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UMOUNT(2) Linux Programmer's Manual UMOUNT(2)


     umount, umount2 - unmount filesystem


     #include <sys/mount.h>
     int umount(const char *target);
     int umount2(const char *target, int flags);


     umount()  and umount2() remove the attachment of the (topmost) filesys-
     tem 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 filesys-
            tems  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 cur-
            rently 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  pro-
            grams that allow unprivileged users to unmount filesystems.


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


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


     MNT_DETACH and MNT_EXPIRE are available in glibc since version 2.11.


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


 umount() and shared mount points
     Shared mount points cause any mount activity on a mount point,  includ-
     ing 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
     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
     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).


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


     This  page  is  part of release 4.16 of the Linux man-pages project.  A
     description of the project, information about reporting bugs,  and  the
     latest     version     of     this    page,    can    be    found    at

Linux 2017-09-15 UMOUNT(2)

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