux 2.3.99-pre7, the call \fIumount(device)\fP was removed,
 221 leaving only \fIumount(dir)\fP (since now devices can be mounted
 222 in more than one place, so specifying the device does not suffice).
 223 .SH SEE ALSO
 224 .BR mount (2),
 225 .BR mount_namespaces (7),
 226 .BR path_resolution (7),
 227 .BR mount (8),
 228 .BR umount (8)