NAME

       umount, umount2 - unmount filesystem

SYNOPSIS

       #include <sys/mount.h>

       int umount(const char *target);

       int umount2(const char *target, int flags);

DESCRIPTION

       umount() and umount2() remove the attachment of the (topmost) filesystem mounted on target.

       Appropriate privilege (Linux: the CAP_SYS_ADMIN capability) is required to unmount filesystems.

       Linux  2.1.116  added  the  umount2()  system  call,  which, like umount(), unmounts a target, but allows
       additional flags controlling the behavior of the operation:

       MNT_FORCE (since Linux 2.1.116)
              Ask the filesystem to abort pending requests before attempting the unmount.  This  may  allow  the
              unmount  to  complete  without waiting for an inaccessible server, but could cause data loss.  If,
              after aborting requests, some processes still  have  active  references  to  the  filesystem,  the
              unmount  will  still  fail.   As  at  Linux  4.12,  MNT_FORCE  is  supported only on the following
              filesystems: 9p (since Linux 2.6.16), ceph (since Linux 2.6.34), cifs (since Linux  2.6.12),  fuse
              (since Linux 2.6.16), lustre (since Linux 3.11), and NFS (since Linux 2.1.116).

       MNT_DETACH (since Linux 2.4.11)
              Perform  a lazy unmount: make the mount point unavailable for new accesses, immediately disconnect
              the filesystem and all filesystems mounted below it from each other and from the mount table,  and
              actually perform the unmount when the mount point ceases to be busy.

       MNT_EXPIRE (since Linux 2.6.8)
              Mark  the  mount point as expired.  If a mount point is not currently in use, then an initial call
              to umount2() with this flag fails with the error EAGAIN, but marks the  mount  point  as  expired.
              The  mount  point remains expired as long as it isn't accessed by any process.  A second umount2()
              call specifying MNT_EXPIRE unmounts an expired mount point.  This flag cannot  be  specified  with
              either MNT_FORCE or MNT_DETACH.

       UMOUNT_NOFOLLOW (since Linux 2.6.34)
              Don't  dereference  target  if  it  is  a symbolic link.  This flag allows security problems to be
              avoided in set-user-ID-root programs that allow unprivileged users to unmount filesystems.

RETURN VALUE

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

ERRORS

       The error values given below result from filesystem type independent errors.  Each  filesystem  type  may
       have its own special errors and its own special behavior.  See the Linux kernel source code for details.

       EAGAIN A call to umount2() specifying MNT_EXPIRE successfully marked an unbusy filesystem as expired.

       EBUSY  target could not be unmounted because it is busy.

       EFAULT target points outside the user address space.

       EINVAL target is not a mount point.

       EINVAL umount2() was called with MNT_EXPIRE and either MNT_DETACH or MNT_FORCE.

       EINVAL (since Linux 2.6.34)
              umount2() was called with an invalid flag value in flags.

       ENAMETOOLONG
              A pathname was longer than MAXPATHLEN.

       ENOENT A pathname was empty or had a nonexistent component.

       ENOMEM The kernel could not allocate a free page to copy filenames or data into.

       EPERM  The caller does not have the required privileges.

VERSIONS

       MNT_DETACH and MNT_EXPIRE are available in glibc since version 2.11.

CONFORMING TO

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

NOTES

   umount() and shared mount points
       Shared  mount  points  cause  any  mount  activity on a mount point, including umount() operations, to be
       forwarded to every shared mount point in the peer group and every slave mount of that peer  group.   This
       means that umount() of any peer in a set of shared mounts will cause all of its peers to be unmounted and
       all of their slaves to be unmounted as well.

       This propagation of unmount activity can be particularly surprising on systems where every mount point is
       shared  by default.  On such systems, recursively bind mounting the root directory of the filesystem onto
       a subdirectory and then later unmounting that subdirectory with MNT_DETACH will cause every mount in  the
       mount namespace to be lazily unmounted.

       To  ensure umount() does not propagate in this fashion, the mount point may be remounted using a mount(2)
       call with a mount_flags argument that includes both MS_REC and MS_PRIVATE prior to umount() being called.

   Historical details
       The original umount() function was called as umount(device) and would return  ENOTBLK  when  called  with
       something  other than a block device.  In Linux 0.98p4, a call umount(dir) was added, in order to support
       anonymous devices.  In Linux 2.3.99-pre7, the call umount(device) was removed, leaving  only  umount(dir)
       (since now devices can be mounted in more than one place, so specifying the device does not suffice).

SEE ALSO

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

COLOPHON

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