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() 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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