mirror of https://github.com/mkerrisk/man-pages
216 lines
6.2 KiB
Groff
216 lines
6.2 KiB
Groff
.\" Copyright (C) 1993 Rickard E. Faith <faith@cs.unc.edu>
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.\" and Copyright (C) 1994 Andries E. Brouwer <aeb@cwi.nl>
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.\" and Copyright (C) 2002, 2005 Michael Kerrisk <mtk.manpages@gmail.com>
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.\"
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.\" %%%LICENSE_START(VERBATIM)
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.\" Permission is granted to make and distribute verbatim copies of this
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.\" manual provided the copyright notice and this permission notice are
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.\" preserved on all copies.
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.\"
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.\" Permission is granted to copy and distribute modified versions of this
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.\" manual under the conditions for verbatim copying, provided that the
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.\" entire resulting derived work is distributed under the terms of a
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.\" permission notice identical to this one.
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.\"
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.\" Since the Linux kernel and libraries are constantly changing, this
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.\" manual page may be incorrect or out-of-date. The author(s) assume no
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.\" responsibility for errors or omissions, or for damages resulting from
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.\" the use of the information contained herein. The author(s) may not
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.\" have taken the same level of care in the production of this manual,
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.\" which is licensed free of charge, as they might when working
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.\" professionally.
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.\"
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.\" Formatted or processed versions of this manual, if unaccompanied by
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.\" the source, must acknowledge the copyright and authors of this work.
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.\" %%%LICENSE_END
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.\"
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.\" 2008-10-06, mtk: Created this as a new page by splitting
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.\" umount/umount2 material out of mount.2
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.\"
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.TH UMOUNT 2 2016-07-17 "Linux" "Linux Programmer's Manual"
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.SH NAME
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umount, umount2 \- unmount filesystem
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.SH SYNOPSIS
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.nf
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.B "#include <sys/mount.h>"
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.sp
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.BI "int umount(const char *" target );
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.sp
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.BI "int umount2(const char *" target ", int " flags );
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.fi
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.SH DESCRIPTION
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.BR umount ()
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and
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.BR umount2 ()
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remove the attachment of the (topmost) filesystem mounted on
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.IR target .
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.\" Note: the kernel naming differs from the glibc naming
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.\" umount2 is the glibc name for what the kernel now calls umount
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.\" and umount is the glibc name for oldumount
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Appropriate privilege (Linux: the
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.B CAP_SYS_ADMIN
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capability) is required to unmount filesystems.
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Linux 2.1.116 added the
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.BR umount2 ()
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system call, which, like
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.BR umount (),
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unmounts a target, but allows additional
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.I flags
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controlling the behavior of the operation:
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.TP
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.BR MNT_FORCE " (since Linux 2.1.116)"
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Force unmount even if busy.
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This can cause data loss.
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(Only for NFS mounts.)
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.TP
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.BR MNT_DETACH " (since Linux 2.4.11)"
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Perform a lazy unmount: make the mount point unavailable for new
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accesses, immediately disconnect the filesystem and all filesystems
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mounted below it from each other and from the mount table, and
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actually perform the unmount when the mount point ceases to be busy.
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.TP
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.BR MNT_EXPIRE " (since Linux 2.6.8)"
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Mark the mount point as expired.
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If a mount point is not currently in use, then an initial call to
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.BR umount2 ()
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with this flag fails with the error
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.BR EAGAIN ,
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but marks the mount point as expired.
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The mount point remains expired as long as it isn't accessed
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by any process.
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A second
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.BR umount2 ()
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call specifying
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.B MNT_EXPIRE
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unmounts an expired mount point.
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This flag cannot be specified with either
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.B MNT_FORCE
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or
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.BR MNT_DETACH .
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.TP
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.BR UMOUNT_NOFOLLOW " (since Linux 2.6.34)"
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.\" Later added to 2.6.33-stable
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Don't dereference
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.I target
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if it is a symbolic link.
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This flag allows security problems to be avoided in set-user-ID-\fIroot\fP
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programs that allow unprivileged users to unmount filesystems.
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.SH RETURN VALUE
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On success, zero is returned.
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On error, \-1 is returned, and
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.I errno
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is set appropriately.
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.SH ERRORS
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The error values given below result from filesystem type independent
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errors.
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Each filesystem type may have its own special errors and its
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own special behavior.
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See the Linux kernel source code for details.
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.TP
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.B EAGAIN
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A call to
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.BR umount2 ()
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specifying
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.B MNT_EXPIRE
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successfully marked an unbusy filesystem as expired.
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.TP
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.B EBUSY
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.I target
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could not be unmounted because it is busy.
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.TP
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.B EFAULT
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.I target
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points outside the user address space.
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.TP
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.B EINVAL
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.I target
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is not a mount point.
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.TP
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.B EINVAL
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.BR umount2 ()
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was called with
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.B MNT_EXPIRE
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and either
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.B MNT_DETACH
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or
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.BR MNT_FORCE .
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.TP
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.BR EINVAL " (since Linux 2.6.34)"
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.BR umount2 ()
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was called with an invalid flag value in
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.IR flags .
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.TP
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.B ENAMETOOLONG
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A pathname was longer than
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.BR MAXPATHLEN .
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.TP
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.B ENOENT
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A pathname was empty or had a nonexistent component.
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.TP
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.B ENOMEM
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The kernel could not allocate a free page to copy filenames or data into.
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.TP
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.B EPERM
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The caller does not have the required privileges.
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.SH VERSIONS
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.BR MNT_DETACH
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and
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.BR MNT_EXPIRE
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.\" http://sourceware.org/bugzilla/show_bug.cgi?id=10092
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are available in glibc since version 2.11.
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.SH CONFORMING TO
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These functions are Linux-specific and should not be used in
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programs intended to be portable.
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.SH NOTES
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.SS umount() and shared mount points
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Shared mount points cause any mount activity on a mount point, including
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.BR umount ()
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operations, to be forwarded to every shared mount point in the
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peer group and every slave mount of that peer group.
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This means that
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.BR umount ()
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of any peer in a set of shared mounts will cause all of its
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peers to be unmounted and all of their slaves to be unmounted as well.
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This propagation of unmount activity can be particularly surprising
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on systems where every mount point is shared by default.
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On such systems,
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recursively bind mounting the root directory of the filesystem
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onto a subdirectory and then later unmounting that subdirectory with
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.BR MNT_DETACH
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will cause every mount in the mount namespace to be lazily unmounted.
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To ensure
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.BR umount ()
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does not propagate in this fashion,
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the mount point may be remounted using a
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.BR mount ()
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call with a
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.I mount_flags
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argument that includes both
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.BR MS_REC
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and
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.BR MS_PRIVATE
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prior to
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.BR umount ()
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being called.
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.SS Historical details
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The original
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.BR umount ()
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function was called as \fIumount(device)\fP and would return
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.B ENOTBLK
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when called with something other than a block device.
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In Linux 0.98p4, a call \fIumount(dir)\fP was added, in order to
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support anonymous devices.
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In Linux 2.3.99-pre7, the call \fIumount(device)\fP was removed,
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leaving only \fIumount(dir)\fP (since now devices can be mounted
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in more than one place, so specifying the device does not suffice).
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.SH SEE ALSO
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.BR mount (2),
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.BR mount_namespaces (7),
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.BR path_resolution (7),
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.BR mount (8),
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.BR umount (8)
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