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)
              Force unmount even if busy.  This can cause data loss.  (Only for NFS mounts.)

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

COLOPHON

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