NAME

       umount, umount2 - unmount filesystem

LIBRARY

       Standard C library (libc, -lc)

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 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 ceases to  be
              busy.

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

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 target is locked; see mount_namespaces(7).

       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.

STANDARDS

       Linux.

HISTORY

       MNT_DETACH and MNT_EXPIRE are available since glibc 2.11.

       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).

NOTES

   umount() and shared mounts
       Shared  mounts  cause  any mount activity on a mount, including umount() operations, to be
       forwarded to every shared mount 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 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 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.

SEE ALSO

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