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)